Dissertations / Theses on the topic 'Magnetic Organic Molecules'

The present thesis is concerned with the theoretical studies on magnetic and photochemical properties of organic molecules. The ab initio and first principles theories were employed to investigate the vibrational effects on the isotropic hyperfine coupling constant (HFCC) known as the critical parameter in electron paramagnetic resonance spectrum, the theoretical simulations of the vibronically resolved molecular spectra, the photo-induced reaction mechanism of α-santonin and the spin-forbidden reaction of triplet-state dioxygen with cofactor-free enzyme. The theoretical predictions shed light on the interpretation of experimental observations, the understanding of reaction mechanism, and importantly the guideline and perspective in respect of the popularized applications. We focused on the vibrational corrections to the isotropic HFCCs of hydrogen and carbon atoms in organic radicals. The calculations indicate that the vibrational contributions induce or enhance the effect of spin polarization. A set of rules were stated to guide experimentalist and theoretician in identification of the contributions from the molecular vibrations to HFCCs. And the coupling of spin density with vibrational modes in the backbone is significant and provides the insight into the spin density transfer mechanism in organic π radicals. The spectral characters of the intermediates in solid-state photoarrangement of α-santonin were investigated in order to well understand the underlying experimental spectra. The molecular spectra simulated with Franck-Condon principle show that the positions of the absorption and emission bands of photosantonic acid well match with the experimental observations and the absorption spectrum has a vibrationally resolved character. α-Santonin is the first found organic molecule that has the photoreaction activities. The photorearrangement mechanism is theoretically predicted that the low-lying excited state 1(nπ*) undergoing an intersystem crossing process decays to 3(ππ*) state in the Franck-Condon region. A pathway which is favored in the solid-state reaction requires less space and dynamic advantage on the excited-state potential energy surface (PES). And the other pathway is predominant in the weak polar solvent due to the thermodynamical and dynamical preferences. Lumisantonin is a critical intermediate derived from α-santonin photoreaction. The 3(ππ*) state plays a key role in lumisantonin photolysis. The photolytic pathway is in advantage of dynamics and thermodynamics on the triplet-state PES. In contrast, the other reaction pathway is facile for pyrolysis ascribed to a stable intermediate formed on the ground-state PES.  The mechanism of the oxidation reaction involving cofactor-free enzyme and triplet-state dioxygen were studied. The theoretical calculations show that the charge-transfer mechanism is not a sole way to make a spin-forbidden oxidation allowed. It is more likely to take place in the reactant consisting of a non-conjugated substrate. The other mechanism involving the surface hopping between the triplet- and singlet-state PESs via a minimum energy crossing point (MECP) without a significant charge migration. The electronic state of MECP exhibits a mixed characteristic of the singlet and triplet states. The enhanced conjugation of the substrate slows down the spin-flip rate, and this step can in fact control the rate of the reaction that a dioxygen attaches to a substrate.
QC 20111220

Topological Insulators (TIs) have become one of the wonder materials of condensed matter physics over the last decade due to their novel properties, possessing an insulating bulk in coexistence with metallic boundaries. They present an inverted band gap consequence of strong spin orbit coupling, which gives rise to the conductive boundary states with linear dispersion, characteristic of Dirac fermions, and helical spin polarization. Numerous materials have been predicted and observed to have TI signatures, holding great perspective for the realization of novel applications in spintronics, quantum computing and metrology. The experimental realization of three-dimensional TIs with the the Fermi Level located well in the bulk band gap is a challenging task because of their relatively small gap of hundreds of meV, and their high sensibility to crystal defects and impurities. These can induce electron doping that activates bulk conduction channels, thus burying the contribution of the surface states to the transport. Molecular Beam Epitaxy (MBE) has been reported to be the most suitable growth method to overcome this hindrance, due to its capability to grow single crystals with fine control over the crystal defects and impurity level. The first part of this thesis deals with the growth of high-quality TIs that maintain their pristine insulating bulk behaviour. By using MBE, we studied the impact of different substrates and growth parameters to the synthesis of Bismuth Telluride (Bi2Te3) thin films, and the growth of the ternary compound Bismuth-Antimony Telluride. We were able to grow insulating Bi2Te3 thin films with complete suppression of the \twin" domains, mirror-symmetric domains that contribute to the self-doping of the crystal. By a combination of the initial interaction with the lattice-matched Barium Fluoride substrate and the high working temperatures, the growth of Bi2Te3 single-crystalline films is achieved already from the first layer. More importantly, the films present low-doping level with the the Fermi Level kept in the bulk band gap. The correlation between the lack of \twin" domains (measured by Re ection High-Energy Electron Diffraction, X-ray Diffraction and Atomic Force Microscopy) and the low-doping level measured by Angle- Resolved Photoemission Spectroscopy (ARPES), indicates the relation between the crystal quality and the capability to preserve the bulk insulating character. This result contrasts to other TIs grown on more conventional substrates, typically presenting large lattice misfits that lead to the formation of an initial polycrystalilne or amorphous seed layer. In parallel, we explored a complemeniii tary approach to the growth of insulating Bi2Te3, based on the addition of Sb at the expense of Te atoms. A sequence of Bismuth-Antimony Telluride films with different x content were measured by X-ray Photoemission Spectroscopy (XPS) and ARPES, showing that the the Fermi Level can be gradually brought to the bulk valence band. The realization of such TIs, with a controlled level of the the Fermi Level position is of special interest for counteracting the n-doping effects typically induced by the addition of magnetic materials. The second and more extended part of this thesis is devoted to the study of interfaces formed by magnetic Metal-organic molecules deposited on the TI thin films. Interfacing TI surfaces with magnetic materials can give rise to novel magnetoelectronic phenomena, involving the manipulation of spin-torques (Inverse Edelstein Effect), or the realization of spin polarized edge states (Quantum Anomalous Hall Effect). The realization of such spin-related effects rely on the capability to control the interfacial magnetic and electronic interactions. The use of organic molecules to cage magnetic ions has been proved to be a versatile approach to engineer inter-ions and ions-surface interactions, due to the exible design that molecules offer and to their ability to form structurally perfect selfassembled structures. Moreover, they can also act as building blocks for covalent or coordination structures via on-surface reactions. As a first approach to tune the interfacial properties with Metal-organic molecules, we showed how the ligand chemistry allows a progressive control over the magnetic interactions between a hosted Co ion and a prototypical Au surface. The spin states and magnetic moments are comprehensively studied thanks to the complementary use of local spectroscopic Scanning Tunneling Spectroscopy and non-local magnetic sensitive X-ray Magnetic Circular Dichroism (XMCD) techniques, which are supported theoretically by Density Functional Theory (DFT). We were able to continuously cover the range of magnetic Co ion-substrate interactions, from a strong interacting scenario where the magnetic moment is quenched, to a gradual decrease of the interaction revealed by a lower Kondo screening of the spin. In addition, by changing the Au surface for a TI surface, the interfacial interactions reach the weakest limit in which the molecular magnetic structure is completely decoupled from the substrate electrons. Thereafter, we explored the electronic and magnetic interactions between the Topological Surface State of the Bi2Te3 thin film and Co ions caged in two different planar molecules such as Cobalt - Tetrakis (4-Promophenyl) Porphyrin (CoTBrPP) and Cobalt - Phthalocyanine (CoPc). We found a Metal-organic / TI interface with unperturbed electronic and magnetic properties. This is assessed by a coverage dependent ARPES study in which the Topological Surface State persists upon the deposition of one (CoTBrPP or CoPc) molecular layer. On the other hand, XMCD and Scanning Tunneling Spectroscopy measurements reveal the preservation of the pristine CoTBrPP magnetic moment and electronic structure respectively. Furthermore, a comprehensive Scanning Tunneling Microscopy (STM) and DFT study of the CoTBrPP adsorption geometry describes weak molecule-surface interactions, and corroborates the electronic decoupling of the Metal-organic layer from the TI surface. In an analogue study with CoPc we find slightly stronger interactions yet within the non-perturbative regime, that suggesting ligand chemistry can be used to tune magnetic interactions without affecting the overall properties of each component of the heterostructure. Subsequently, the Br-functionalized CoTBrPP on Bi2Te3 system was used to induce on-surface synthesis of Metal-organic coordination networks on TI. These more entangled structures are of great interest as a framework in which magnetic ions could arrange in ordered and mechanically stable arrays. Two different coordination phases are selectively created after CoTBrPP dehalogenation upon thermal activation. We track the chemical reaction by XPS, and investigate the morphological and electronic properties of the final products by combining Scanning Tunneling Spectroscopy (STS) and DFT calculations. We conclude that the resulting structures consists of CoTPP coordinated with Te atoms incorporated from the substrate, and thanks to the supporting DFT calculations, we are able to explain the presence of linear chains and irregular coordinated networks. In parallel, the presence of unperturbed Topological Surface State upon the formation of the Metal-organic structures is confirmed by a coverage-dependent ARPES study. Overall, the first part of the thesis constitutes an extensive study of MBE grown of Bi2Te3 thin films, in which different substrates and growth conditions are discussed. Furthermore, the results provide a route for the enhancement of the crystal quality of simple diatomic TIs, crucial for the preservation of their bulk insulating behaviour. The results presented in the second part conceive the capabilities of organic molecules to tune magnetic interactions between Co atoms and Bi2Te3 films, and pave the way for the on-TI surface synthesis of magnetic supramolecular structures.

Comprendre les propriétés des interfaces molécules/métaux est d’une importance capitale pour la spintronique organique. La première partie porte sur l’étude des propriétés magnétiques de molécules de phtalocyanine de manganèse. Nous avons montré que les premières couches moléculaires forment des colonnes avec un arrangement antiferromagnétique sur la surface de Co(100). Ces dernières mènent à de l’anisotropie d’échange. La seconde partie porte sur l’étude d’une molécule à transition de spin, la Fe(phen)2(NCS)2, sublimée sur différentes surfaces. Nous avons identifié les états de spin d’une molécule unique sur du Cu(100). De plus, nous avons commuté l’état de spin d’une molécule unique pourvu qu’elle soit suffisamment découplée du substrat
Understanding the properties of molecules at the interface with metals is a fundamental issue for organic spintronics. The first part is devoted to the study of magnetic properties of planar manganese-phthalocyanine molecules and Co films. We evidenced that the first molecular layers form vertical columns with antiferromagnetic ordering on the Co(100) surface. In turn, these molecular columns lead to exchange bias. The second part is focused on the study of a spin-crossover complex, Fe(phen)2(NCS)2 sublimed on different metallic surfaces. We identified the two spin states of a single molecules on Cu(100). By applying voltages pulses, we switched the spin state of a single molecule provided that it is sufficiently decoupled from the substrate

The study of the interaction between matter and electromagnetic radiation which procreated the wide branch named as Spectroscopy has gained tremendous attention since the last century. Atoms and molecules respond to electromagnetic radiation to produce their unique spectra which can be used to detect, identify and quantify valuable information about the substance under study. Since, its conception, spectroscopy has been widely used in physical and analytical chemistry and has ramified various techniques depending on the different types of radiation. This dissertation focuses on implementation of various spectroscopic techniques such as Infrared, Raman and Nuclear Magnetic Resonance (NMR) spectroscopy in order to determine theoretical, conformational, qualitative and quantitative properties of different molecules under study.

Understanding the physical structure of a molecule is fundamental for function, dynamic, and mechanism studies. Infrared and Raman spectroscopy are two of the most widely used and powerful techniques for the accurate determination of molecular symmetry and conformational stability. They provide information of molecular vibrations and the two techniques complement each other to yield more complete information about the molecular structure than when they are evaluated separately. One of the focus of this dissertation is the determination of the structural parameters, conformational stability, vibrational assignments and ab initio calculations of organic molecules containing five membered ring and phosphorous by utilizing infrared and Raman spectral techniques. The findings of my spectroscopic, structural, and theoretical studies are based on infrared and/or Raman spectra of gas, liquid, solid as well as variable temperature xenon solutions, and microwave spectrum which are supported by ab initio and DFT calculations.

Nearly four decades ago the potential of Nuclear Magnetic Resonance (NMR) spectroscopy for the quantitative analysis of organic chemicals was first demonstrated. Along with solution state NMR, for past two decades solid state NMR spectroscopy has also come to the forefront of quantitative analytical techniques in pharmaceutical research, as, both of these techniques have been successfully applied to the study of polymorphism in pharmaceutical drugs at both the qualitative and quantitative levels. The investigation of our research presented in this dissertation was initiated by selecting AIDS, the predominant pandemic of twenty-first century and Tenofovir (TFV), a well-tested antiretroviral drug that has proven its mettle against HIV/AIDS. In order to be able to accurately quantify the amount of drug being delivered in human body is a crucial requirement of any drug development process. We specifically focused on phosphorous containing drugs and hence, a part of this dissertation describes about the development and implementation of a general ³¹ P qNMR method to achieve direct, real time quantification of in vitro drug release. We have effectively utilized both solution state and solid state ³¹P qNMR spectroscopic techniques to establish the kinetics of drug release and to determine the encapsulation efficiency of nano-formulation for a particular drug under study, respectively. The in vitro drug release profile has been studied in various human body fluids such as simulated vaginal & seminal fluids, plasma etc. depending on the drug under study. The results of method validation parameters for TFV in simulated vaginal & seminal fluid and human plasma obtained by using ³¹P solution state qNMR spectroscopy are presented in this dissertation.

Another chapter of this dissertation explains the analysis of calcined clay as supplementary cementitious material, obtained from Ghana, a West African nation, which does not have an abundance of commonly used SCMs such as fly ash, silica fume, metakaolin, and slag. However, the abundance of clay minerals in the country could provide a sustainable alternative with respect to SCMs application. Qualitative techniques such as Thermal Gravimetric Analysis (TGA) and Forrier Transform Infrared Spectroscopy (FTIR), and quantitative tools like Nuclear Magnetic Resonance (NMR) are able to provide meaningful characterizing of thermally activated clays. In this study, clay from Ghana was thermally activated at temperatures of 600, 700, 800, 900 and 1000°C. The main objective was to characterize calcined clay using TGA, FTIR, NMR, and their relation to pozzolanic activity to best understand the potential of this abundant resource to alleviate cement supply burdens.

The controlled assembly of molecular building blocks is an emerging strategy that allows for the preparation of materials with tailor-made properties. This involves the precise combination of molecular subunits that interact with one another via specifically designed reactive sites. Such a strategy has produced materials exhibiting remarkable properties, including those based on metal-organic frameworks and single-molecule magnets. The present Thesis aims to highlight how such metal-organic assemblies can be engineered at the molecular level to promote certain desired functionalities. Specifically, Chapter 2 will focus on the confinement effects of a crystalline sponge on a ferrocene-based guest molecule that is nanostructured within the porous cavities of a host material. In doing so, we evaluate how one can exert some level of control over the binding sites of the guest molecule, through the addition of electron-withdrawing groups, as well as tuning the physical properties of the guest itself through molecular encapsulation. Notably, we demonstrate a distinct change in the dynamic rotational motion of the ferrocene molecules once confined within the crystalline sponge. In Chapter 3, we investigate the generation of slow relaxation of the magnetization from a Co(II)-based metal-organic framework. We compare this to a closely related 2D Co(II) sheet network, and how slight changes in the crystal field, probed through computational methods, can impact the magnetic behaviour. This type of study may be particularly beneficial in the optimization of single-ion magnets, by sequestering metal centres in select chemical environments, and minimizing molecular vibrations that may offer alternative magnetic relaxation pathways. We extend these principles in Chapter 4, through the use of a nitrogen-rich ligand that acts as a scaffold for Ln(III) ions, thereby yielding 0D and 1D architectures. The coordination chemistry of Ln(III) ions with N-donor ligands remains scarce, especially when evaluated from a magnetic perspective, and therefore, we sought to determine the magnetic behaviour of such compounds. The monomeric unit displays clear single-molecule magnet behaviour with an energetic barrier for the reversal of the magnetization, while the 1D chain displays weaker magnetic characteristics. Nevertheless, such compounds incorporating nitrogen-rich ligands offer much promise in the design of environmentally-friendly energetic materials. In Chapter 5, we take a look at different two different systems that involve the formation of radical species. On one hand, we can promote enhanced magnetic communication between Ln(III) ions, which is typically quite challenging to achieve given the buried nature of the 4f orbitals, and on the other hand, we rely on a redox-active ligand to design stimuli-responsive metal-organic assemblies. The latter case provides access to “smart” molecular materials that can respond to changes in their environment. Here, a multi-stimuli responsive nanobarrel was studied, which displayed sensitivity to ultraviolet radiation, heat and chemical reduction. Lastly, Chapter 6 provides a new method for the systematic generation of cationic frameworks, termed Asymmetric Ligand Exchange (ALE). This strategy focuses on the replacement of linear dicarboxylates with asymmetric linkers that features one less negative charge, in order to tune the ionicity of porous frameworks. This allows for the retention of the structural topology and chemical reactivity of the original framework, representing distinct advantages over other similar strategies. Methods to retain permanent porosity in such cationic frameworks are also proposed. Altogether, these studies highlight how the directed assembly of ordered networks can generate varied properties of high scientific interest.

ET charge transfer salts (where ET is bis(ethylenedithio)- tetrathiafulvalene) have relatively simple quasi two-dimensional Fermi surface topologies, making them ideal for the study of the relationship between bandstructure and properties such as superconductivity. Experimental studies of the Fermi surface areas and associated effective masses have been carried out using the Shubnikov-de Haas (SdH) and de Haas-van Alphen (dHvA) effects. By comparing the experimental results to theoretical bandstructure calculations the strength of many body interactions has been estimated. High pressure magnetotransport experiments have been carried out on the superconductor κ-ET₂Cu(NCS)₂. The observation of SdH and magnetic breakdown oscillations has allowed the pressure dependences of the Fermi surface topology and effective masses to be deduced and compared with simultaneous measurements of the superconducting critical temperature. The data strongly suggest that the enhancement of the effective mass and the superconducting behaviour are directly connected. The results are compared with several current theories of superconductivity. The dHvA effect has been used to probe the superconducting mixed state of κ-ET₂Cu(NCS)₂. A recent model of the superconducting mixed state is applied to the experimental data in an attempt to determine the value and symmetry of the superconducting energy gap. SdH measurements up to 30 T have been used to study spin densitywave formation in α-ET₂KHg(SCN)₄, and the reasons why a very slight increase of the unit cell volume (i.e. replacing the K in α-ET₂KHg(SCN)₄ by NH₄) stabilises a superconducting state. Galvanomagnetic techniques have been used to measure the quasi onedimensional Fermi surface orientation below the spin-density-wave transition, and to accurately determine the shape of the quasi twodimensional Fermi surface above it. The application of pressure has been used to gradually reduce the onset temperature of a metal-insulator transition and to eventually stabilise a superconducting state in ET₃Cl₂2H₂O. The bandstructure of ET₃C1₂2H₂O has been investigated using the SdH effect whilst hydrostatic pressure has been used to pass through the superconducting part of the phase diagram.

Several new families of organic acceptors that are candidates as building blocks of molecule-based ferrimagnets were synthesized and characterized. These families include fluorodicyanostilbenes, a tetrachlorodicyanostilbene, naphthyltricyanoethylenes, bromophenyltricyanoethylenes, and an anthryltricyanoethylene. The magnetic networks were synthesized by reacting each acceptor with V(CO)6. The magnets synthesized in this study were characterized using a SQUID magnetometer, elemental analysis, and infrared spectroscopy. Although some combinations failed to yield magnetically ordered materials, others exhibited ordering temperatures in the range of 95 K – 260 K. The ordering temperatures and saturation magnetizations were compared among families of acceptors and correlated with individual properties of the acceptors such as reduction potential and structure.
Doctor of Philosophy
Several new families of organic molecules have been created and examined for use as building blocks of molecule-based magnets. These families include fluorodicyanostilbenes, a tetrachlorodicyanostilbene, naphthyltricyanoethylenes, bromophenyltricyanoethylenes, and an anthryltricyanoethylene. The 3-D magnetic scaffoldings were created by combining an individual organic molecule in one of the families listed above with vanadium. The magnets created in this study were examined using a SQUID magnetometer, elemental analysis, and infrared spectroscopy. Some of the combinations of the organic molecules with vanadium failed to result in a 3-D magnetic scaffolding and showed no magnetic properties. Others showed magnetic properties in the below certain temperatures in the range of 95 K – 260 K. The magnetic properties were compared among families of molecules and correlated with individual properties of each molecule such as electronic effects and structure.

This dissertation presents the investigation of magnetic exchange and anisotropy in novel metal-organic complexes containing minimum number of magnetic ions. Such complexes can serve as a model system to understand the exciting magnetic phenomena in such class of materials and also can put forward as candidates for the so called molecular nanomagnets. A direct assessment of the effective magnetic moment and the effective interaction between the metal ions in the complex can be done using magnetization measurements. Here the magnetization studies are performed as a function of temperature and field using a SQUID magnetometer. Yet another powerful tool to characterize and determine the spin levels, the ESR spectroscopic methods, has also been exploited. The study of the dynamical properties of this class of materials was relevant to understand the relaxation mechanism in the low temperatures. For this a new ac susceptometer has been built in house which was another main objective of this dissertation work. The design, fabrication, calibration and automation done on this device is presented in this thesis. The device has been tested using the known molecular magnet Mn12 acetate, and the antiferromagnet Dy2PdSi3. The present work is mainly focused on the magnetic properties of Mn, Ni and Fe based organometallic complexes. The studied Mn dimer with different acceptor and donor ligands exhibit the fine tuning of the electron density at the core of molecular complex by variation in ligands. This in turn shows that the change in peripheral ligands can control the magnetism of the molecule. The influence of the change in Ni-S-Ni bond angle in the magnetic exchange interaction is studied in a Ni(2) dimer and a Ni(2) trimer complex. The Ni dimer complex shows a ferromagnetic interaction (J = -42K) whereas trimer shows an antiferromagnetic interaction (J = 140K). Another Ni based complex bridged via phosphorous has been studied which shows the existence of glassy nature at low temperature. Also a polymeric chain compound based on Fe is studied and presented. All these phosphorous or sulphur bridged complexes are novel materials and these are the first data on these complexes.

The work presented in this thesis focuses on exploring the versatile chemistry of 4-aryl substituted 1,2,4-triazole derivatives. The ligands 4-(4'-nitrophenyl)-1,2,4-triazole (npt) and 4-(4'-carboxyphenyl)-1,2,4-triazole (Hcpt) were prepared following a modified known synthetic strategy. Reaction of either of these ligands with transition metal or lanthanide precursor salts resulted in two novel complexes, namely [FeII3(npt) 6(EtOH)4(H2O)2](ptol)6·4(EtOH) (1) and [DyIII4(mu3-OH) 2(mu3-O)2(cpt)6(MeOH)6(H 2O)]2·15,(MeOH) (5), and of five analogous compounds. In the case of 1, the structural analyses and the magnetic properties indicated that the complex consisted of a linear trinuclear Spin Crossover FeII compound with a T1/2 of 148 K. For this complex, the SC-XRD analyses were performed at 100 and 181 K in order to characterize the structural changes occurring during the spin transition. For 5, the magnetic and structural data indicated that the complex was a dumbbell-shaped cubane dimer {DyIII 4}2 for which each cubane unit is a Single-Molecule Magnet with a small effective energy barrier.

The purpose of devising and validating models for intramolecular motions for FMOC amino acids is to quantify side chain motion in proteins which plays an important role in understanding biological structure function relations of proteins. In this thesis, spin lattice relaxation times (Ti) of FMOC amino acids were m easured under both static and magic angle spinning (MAS) condition at variable tem peratures. Lower activation energies of the relaxation times than the normal amino acids observed indicate a less sterically crowded environment for the rotation methyl group. A three-site jump model for the methyl group w as developed to fit the Tiz and Tiq anisotropy under static condition. Under MAS, Multiple deuterated sites can be resolved and studied independently. Finally, a tem perature model for the spinning rotor w as developed to account for the tem perature gradient across the rotor. A com parison of using the single most probable tem perature and the tem perature distribution in the simulation of relaxation times concludes the difference between th ese two approaches is minimal.

Les molécules-aimants ou Single–Molécule Magnets (SMM) ont attiré une attention croissante au cours des dernières années en raison de leur potentiel attrayant en tant que dispositifs de stockage magnétique à haute densité. Beaucoup d'efforts ont été faits pour améliorer la performance magnétique de ces molécules à l’aide des techniques de chimie de coordination.Dans cette thèse, le travail est organisé en deux parties principales. La première partie est constituée des chapitres 2 et 3 qui se concentrent principalement sur les familles Lanthanide-radicaux zéro- et mono-dimensionnelles. La deuxième partie contient les chapitres 4 et 5, ou des ligands diamagnétiques sont utilisés afin concevoir des matériaux multifonctionnels.Dans la première partie, neuf radicaux TEMPO-R (R représente le substituant) sont utilisés. Leur structure cristalline, ainsi que leurs propriétés magnétiques ont été caractérisées. Dans le chapitre 2, quatre radicaux (TEMPO-OCH3, TEMPO-NH2 TEMPO-Acetamido et TEMPO-OCH2CCH) sont utilisés pour synthétiser des complexes zéro-dimensionnels, dont trois présentent des propriétés de SMM. Dans le cas particulier de TEMPO-OCH3, un très rare comportement de SMM avec lanthanides légers est observé (CeIII, PrIII et NdIII). La première SMM à base de PrIII est ainsi reportée. Dans le chapitre 3, l'objectif est de concevoir des SMM organisés mono-dimensionnellement dans l’empilement cristallin. Cinq autres radicaux sont utilisés: TEMPO-Méthacrylate, TEMPO-OCOPh, TEMPO-oxo, TEMPO-OH et TEMPO-CN. Parmi toutes les chaînes obtenues, [Pr(hfac)3(H2O)(TEMPO-OH)]n (17), [Dy(hfac)3-TEMPO-OH)]n (18) et [Tb(hfac)TEMPO-CN)]n (22) sont identifiés comme SMM. 22 présente la relaxation magnétique la plus lente parmi tous les SMM 4f-2p obtenues dans cette thèse, avec une d'hystérèse magnétique à basse température. Son analogue à base GdIII (23) présente une des plus grandes valeurs d'échange dans les composés de Gd-2p. Enfin un très rare exemple de réseau bidimensionnel 4f-2p de formule [(Ce(hfac)3)3(Oxo-TEMPO)4]n (15) est obtenu. Dans la deuxième partie, une chaine de dimère est obtenue par réaction d'un ligand carboxylique photo-commutable avec des sels d’ions LnIII. La photo-sensibilité du ligand sous irradiation UV a été testée et des mesures magnétiques en solution ont été entreprises. En outre, un composé de type Metal-Organic-Framework (MOF) présentant un comportement de SMM de {[Dy2(o-PDA)3(H2O)2]2H2O}n (28) a été conçu et caractérisé. Une interaction ferromagnétique Ln-Ln a été observée dans ce MOF-SMM et le dopage diamagnétique démontre que, contrairement à ce qui est observé sur [Ln(AZO)3(DMSO)(H2O)]2•4DMSO, cette interaction favorise le comportement de SMM
Single-molecule-magnet (SMM) has attracted increasing attention in recent years due to their appealing potential for high-density storage devices. Much effort has been made to improve the magnetic performance through flexible coordination chemistry strategy.In this thesis, the work is organized in two main parts. The first part is constituted of chapter 2 and chapter 3, primarily focus on the Ln-Radical families aiming at designing zero-dimensional and one-dimensional single-molecule-magnet (SMM). The second part contains chapter 4 and chapter 5, in which the ligands are replaced by diamagnetic ones for the purpose of designing the multifunctional materials.In the first part, nine TEMPO-R (R represents the substituent) radicals are employed to construct zero-dimensional and one-dimensional complexes. These kinds of compounds were prepared by reactions in the dichloromethane/n-heptane co-solvents between the precursor [Ln(hfac)3(H2O)2] and TEMPO radicals. Subsequently their molecular structure as well as magnetic properties have been characterized and described. In chapter 2, four radicals (TEMPO-OCH3, TEMPO-NH2 TEMPO-Acetamido and TEMPO-OCH2CCH) are used to synthesize monometallic or dimetallic complexes, among which three are successful to construct the SMM. For the special case of TEMPO-OCH3 a rare light lanthanide ions (CeIII, PrIII and NdIII) SMM behavior is reported. The PrIII derivative is the first PrIII-based SMM ever reported. In chapter 3, the target is to design SMM in one dimension by using another five radicals: TEMPO-Methacrylate, TEMPO-OCOPh, TEMPO-oxo, TEMPO-OH and TEMPO-CN. Among all the chains, [Pr(hfac)3(H2O)(TEMPO-OH)]n (17), [Dy(hfac)3-TEMPO-OH)]n (18) and [Tb(hfac)3(TEMPO-CN)]n (22) are identified as chains of SMM. 22 exhibits the slowest magnetic relaxation among all the 4f-2p SMMs obtained in this thesis, with a small opening of magnetic hysteresis. Its analogue of [Gd(hfac)3(TEMPO-CN)]n (23) even exhibits one of the largest exchange values in Gd-2p compounds. Last a very rare example of bidimensional 4f-2p network of formula [(Ce(hfac)3)3(Oxo-TEMPO)4]n (15) is obtained.In chapter 4, the salt of a photo-switchable carboxylic ligand was reacted with LnIII ions to afford a chain-like arrangement of dinuclear complexes of formula [Ln(AZO)3(DMSO)(H2O)]2•4DMSO. Photo-sensitivity of the ligand under the irradiation of UV has been tested together with magnetic measurements in solution. In chapter 5, a Metal-Organic-Framework (MOF) (28) has been designed and characterized. Ln-Ln ferromagnetic interaction has been observed and diamagnetic doping highlight that, contrary to what observed on [Ln(AZO)3(DMSO)(H2O)]2•4DMSO, this interaction promote SMM behavior in a so-called MOF-SMM

The objective of this PhD thesis has been the study of the mechanisms that stabilize high-spin states in organic molecule-based magnetic materials. These materials require organic radicals with permanent magnetic moment as building units. Additionally, these molecules need to interact ferromagnetically, and expand that interaction along all three directions of the space. We have carried out our studies using computational chemistry techniques, mostly ab- initio methods (MP2, CASSCF and CASMP2), and DFT-based methods (B3LYP or M06L). Besides, we have also used the hybrid Molecular Mechanics Valence Bond method (MMVB) for alternant hydrocarbons with high number of active electrons. We have demonstrated that in organic radicals the energy gap between two spin states is usually higher when the stabilization of the spin centers occurs by means of through- bond (TB) instead of through-space (TS) interactions. As a result, alternant hydrocarbons (π-delocalized, with TB interactions) are more stable than non-alternant hydrocarbons (π-localized, with TS interactions). Therefore, alternant hydrocarbons would be preferable in the design of permanent molecular magnets. Polymerization of high-spin radicals leads to high-spin systems. However, our research showed that the gap of energy between the first and second spin states decreases with the number of units bonded when these are alternant hydrocarbons. On the other hand, it has been proved that, when the synthesis of macro-radicals follows the SU-CU- SU methodology (SU=spin-containing unit; CU=coupling unit), the SU and CU units keep their multiplicity once coupled. In that case, the energy gap between the spin states of the system can be described considering the energy gap of the spin states of the constitutive units. McConnell-I theory is widely applied to describe ferromagnetic intermolecular interactions. Our research has revised systematically this approach. We have explored the existence of a magneto-structural relationship using pairs of well-known radicals (H2NO·, ·CH3 and ·C3H5) at different geometrical orientations. We demonstrated that McConnell-I model predicts correctly the spin preference of the ground state when the interacting spin-containing radical centers are placed in parallel planes and there are mainly TS interactions between them. However, in other cases, the prediction of the spin preference becomes very complex, and more detailed quantum calculations are required. Overall, we have demonstrated that this model must be used carefully when predicting the multiplicity of the through-space interaction between two radicals. Further, we evaluated whether McConnell-I theory could be applied to assess the magnetic character of real crystals on the subset of experimentally FM crystals of the α- nitronyl nitroxide (α-NN) family. We analyzed the closest contacts between two intermolecular ONCNO groups (atoms where the spin densities are mainly located) for each chosen crystal. We concluded that the ONCNO interactions do not describe entirely the observed macroscopic magnetic property for all the systems. Consequently, TS interactions not considered in the simplistic ONCNO model must play an important role defining the magnetic character. Secondly, we proved there is not a simple magneto-structural relationship, such as the one suggested in McConnell-I model, that can be applied to all through-space interactions in the crystals. This conclusion was reached after a twofold statistical analysis (namely, factor and cluster analyses) of the geometrical parameters as a function of the calculated energy gap ΔES-T. Charge-transfer salts are successful examples of molecular magnets. However, the formation of diamagnetic dimers of the donor species, [D]22+, or the acceptor species, [A]22-, causes the loss of the magnetic properties. We studied the causes of this dimerization studying the formation of TCNE dimers, [TCNE]22-], as a prototypical example of an organic acceptor. The Eint of two charged molecules has two components: the Coulomb contribution (Ecoul > 0, for molecules with the same charge) and the bonding energy (Ebond < 0). If the repulsion energy is higher than the bonding energy in absolute value (|Ecoul|>|Ebond|) the two molecules will repel and the formation of the dimer will not be stable (Eint > 0). However, if there is any force that counterbalances the repulsion between the two charged molecules, the bonding energy could overcome the repulsion energy in absolute value (|Ecoul|<|Ebond|), and the metastable minima would become stable (Eint < 0). The calculations performed described three metastable minima that agree with those observed experimentally. Besides, the spectroscopic features of each class of these three dimers have been calculated and are in agreement with the available experimental data. Extended calculations performed in the presence of cations or polar solvents resulted in the stabilization of the dimers, which demonstrates that counterbalance of the repulsive energy is needed for the formation of these long multicenter bonds. The two electrons - four centers (2e-/4c) bond described is unique since it involves 2e- and takes place among four carbon atoms chemically equivalent.
El objetivo de esta tesis ha sido estudiar computacionalmente las bases teóricas del magnetismo molecular para poder utilizar el conocimiento adquirido en el diseño de materiales magnéticos moleculares. Hemos analizado los mecanismos a través del enlace (TB: through-bond) y a través del espacio (TS: through-space) que estabilizan moléculas de alto spin (radicales) y sus interaccionan intermoleculares ferromagnéticas. Para llevar a cabo dichos estudios se han utilizado métodos híbridos como el Molecular Mechanics Valence Bond (MMVB), métodos DFT como el B3LYP y métodos ab-initio como MP2, CASSCF, y CASMP2. Así pues, por un lado, se ha estudiado la estabilidad de moléculas orgánicas de alto spin y su posible polimerización manteniendo su alta multiplicidad de spin. Se ha llegado a la conclusión que el mecanismo TS es de menor coste energético que el TB. Por lo tanto, los radicales cuyos centros de spin se estabilizan a través del enlace TB son más estables. Asimismo, compuestos que presentan ambos mecanismos, los estados de spin de los estados fundamental y primer excitado vendrán determinados por el mecanismo TS. Por otro lado, se estudiaron las interacciones intermoleculares entre radicales, con el objetivo de establecer las condiciones que favorecen las que son ferromagnéticas. En este contexto, se evaluó la teoría denominada McConnell-I. Tras metódicos estudios de la interacción entre dos radicales (H2NO·, ·CH3 y ·C2H6) en diferentes orientaciones en el espacio, se concluyó que el ámbito de aplicación de esta teoría está limitado a cuando los centros de spin interaccionan en planos paralelos y existe una interacción TS predominante. Estudios adicionales en cristales de la familia α-nitronil nitróxido demostraron que la teoría de McConnell-I no se puede aplicar de forma general a cualquier interacción intermolecular entre radicales. Se observó que esta teoría no predice correctamente el comportamiento magnético de cristales cuando se analiza sólo la interacción entre los átomos que contienen mayoritariamente la densidad de spin (ONCNO). Así pues, el estudio se debe ampliar a otros contactos entre las moléculas para poder describir correctamente el comportamiento magnético observado. Finalmente hemos establecido que, en sales de transferencia de carga, se dan casos de dimerización de las especies constituyentes, por ejemplo tetracianoetileno (TCNE), cuando la repulsión entre especies de la misma carga se minimiza por la presencia de contra-iones o disolventes polares. De esta manera, se favorece la formación del enlace en el dímero al permitir la interacción de los electrones desapareados.

In the past decade, the field of NMR spectroscopy has seen the emergence of ever more powerful superconducting magnets, which has opened the door for the observation of many traditionally challenging or non-receptive nuclei. In this dissertation, a variety of ionic solids with organic coordination environments are investigated using quadrupolar solid-state NMR experiments with an ultrahigh-field magnet (21.1 T). Two general research directions are presented including a 79/81Br solid-state NMR study of a series of 6 triphenylphosphonium bromides for which single-crystal X-ray structures are reported herein. A second research direction is also presented wherein alkaline-earth metal (25Mg, 43Ca, and 87Sr) solid-state NMR is used to characterize a systematic series of 16 aryl and alkyl carboxylates. In both studies, the quadrupolar nuclei studied are deemed “exotic” due to their unreceptive nature to NMR spectroscopic analysis including low natural abundances, large quadrupole moments, or low resonance frequencies. A variety of coordination modes to alkaline-earth metals, including N-atom coordination, are characterized herein for the first time using alkaline-earth metal solid-state NMR. In all cases, the electric field gradient (EFG) and chemical shift (CS) tensors are characterized and correlated to structural features such as interatomic distances measured from the crystal structure of the compound under study. In all of the projects undertaken herein, the gauge-including projector-augmented-wave density functional theory (GIPAW DFT) method is used, which allows for the prediction and rationalization of the experimental EFG and CS tensor parameters based on the input crystal structure. In the case of 43Ca solid-state NMR experiments reported in this dissertation, a linear correlation between the calculated and experimental 43Ca quadrupolar coupling constants, CQ, is used as a calibration curve for GIPAW DFT calculations performed on the 18 structural models currently available for the vaterite polymorph of CaCO3. Vaterite cannot be fully characterized by X-ray diffraction alone; therefore an NMR crystallography protocol is used in order to identify the model that best accounts for 43Ca solid-state NMR experiments performed on vaterite. It is expected that the conclusions from this dissertation can be used for future studies involving structural refinement and elucidation of solid materials containing challenging quadrupolar nuclei.

Les commutables moléculaires sont des molécules qui peuvent moduler leurs propriétés (physiques, chimiques) sous l'influence d'un stimulus externe). Les remarquables propriétés des objets ont suscité un vif intérêt dans l'électronique moléculaire, et plus généralement, dans le domaine des matériaux multifonctionnels. Ce projet de thèse s'est inscrit dans le cadre d'une première collaboration entre les équipes ERMMES et Polymères de l'Institut Parisien de Chimie Moléculaire (IPCM). L' objectif a consisté à développer des voies de synthèses générales de (nano)matériaux hybrides à partir de commutables moléculaires magnétiques. Ce travail s'est essentiellement appuyé sur l'utilisation de complexes à transition de spin ainsi que sur une famille de composés cyanurés Fe/Co photomagnétiques. Ces systèmes ont été fonctionnalisés à leur périphérie, sur les ligands "scorpionates" de type tris(pyrazolyl)borate utilisés pour leur préparation. L'influence du groupement fonctionnel sur les propriétés des différents commutables a ensuite été étudiée, afin de déterminer les meilleurs candidats pour la conception des matériaux hybrides. Finalement, la déposition ou l'intégration des commutables magnétiques fonctionnalisés sur surfaces ou dans des polymères organiques respectivement, a été approchée par différentes méthodes en solution. Ce manuscrit reprend ainsi l'ensemble de ces études
Molecular switchesare molecules that can asdjust their(chemical, physical) properties in response to an external stimulus. The fascinating properties of molecular switches have drawn most attention in molecular electronics and more generally in advances materials research. This PhD project was developped at the frontiers of ERMMES and Polymeres research themes, in the framework of a first collaboration. Our major interest was to establish reliable synthetic routes for the design of hybrid (nano)materials based on magnetic molecular switches. The work was particularly focused on spin crossover complexes and a family of photomagnetic cyanide-bridged Fe/Co cages. These switches were first functionalized at their periphery, on the tris(pyrazolyl)borate capping ligand used for their synthesis. The functionalization influence on the switches properties were carried out to target the best candidates for hybrid materials design. Then, we dedicated efforts on the intergration of magnetic molecular switches into two main classes of materials, (i) surfaces and (ii) organic polymers, through wet-chemistry approaches. This manuscript combines this set of studies

Di-n-propyl dicyanofumarate (DnPrDCF) and di-isopropyl dicyanofumarate (DiPrDCF) have been used as one-electron acceptors in the synthesis of charge-transfer salt magnets with decamethylmetallocenes, MCp*2 (M = Mn, Cr). Salts of each acceptor with each metallocene have been characterized and the structures of the chromium analogues have been solved. The two acceptors are structurally similar to dimethyl dicyanofumarate (DMeDCF) and diethyl dicyanofumarate (DEtDCF), which have been previously studied and found to form charge-transfer salt magnets with the aforementioned decamethylmetallocenes. A typical structural motif is present in these types of charge-transfer salts which allows for the comparison of magnetic properties based on the length or size of the alkyl group of the dialkyl dicyanofumarate. Some trends were established based on the magnetic properties of the homologous series including ordering temperature/bulkiness of the alkyl group and intrastack distances/theta values. Correlation of magnetic and structural properties may give some insight into "through-space" magnetic coupling, of which little is understood.
Master of Science

The study of two pharmaceutically active systems that each display polymorphism has provided a platform upon which to develop and apply solid-state NMR techniques in order to increase the understanding of the solid-state structure of small organic molecules. The multidisciplinary approach adopted has highlighted the advantages of solid-state NMR as a non-invasive probe of molecular conformation and crystallographic packing.Carbon-13 CP/MAS spectra of the two polymorphs of BRL55834 - a fluorinated benzopyran derivative - immediately suggest the presence of one and three molecules in the asymmetric unit. A lack of crystals suitable for single-crystal XRD has catalysed the application of high-power powder X-ray diffraction studies. Subsequent attempts at structure solution using Genetic Algorithm techniques are showing preliminary results that reinforce predictions made from solid-state NMR. Novel triple-channel techniques have aided assignment and resolution of die complex (^13)C CP/MAS spectra. Enrichment of the (^15)N site appears to have resulted in the formation of a new polymorph. Techniques for the analysis of detection Units have been developed using solid-state Raman spectroscopy and chemometric analysis. The aminoxanthine derivative, BRL61063, provides interesting inter-form variations in molecular disorder, solid-state packing, and hydrogen bonding. A previously basic understanding of the single-crystal XRD data has been further evaluated through the course of this Ph.D. and solid-state NMR spectral editing techniques have been developed and applied to identify these phenomena. Recrystallisation studies have produced two samples that appear to exist in an intermediate state between the rigid and mobile structural limits. Temperature variation causes interesting changes in the relaxation characteristics and natural abundance (^15)N and (^13)C CP/MAS spectra. Residual dipolar coupling effects vary in their manifestation within the (^13)C CP/MAS spectra of the polymorphic systems studied and comparison with the literature yields important information regarding molecular conformation. Nitrogen-15 enrichment and operation at higher magnetic field have been applied to reduce these second order effects. Finally, some distance has been travelled along the path towards decoupling (^14)N. Future development of this technique holds potential for resolution enhancement in the solid state spectra of most naturally occurring, nitrogen-containing molecules.

Neste trabalho estudamos a dinâmica molecular e o empacotamento em semicondutores orgânicos com diferentes tamanhos de cadeias conjugada usando uma estratégia de multi-técnicas, em particular Ressonância Magnética Nuclear (RMN), espalhamento de Raios-X de alto ângulo (WAXS), Calorimetria Exploratória Diferencial (DSC), espectroscopia Raman e espectroscopias Ópticas de absorção UV-Vis e fluorescência. Nestes estudos utilizamos oligômeros de fluorenos, com 3, 5 e 7 unidades repetitivas e copolímeros multibloco conjugados/não-conjugados com as unidades conjugadas constituídas por unidades de fenileno de vinileno (PV) e as não-conjugadas formadas por unidades metilênicas. No estudo com oligômeros, foi mostrado que a capacidade e a forma de ordenamento das cadeias dependem do número de unidades repetitivas, com o Pentâmero possuindo uma tendência muito maior de cristalização. Essa conclusão foi suportada por cálculos teóricos ab-initio, que mostraram que a conformação de menor energia do pentâmero favorece as interações intercadeias e, portanto, o ordenamento de longo alcance. Os resultados referentes aos estudos de dinâmica molecular corroboraram essas características e mostraram que a ativação dos movimentos moleculares nas fases amorfas dos oligômeros são predominantemente dependentes dos comprimentos das cadeias oligoméricas, em concordância com o comportamento encontrado para as suas Tg´s. No estudo referente aos copolímeros multiblocos, foi encontrado que a presença dos grupos espaçadores alifáticos inibem a forte tendência de cristalização das unidades de PV, porém não impedem a agregação dessas unidades. Foi verificado que, a dispersão de tamanhos das unidades agregadas afeta fortemente as características de emissão dos copolímeros, onde a emissão nas cadeias maiores é privilegiada. No que diz respeito a dinâmica molecular, foi observado que a presença de movimentos na região alifática contribui para o aparecimento de processos de relaxação não radiativos o qual inibem a emissão dos copolímeros e provocam alargamento das bandas vibrônicas. Por fim, foi observado que movimentos isotrópicos das cadeias de PV são responsáveis pela transição vítrea dos copolímeros, sendo que as energias necessárias para ativar esses movimentos aumentam com o tamanho da cadeia. Portanto, de forma geral, nossos resultados indicam que mesmo em sistemas com comprimento de cadeias muito bem controlados, as fortes interações intermoleculares presentes em polímeros conjugados, podem tornar a morfologia em estado sólido desses sistemas bastante complexa, sendo que muitas das propriedades ópticas (e provavelmente também elétricas) são afetadas pela forma de empacotamento, desordem conformacional e térmica, além da própria constituição das cadeias.
In this dissertation we present a study of the molecular dynamics and packing in organics semiconductor with different conjugated chains lengths using a of multi-techniques approach, in particular, Nuclear Magnetic Resonance (NMR), Wide Angle X-ray Scattering (WAXS), Differential Scanning Calorimetry (DSC), Raman spectroscopy, UV-Vis absorption and fluorescence spectroscopy. The studies were carried-in fluorene oligomers with 3, 5 and 7 repeat units and multi-block conjugated/non-conjugated copolymers with the conjugated part formed by phenylene-vinylene units (PV) and the non-conjugated block formed by methylene units. Concerning the oligomers studies, it was shown that the ability of the chain to form ordered domains as well as the domain structure depend on the number of repeat units, with the pentamer having a higher tendency to crystallization. This conclusion was supported by theoretical ab-initio calculations, which showed that the pentamer conformation favors inter-chain interactions and therefore long-range ordering. The molecular dynamics studies support these characteristics and showed that the activation of molecular motions in oligomers amorphous phase are predominantly dependent on the oligomeric chain lengths, in agreement with the behavior observed for their glass transitions (Tg´s). In the study concerning the multi-block copolymers, it was found that presence of the aliphatic chains inhibit the strong tendency to crystallization of the PV units, but do not prevent their aggregation. It was found that the dispersion in aggregated units sizes strongly affects the copolymers emission, with the emission of larger chains being privileged. Regarding the molecular dynamics, we observed that the presence of motion on aliphatic region contributes to the appearance of non-radiative relaxation processes that inhibit the emission of the copolymers and produce broadening of the vibronic bands. Finally, we observed that isotropic motions of the PV chains are responsible for the copolymers glass transition and the energy required to activate these movements increase with length of the chain. In summary, our results indicate that even in systems with well controlled chains length, the strong intermolecular interactions present in conjugated polymers, can make the solid state morphology of these systems quite complex, which may affect many optical (and probably electric) properties are affected by the packaging structure, thermal and conformational disorder, in addition to the constitution of the chains composition.

This thesis combines synthetic studies for isotopic enrichment with solid-state characterisation techniques to investigate two classes of microporous materials: zeolites and metal-organic frameworks (MOFs). These materials have a wide range of successful applications, from industrial catalysis to medicine, resulting in the increasing need for both a complete understanding of their unique structural features and synthetic methods to target new frameworks. Nuclear magnetic resonance (NMR) spectroscopy, thanks to its sensitivity to the local, atomic-scale, environment and its element specificity, is applied, in combination with powder X-ray diffraction (PXRD), electron microscopy, N2 adsorption and mass spectrometry, to the study of these materials. Oxygen atoms play a crucial role in the structure and chemistry of zeolites and MOFs, making 17O NMR an excellent tool for chemical and structural investigations. However, the low natural abundance of this isotope (0.037%) and the cost of 17O-enriched reactants require the development of atom-efficient synthetic processes for isotopic enrichment. In the first part of this work, the unconventional assembly-disassembly-organisation-reassembly (ADOR) method is applied to the Ge-doped UTL framework and optimised in reduced-volume conditions for economic enrichment to obtain 17O- and 29Si-enriched UTL-derived zeolites. In situ and ex situ solid-state characterisation studies show that isotopic enrichment not only enables a more detailed spectroscopic investigation, but also provides new insights into the mechanism of the ADOR process and its sensitivity to experimental conditions. In the second part of this work, dry gel conversion synthesis and a novel steaming procedure are studied as cost-effective 17O-enrichment pathways for Al, Ga and Sc mixed-metal terephthalate MOFs. 17O solid-state NMR spectroscopy, in combination with PXRD and electron microscopy, is employed to investigate cation disorder and 17O NMR spectra are shown to be sensitive to substitution of metal centers and conformational changes upon interaction with guest molecules.

One of the most challenging goals in nanoparticle research is to develop successful protocols for the large-scale, simple and possibly low-cost preparation of morphologically pure nanoparticles with enhanced properties. The work presented in this thesis was focused on the synthesis, characterisation and testing of magnetic nanoparticles and their potential applications. There are a number of magnetic nano-materials prepared for specific applications such as metal oxide nanoparticles encapsulated with various porous materials including Fe₃O₄/Fe₂O₃ coated with soft bio-organic materials such as glycol chitosan and bovine serum albumin and hard materials such as silica (SiO₂) and zinc sulphide (ZnS). The preparation of these materials was achieved principally by bottom-up methods with different approaches including micro-emulsion, precipitation, electrostatic and thermolysis processes. The thesis also presents the uses of various analytical techniques for characterising different types of nano-materials including Attenuated Total Reflection Fourier Transformer Infrared Vibrational Spectroscopy (ATR-FTIR), Ultraviolet Visible- Near Infrared (UV-Vis-NIR) Spectroscopy, Zeta Potentiometric Surface Charge Analysis, Superconducting Quantum Interference Device (SQUID) and Vibration Sample Magnetometry (VSM) for magnetic analysis and powder X-Ray Diffraction (XRD) for crystallographic pattern analysis. There are many applications of magnetic nanoparticles, including nano-carriers for biological and catalytic reagents. The magnetic nanoparticles can facilitate separation in order to isolate the carriers from solution mixtures as compared to many inefficient and expensive classic methods, which include dialysis membrane, electrophoresis, ultracentrifugation, precipitation and column separation methods. There are six key chapters in this thesis: the first chapter introduces the up-to-date literature regarding magnetic nano-materials. The uses of magnetic nano-materials in drug binding and for protein separation are discussed in the second and third chapters. The fourth chapter presents the use of magnetic nanoparticle in conjunction with a photo-catalytic porous overlayer for the photo-catalytic reduction of organic molecules. The fifth chapter describes different analytical techniques used for the characterisation of nanoparticles and the underlying principles and the experimental details are also given. The sixth chapter summarises the results and provides an overview of the work in a wider context of future applications of magnetic nanoparticles.

D. Tech. (Department of Chemistry, Faculty of Applied and Computer Sciences), Vaal University of Technology
In the present era of water resources scarcity, efficient treatment of wastewater is a major prerequisite especially for growing economy. Numerous approaches have been studied for the development of cheaper and more effective adsorbents for removal of both organic and inorganic pollutants from wastewater. The present study seeks to harness the potential of biosorption and nanotechnology by producing more efficient, selective, mechanically stable and effective adsorbents for removal of organic and inorganic pollutants. The biosorbent-magnetic nanomaterial was synthesized by coating magnetite nanoparticles with sodium hydroxide treated pine cone by co-precipitation method. Magnetite coated pine bio-composite was then modified by cross-linking with hexamethylene diisocyanate and epichlorohydrin to the molecular recognition compound “cyclodextrin”. These novel biosorbent-magnetic nanoparticle materials were explored in overcoming the drawbacks of the biosorbent alone and selectively remove inorganic and organic pollutants from complex matrices. The synthesized materials were characterized by several analytical techniques including, Fourier Transformed Infrared Spectroscopy (FTIR), Thermogravimetric analysis (TGA), X-Ray Diffraction (XRD), Scanning Electron Microscopy-Energy Dispersive X-Ray (SEM–EDX), Transmission Electron Microscopy (TEM), Brunauer, Emmett and Teller Isotherm (BET) surface area analysis, X-ray Photoelectron Spectroscopy (XPS), Vibrating Sample Magnetometer (VSM) and pH at point of zero charge (pHpzc). Sodium hydroxide treated pine cone and magnetite coated pine cone were applied for both chromium(VI) and arsenic(III) adsorption while the magnetite coated pine cone, magnetite coated pine cone crosslinked to cyclodextrin using both epichlorohydrin and hexamethylene diisocyanate were applied for 4-nitrophenol removal from aqueous solution. Batch adsorption studies were performed to optimize operating parameters such as solution pH, adsorbent dose, contact time, temperature and initial concentration. Pseudo first, pseudo second, intraparticle diffusion, pore and film diffusion kinetic models were determined to investigate the mechanism of adsorption process. Coefficient of correlation, r2, and variable error, methods were also applied in the determination of the best fit of the kinetic method. Structural characterization of magnetite coated pine cone and the magnetite coated pine crosslinked to cyclodextrin using 1,6-hexamethylene diisocyanate and epichlorohydrin were confirmed by characterization techniques applied. The adsorption of Cr(VI), As(III) and 4-nitrophenol was found to be dependent on the solution pH, adsorbent dose, initial concentration, temperature and ionic strength. Kinetic modelling revealed that the adsorption of Cr(VI), As(III) and 4-nitrophenol is controlled by pseudo second order kinetic model suggesting surface adsorption and intraparticle diffusion model. Intraparticle, pore and film diffusion models gave further insight into the controlling diffusion mechanism involved in the adsorption process for all pollutants investigated. Equilibrium studies indicated that the adsorption of all pollutants followed Langmuir isotherm indicating that adsorption sites are homogeneous in nature. The obtained thermodynamic parameters demonstrated that the adsorption of Cr(VI), As(III) and 4-nitrophenol were spontaneous, favourable and endothermic in nature. Anionic effect positively affected Cr(VI) and As(III) removal but had a negative effect on the 4-nitrophenol adsorption. Adsorption of 4-nitrophenol onto the nanocomposite adsorbents was attributed to multiple adsorbent-adsorbate interactions such as hydrogen bonding, hydrophobic attraction and guest host interaction. Magnetite coated pine better removed Cr(VI) and As(III) from aqueous solution than NaOH treated pine cone biomaterial while the magnetite coated pine crosslinked to cyclodextrin using 1,6-hexamethylene diisocyanate exhibited better adsorption performance for 4-nitrophenol removal than the nanocomposite crosslinked using epichlorohydrin and the magnetite coated pine cone.

Les aimants moléculaires de type TbPc2 avec leur anisotropie magnétique élevée associée à des temps de relaxation longs de l’aimantation sont de bons candidats pour le codage et l’enregistrement de l’information. Leur robustesse et leur propension à s’arranger en réseaux se prêtent bien à une étude de leur structure électronique et magnétique par STM/STS à très basse température (4.5 K). Dans ce travail de thèse, il a été possible de mettre en évidence un écrantage Kondo des électrons π et 4f de la molécule de TbPc2 par les électrons du substrat métallique. Les propriétés magnétiques de la molécule dépendent de leur degré d’interaction avec le substrat et des interactions latérales avec les autres molécules. Ainsi le radical π de la molécule est préservé sur Au(111) mais disparait sur Ag(111) sous l’effet d’un transfert de charge entre le substrat et la monocouche moléculaire. Enfin, lorsque la molécule est en interaction forte avec le substrat de Cu(111), un accès direct aux états 4f de l’ion central de Tb est observé par STM comme le montre la détection d’un effet Kondo sur l’ion central de Tb
With their large magnetic anisotropy associated with long relaxation times of the magnetization, TbPc2 molecular magnets are good candidates for encoding and recording data. Their robustness and their propensity to arrange into networks lend themselves well to a study of their electronic and magnetic structure by STM/STS at very low temperature (4.5 K). In this thesis work, it was possible to highlight a Kondo screening of the π and 4f electrons of the TbPc2 molecule by the electrons of the metal substrate. The magnetic properties of the molecule depend on their degree of interaction with the substrate and lateral interactions with other molecules. Thus, the π radical of the molecule is preserved on Au (111) but disappears on Ag (111) under the effect of a charge transfer between the substrate and the molecular monolayer. Finally, when the molecule is in strong interaction with the Cu(111) substrate, a direct access to the 4f states of the central Tb ion by STM is possible as shown by the detection of a Kondo effect on the central Tb ion

La chimie de coordination des radicaux libres nitronyl nitroxydes est propice aux composes moleculaires magnetiques (aimants moleculaires ou transitions de spin). Cependant le caractere base de lewis faible du groupe nitroxyde >n-o limite dans de nombreux cas leur utilisation aux ions metalliques porteurs de ligands attracteurs. Pour eviter ce probleme, nous avons choisi de substituer les radicaux par des groupes donneurs. Ce travail concerne la synthese, la caracterisation structurale et l'etude des proprietes magnetiques de complexes d'ions metalliques avec les radicaux nitronyl nitroxydes porteurs des groupes imidazolyle (nitimh) ou benzimidazolyle (nitbzimh). La premiere partie est consacree a la complexation des radicaux non deprotones. Avec les ions metalliques de transition mn#2#+, ni#2#+, zn#2#+ et de terres rares gd#3#+, dy#3#+, plusieurs molecules mononucleaires qui sont des entites discretes, ont ete isolees. Leurs structures determinees par diffraction des rayons x sur monocristal montrent pour la premiere fois des centres metalliques entoures exclusivement de radicaux nitroxydes ; m(l)#3(mn#2#+, ni#2#+, zn#2#+) ou m(l)#4 (gd#3#+, dy#3#+)(l = nitimh ou nitbzimh). L'etude de leurs proprietes magnetiques revele des comportements varies. La seconde partie du travail decrit une serie de composes etendus du manganese(ii), obtenus lorsque les radicaux sont deprotones ; nitim#- ou nitbzim#-. La dimension des composes depend alors des conditions experimentales de deprotonation et du type de contre anions utilises. Quatre chaines ferrimagnetiques dans lesquelles alternent les ions manganese(ii) et les radicaux ont ete isolees, et leurs structures determinees. L'etude de leurs proprietes magnetiques met en evidence une aimantation spontanee au-dessous de temperature comprises entre 2 k et 4,5 k. Selon des conditions experimentales differentes, une serie de composes bidimensionnels a egalement ete synthetisee. La determination structurale de l'un d'entre eux montre qu'il s'agit de composes lamellaires constitues de feuillets mn#2(l)#3#+, separes par les contre anions. Ces composes sont des aimants en dessous de temperatures allant de 1,4 k a 40 k. Cette strategie est donc prometteuse et devrait ouvrir de nouvelles perspectives pour la construction d'edifices tridimensionnels.

Ce travail porte sur la synthèse et la caractérisation de matériaux moléculaires fonctionnels basés sur la molécule anilate et présentant des propriétés de conductivité, de magnétisme et de luminescence. Les anilates sont des dérivés de la 2,5-dihydroxy-1,4-benzoquinone substitués en positions 3 et 6 par une variété d’éléments (H, F, Cl, Br, I, CN, etc). Parmi eux, le seul composé hétérosubstitué ClCNAn2- a été choisi pour préparer une nouvelle famille de polymères de coordination bidimensionnels (PC 2D) avec des métaux de transition ou des ions lanthanides : i) un PC à valence mixte FeIIFeIII, de formule [TAG][FeIIFeIII(ClCNAn)3], contient pour la première fois le cation triaminoguanidinium dans un réseau de coordination.ii) Des PC basés sur le ligand ClCNAn2- et des ions lanthanides émettant dans le proche infrarouge (YbIII, NdIII, ErIII). Ces composés ont été exfoliés en monocouches, et des études de photoluminescence ont été menées à la fois sur les cristaux et les monocouches. iii) Une famille de PC hétéroleptiques basés sur des ions lanthanides et sur deux types de ligands pontants, le ligand ClCNAn2- et des ligands de type carboxylates (DOBDC et F4-BDC). iv) Une famille de PC basés sur des ions DyIII ont été préparés afin d’étudier leur propriétés magnétiques. v) Finalement, la capacité des ligands anilates à se combiner à des conducteurs moléculaires basés sur le BEDT-TTF a été démontrée à travers la synthèse et l’électrocristallisation de semiconducteurs organiques et de conducteurs magnétiques hybrides avec l’anion [Fe(ClCNAn)3]3-
This work reports on the design, synthesis and characterization of novel anilate-based functional molecular materials showing luminescent, magnetic and/or conducting properties. The family of anilate ligands comprises several derivatives obtained by introducing various substituents (H, F, Cl, Br, I, CN, etc.) at the 3 and 6 positions of the common 2,5-dihydroxy-1,4-benzoquinone framework. Among the anilate ligands, the only known heterosubstituted anilate with Cl/CN substituents at the 3,6 positions, ClCNAn2-, have been selected for preparing a novel family of 2D layered coordination polymers (2D CP) with both 3d metal ions and 4f lanthanide ions, through a general and straightforward synthetic strategy. i) Mixed-valence FeIIFeIII 2D CP, formulated as [TAG][FeIIFeIII(ClCNAn)3], containing, the tris(amino)-guanidinium (TAG) cation for the first time in such 2D networks has been synthesized and thoroughly characterized. ii) 2D CPs based on NIR-emitting lanthanides (YbIII, NdIII, ErIII) and the ClCNAn2- ligand, have been prepared and characterized. These layered compounds were exfoliated to nanosheets, by sonication-assisted solution synthesis. Time-resolved photoluminescence studies performed on both the bulk and nanosheets are also highlighted. iii) Novel family of heteroleptic 2D CPs based on NIR-emitting lanthanides and mixed ligands (ClCNAn2- and carboxylate ligands (DOBDC and F4-BDC)), were prepared and characterized. vi) Novel family of 2D CPs based on DyIII and ClCNAn2- were prepared in order to investigate their magnetic properties. v) Furthermore, the ability of anilate ligands to work as components of BEDT-TTF- based molecular conductors have been demonstrated through the synthesis, via electrocrystallization technique. vi) П-d hybrid multifunctional paramagnetic molecular conductors BEDT-TTF and [Fe(ClCNAn)3]3-) were also studied

Dans une première partie, nous avons étudié quelques propriétés de molécules magnétiques impliquant des radicaux organiques (seuls ou conjointement avec des terres rares). Nous avons ainsi pu interpréter l'évolution de la susceptibilité magnétique et de l'aimantation en fonction de la température en évaluant par des approches ab initio fonctions d'onde les constantes d'échange ou le tenseur g au sein de ces matériaux. De plus, nous avons chercher à définir les conditions pour que des matériaux à base de radicaux organiques présentent simultanément des propriétés magnétiques et conductrices. Nous avons ainsi examiné différentes familles de composés et l'influence de la structure géométrique et chimique des radicaux organiques utilisés. Pour cette partie, nous avons extrait les intégrales physiques pertinentes par la méthode des Hamiltoniens effectifs.Dans une deuxième partie, nous avons utilisés ces quantités physiques (intégrale de saut, répulsion sur site, échange) pour décrire le phénomène de transport dans des jonctions pour lesquelles les effets de la corrélation électronique ne peuvent être écartés. Munis de ces paramètres ab initio, nous avons développé un modèle phénoménologique permettant de décrire la conduction moléculaire à l'aide d'un jeu d'équations maîtresses. Nous avons ainsi cherché à mettre en évidence l'intérêt des approches post Hartree-Fock empruntant une fonction d'onde corrélée et de spin adapté dans la description du transport électronique. Que ce soit dans le cas de transport polarisé en spin ou non, l'approche utilisée (mono ou multi-déterminentale) conditionne qualitativement et quantitativement la caractéristique courant/tension.

La famille des ligands oxamate N-aromatiques est considérée comme idéale pour l’obtention de structures de dimensionnalité et propriétés magnétiques contrôlées. Au cours de ces travaux de thèse, nous avons incorporé au ligand phenyloxamate des groupes coordinants additionnels (hydroxyde et carboxylique), afin d’obtenir de nouveaux polymères de coordination magnétiques aux structures originales. Les études de la réactivité de ces ligands hétérotopiques ont été menées en solution et en conditions solvothermales, vis-à-vis de différents ions métalliques de transitions (CuII, CoII, NiII, MnII). En solution, de nombreuses architectures ont pu être isolées, comme des métalloligands à base de CuII, un complexe à valence mixte de cobalt, et une chaîne hélicoïdale de CuII. Des chaînes bi- et tri-métalliques, un composé hétérométallique 2D et trois polymères de coordination 3D ont été obtenus par voie solvothermale, une technique rarement utilisée dans la chimie des ligand oxamate. Les propriétés thermiques et magnétiques ont été étudiées, une transition de phase, un comportement de molécule aimant et un phénomène d’ordre magnétique ont notamment pu être observés, confirmant le potentiel de notre approche
The family of N-substituted aromatic oxamate ligands is considered as ideal for obtaining structures of predictable dimensionality and magnetic properties. During this PhD, we have introduced additional coordinating groups (hydroxyl and carboxylic acid) on the phenyloxamate ligand, in order to obtain novel magnetic coordination polymers with original architectures. Reactivity studies for these heterotopic ligands have been performed in both bench and solvothermal conditions, towards different metal ions (CuII, CoII, NiII, MnII). On the bench, various architectures have been isolated, including Cu-based metalloligands, a mix-valence Co-based complex, and a 1D Cu-based helix. Bi- and tri-metallic chains, a 2D heterometallic compound and three 3D coordination polymers have been obtained solvothermally, a technique seldom used in oxamate chemistry. Thermal and magnetic properties were studied, temperature dependent SC to SC transition, single-ion magnet behavior or magnetic ordering were observed, supporting the potential of our approach

Le travail presente dans cette these est une contribution aux etudes de cristaux de radicaux organiques par diffraction de neutrons, surtout aux mesures de densite de spin par diffraction de neutrons polarises. Des calculs theoriques ab initio ont ete aussi effectues. Une partie importante est consacree au developpement des methodes de reconstruction des densites de diffusion a partir de donnees de diffraction par la methode du maximum d'entropie. Les quatre premiers chapitres introduisent le magnetisme moleculaire, les methodes experimentales, les procedures de traitement de donnees et les methodes de calculs theoriques. Les chapitres suivants sont en fait des articles publies ou soumis. Le chapitre 5 contient les resultats experimentaux et theoriques obtenus par les nitronyl phenyl et para-nitrophenyl. Les chapitres 6 et 7 sont consacres aux etudes du meta-nitrophenyl imino nitroxide et du radical ion tetracyanoethylenide. Les manuscrits relevant du developpement de la methode de maximum d'entropie font la matiere du chapitre 8

Etude parrpe et par des mesures de la conductivite dc et de la reponse dielectrique ac hyperfrequence. Mise en evidence de la nature radicalaire de type pi du systeme, a l'etat cristallin comme en solution; effet du solvant et de la temperature; observation de sauts de l'electron non apparie d'un macrocycle a l'autre a une frequence voisine de 1 mhz. Mise en evidence de la possibilite de formation de deux systemes cristallins (empilement de chaines ou de plans paralleles) et de leur caractere 1d ou 2d au moyen du modele de richards. La susceptibilite du systeme mesomorphe indique un desordre magnetique comparable a celui de la phase liquide; la conductivite est celle des systemes desordonnes. Les cristaux solvates sont des semiconducteurs moleculaires intrinseques

Mesures, par la méthode piezothermique, du coefficient de dilatation thermique et du coefficient de compressibilité isotherme, dans l'intervalle 200-450 k, sous des pressions atteignant 4000 bars; ajustement des résultats, établissement d'une équation phénoménologique du coefficient de dilatation comme fonction de la pression et de la température, permettant la détermination des propriétés thermodynamiques. Discussion de l'organisation moléculaire à la lumière des résultats obtenus et de données spectroscopiques (RMN, diffusion de la lumière).

博士
國立成功大學
光電科學與工程學系
106
This dissertation presents the magnetic field effects (MFEs) in conjugated polymer/ organic molecules-based diodes, including the phenyl-substituted poly(p-phenylene vinylene) copolymer (super yellow, SY-PPV)-based polymer light-emitting diodes (PLEDs), tetracene- and pentacene-based diodes. For the first topics in the first part, the effect of device architecture (current injection through the diodes) and operating condition (the external applied bias) on magnetoconductance (MC) response was investigated and analyzed by the fitting analysis in SY-PPV-based PLEDs. Using the mathematical analysis to fit the curves with two empirical equations of a non-Lorentzian and a Lorentzian function, we are able to extract the hidden negative MC component from the positive MC resposes in charge-unbalance SY-PPV-based PLEDs. We attribute the negative MC component to the triplet excitons-charge reaction. The negative MC component can be further increase by increasing the concentration of free hole carriers in hole-blocking SY-PPV-based PLED. Thus, the negative MC component corresponds for the line shape broadening of MC curves. In the next part, we investigate the triplet-triplet annihilation (TTA) process in the charge-balanced SY-PPV-based PLEDs. We found that the temperature and current density may induce the TTA process in SY-PPV-based PLEDs. The TTA process may harvest the energy from triplet to singlet excitons in SY-PPY active layer and in part contribute the emission to fluorescence in PLEDs especially in the high current density regime. In the second topics, we study the magnetic field effect in tetracene-based diodes. We found the singlet fission (SF) reaction occurs in tetracene-based diodes based on the magnetophotocurrent (MPC) and magnetophotoluminescence (MPL) characterization. By depositing the fullerene (C60) on the tetracene active layer to yield a planar heterojunction device, we found that the MPC response show the sign-change and the PL spectra of tetracene/C60 PHJ-based diode show almost completely quenched. It indicates that the charge separation by charge transfer (CT) complex states is more effective than the SF reaction. Consequently, the singlet fission reaction is suppressed by this charge separation of the opposite charge carriers at the donor/acceptor interfaces. Finally, at the third topics, we investigate the MPC response of pentacene-based diodes in different magnetic fields orientation either perpendicular (90°) or parallel (0°) orientation. We found that the MPC magnitude is magnetic field-orientation dependent. We attribute the change of MPC magnitude under magnetic field-orientation to the interaction of polaron pair’s spins (dipole-dipole or exchange interaction). Depositing C60 on pentacene layer in addition to the change of MPC magnitude, it also narrows the MPC line shape. Due to the weaker exchange interaction in CT complex states from pentacene/C60 interface, the external applied magnetic field can modulate this interaction result in the modulation in MPC line shape. The MPC line shape narrowing is observed at low magnetic field regime (B 〈 300 Oe) in pentacene/C60-based diode by changing the magnetic field orientation from 90° to 0°. We contribute this MPC line shape narrowing to the suppression of hyperfine interaction to induce the intersystem crossing. Our experimental results strengthen the previous studies that the dipole-dipole, exchange or hyperfine interactions between polaron pairs or CT complex states are responsible for the magnetic field orientation dependence of organic magnetoresistance.

The focus of this work was the synthesis and coordination of t-butyl nitroxide radicals with various paramagnetic transition metal dications to investigate the magnetic interactions in the products. The crystal structure of each coordination complex was studied to elucidate possible structural pathways for magnetic exchange. A silicon centered tetraradical, tetrakis(N-oxyl-2,2,6,6-tetramethylpiperidin-4-oxyl)silane, was synthesized and coordinated to various metal ions to yield one-dimensional coordination polymers joined through nitroxide-metal-nitroxide (3-spin) units. The Mn(II) complex showed strong antiferromagnetic (AFM) coupling within the 3-spin unit, while the Cu(II) complex showed modest ferromagnetic (FM) coupling within the unit. Both complexes showed weak AFM interactions between 3-spin units. The new radical 5-(3-[N-tert-butyl-N-aminoxyl]phenyl)pyrimidine was synthesized and coordinated to metal dications, and formed both cyclic 2:2 dimers and extended network complexes that were isolated as stable crystalline solids. The Mn(II) dimer showed strong AFM exchange within the dimer and weak FM exchange between dimer units, based on magnetic analysis by a rectangular four-spin model. A linear Curie-Weiss model for the Co(II) dimer showed strong AFM exchange within these dimer units. The Cu(II) dimer exhibited crystallographic Jahn-Teller distortion accompanied by reversible solid-state thermochromic effects that correlate qualitatively with changes in magnetic behavior with temperature. 5-(3-[N-tert-Butyl-N-aminoxyl]phenyl)pyrimidine formed zero- to three-dimensional complexes with Cu(hfac)2. Most of these were 3:2 Cu:radical complexes with different magnetic behaviors. Two-dimensional complex 25 shows a slight ferromagnetic rise, then antiferromagnetic downturn in the χT(T) plot; it also exhibits multistep behavior in the M( H) plot, with a magnetic moment higher than any comparable complex in this series. Two-dimensional complex 26 undergoes a reversible crystal lattice change from orthorhombic to monoclinic, and shows strong AFM coupling between metal-radical units, with additional AFM coupling between units. Three-dimensional complex 27 shows multi-stage magnetic M(H) behavior. Jahn-Teller distortion of Cu-radical units was noted in complexes 26 and 27. 3-(N-tert-butyl-N-aminoxyl)benzoic acid was isolated as a stable crystalline solid that forms one-dimensional chains of hydrogen bonded dimers. The radical shows antiferromagnetic exchange behavior that could be roughly fit to a Bleaney-Bowers spin pairing model. The radical adsorbs strongly but reversibly to silica based materials from nonpolar solvents.

A variety of molecules were investigated to help determine structure-property relationships governing the limits of unpaired electron delocalization and interaction. Nitreno-radicals were synthesized and examined by ESR. Connectivites of nitrene groups were correlated to ZFS :D/hc: and :E/ hc: values. Meta-connected quartet state systems showed larger :D/hc: compared to para-connectivity. Calculations did not show more delocalization in either connectivity. Prepared quartets were studied by ESR Curie analysis over 4–70 K. Nitrosonium perchlorate complexes of azidophenyl nitronyl nitroxides were studied by X-ray crystallography and found to be more planar than their radical precursors. Magnetic susceptibility measurements showed little response, confirming oxidation to cation. Prepared para-connected rigid dinitrenes were studied by ESR and Curie analysis. 1,4-Bis-(nitrenophenyl)-1,3-butadiyne (29 ) showed an excited-state triplet biradical spectrum, and is presumably a ground-state singlet. This proves that the unpaired π-electrons in 29 are strongly exchange coupled and strongly delocalized. 1,4-Bis-(4-nitrenophenylethynyl)-benzene (30) showed an excited-state quintet spectrum but is also presumed to be a ground-state singlet. The quintet spectrum indicates that the unpaired π-electrons in 30 are not strongly exchange coupled, and do not delocalize to the central phenyl ring. A rigid dinitrene, 1,5-dinitrenonaphthalene ( 113), was found to rearrange to a cumulated alkene with terminal nitrile groups. No biradical state was observed. Apparently 113 exhibits strong exchange delocalization of its unpaired π-electrons to favor a singlet state over the triplet, since the triplet is not observed: computations support a singlet ground-state. Septet trinitrene 1,3-dicyano-5-methyl-2,4,6-trinitrenobenzene ( 115) was prepared and studied by ESR and Curie analysis. A high-spin ground state was observed with estimated ZFS of :D/hc: = 0.090 cm−1 and :E/hc: ≈ 0 cm −1. The azide groups photolyze in step-wise fashion giving triplet mononitrene, quintet, and septet. There is sufficient delocalization of the nitrene π-electrons to favor the septet state in the overall system. An iminoyl nitroxide with a hydrogen-bonding 6-uradinyl group ( 119) was prepared. X-ray crystallography showed planarity dominated by two-point hydrogenbonding. Room temperature solution ESR shows seven lines (aN = 8 G, 3 G) compared to five lines (typical aN = 7 G) that result from a nitronyl nitroxide.

Many technological aspects of everyday life are based on practical applications of the magnetic properties of the materials. Miniaturization is a key technological aspect; electronic circuits and storage devices are nowadays steadily decreasing in size and will eventually reach molecular dimensions. The understanding and predictions of the properties of matter at atomic levels represents one of the great achievements of the last years in science. In the present thesis, the aim is to present a complete study of the electronic structure of selected materials, by means of experimental and theoretical methods. The class of materials which are presented in this thesis, are belonging to the magnetic molecules and intermetallic compounds. The electronic structure of the single molecule system named ferric star molecule has been studied. From the resonant X-ray emission study the trend observed for the FeFe3 star gives a signature for the high-spin structure, or more precisely of strong magnetic systems like FeO or Heusler alloys. For the case of intermetallic alloys and compounds, the Mn 2p core-level presents a visible split structure, which is arising from the exchange interactions between the core-hole and the unpaired 3d electrons. The interpretation of this splitting can be regarded as an evidence of local magnetic moments belonging to the Mn site.

The thesis entitled “Exploring Diverse Facets of Small Molecules by NMR Spectroscopy” consists of six chapters. The main theme of the thesis is to exploit one and two dimensional NMR methodologies for understanding the diverse facets of small organic molecules, such as, weak intra- and inter- molecular interactions, chiral discrimination, quantification of enantiomeric excess and assignment of absolute configuration. Several new pulse sequences have also been designed to solve specific chemical problems, in addition to extensive utility of existing one and two dimensional NMR experiments. The results obtained on different problems, are discussed under six chapters in the thesis. The brief summary of each of these chapters is given below. Chapter 1 begins with the discussion on the importance of small molecules and their various facets, the analytical techniques available in the literature to study them. The role of NMR spectroscopy as powerful analytical technique to understand the diverse facets of organic molecules and their importance is set out in brief. A short introduction to the basic principles of NMR, the interaction parameters, the commonly employed one and two dimensional homo- and herero- nuclear NMR experiments are also given. The basic introduction to product operators essential for understanding the spin dynamics in the developed pulse sequences is given. The application of diffusion ordered spectroscopy (DOSY), the general problems encountered in the analysis of combinatorial mixtures and the matrix assisted method in circumventing such problems are discussed. Chapter 2 focuses on the chiral discrimination and the measurement of enatiomeric excess. The NMR approach to discriminate enantiomers using chiral auxiliaries such as, solvating agents, derivatizing agents, lanthanide shift reagents, the choice of such auxiliaries and the limitations are discussed in detail. The in-depth discussion on the new protocols developed using both the solvating and derivatizing agents for enantiomeric discrimination of chiral amines, hydroxy acids and diacids are discussed. The new three-component protocols that serve as chiral derivatizing agents for the discrimination of primary amines, diacids and hydroxy acids are discussed. Also the role of organic base such as DMAP in the chiral discrimination is explored for discrimination of acids using BINOL as a chiral solvating agent. Accordingly the discussion is classified into two sections. In the first section the protocol developed utilizing an enantiopure mandelic acid, a primary amine substrate and 2-formylphenylboronic acid that is ideally suited for testing the enantiopurity of chiral primary amines is discussed. The broad applicability of the protocols for testing enantiopurity has been demonstrated on number of chiral molecules using 1H and 19F NMR. The second section contains the results on the new concept developed for discrimination of hydroxy acids. The strategy involves the formation of three component protocol using chiral hydroxy acid, R-alphamethylbenzylamine and 2-formylphenylboronic acid for 1H-NMR discrimination of diacids. The section also includes the utility of ternary ion-pair complex for the discrimination of acids. The ternary ion-pair not only permitted the testing of enantiopurity of chiral acids, but is also found useful for the measurement of enantiomeric excess. Chapter 3 discusses the utilization of the developed three-component protocols for the assignment of absolute configurations of molecules of different functionality. The protocols for the assignments of absolute configuration of primary amines using 2-formylphenylboronic acid and mandelic acid yielded the substantial chemical shift differences between diastereomers. The consistent trend in the direction of change of chemical shifts of the discriminated proton(s) gave significant evidence for employing them as parameters for the assignment of spatial configuration of primary amines. Another protocol using 2-formylphenylboronic acid, hydroxy acids and enantiopure alphamethylbenzylamine permitted their configurational assignment. In the second section a novel solvating agent, obtained by the formation of an ion-pair complex among enantiopure BINOL, DMAP and chiral hydroxy acid for the assignment of the spatial configuration of hydroxy acids is discussed. Chapter 4 focuses on the development of novel NMR methodologies, and also the utility of existing two-dimensional experiments for addressing certain challenging problems. This chapter has been divided into three sections. In Section-I the utilization of well-known homonuclear 2D-J-resolved methodology for unravelling the overlapped NMR spectra of enantiomers, an application for chiral discrimination and the measurement of enantiomeric excess is discussed. The utilization of the chiral auxiliaries, such as, chiral derivatizing agents, chiral solvating agents and lanthanide shift reagents permits enantiodiscrimination and the measurement of excess of one form over the other. Nevertheless many a times one encounters severe problems due to small chemical shift difference, overlap of resonances, complex multiplicity pattern because of the presence of number of interacting spins, and enormous line broadening due to paramagnetic nature of the metal complex. This section is focused on combating such problems utilizing 2D-J-1JNH resolved spectroscopy where a 450 tilting of the spectrum in the F2 dimension, yielded the pure shift NMR spectrum. The method circumvents several problems involved in chiral discrimination and allows the accurate measurement of enantiomeric excess. In Section-II, the development of novel NMR experimental methodology cited in the literature as C-HetSERF and its application for the study of symmetric molecules, such as, double bonded cis- and trans- isomers, and extraction of magnitudes and signs of long range homo- and hetero- nuclear scalar couplings among chemically equivalent protons in polycylic aromatic hydrocarbons is discussed. The extensive utility of the new pulse sequence has been demonstrated on number of symmetric molecules, where the conventional one dimensional experiment fails to yield spectral parameters. In section III, yet another novel pulse sequence called RES-TOCSY developed for unravelling of the overlapped NMR spectrum of enantiomers and the measurement of enantiomeric contents, has been utilized for the accurate measurement of magnitudes and signs of 1H-19F couplings in fluorine containing molecules. The method has distinct advantages as the strengths of the couplings and their relative signs could be extracted on diverse situations, such as, couplings smaller than line widths, the spectrum where the coupling fine structures are absent. Chapter 5 covers the study of nature of intra- and inter- molecular hydrogen bond in amide and its derivative. The chapter is accordingly divided into two sections. In the first section the study of acid and amide hydrogen bonding is discussed and the hydrogen bonded interactions are probed by extensive utility of 1H, 13C and 15N-NMR. The temperature perturbation experiments, measurements of the variation in the couplings, monitoring of diffusion coefficients and the association constants, detection of through space correlation have given unambiguous evidence for the hydrogen bond formation. The results were also supported by DFT calculations. Similar interaction in the solid state has also been derived by obtaining the crystal structure of complex phenylacetic acid with benzamide. In the second section of the chapter the hydrogen bond interaction of organic fluorine in trifluoromethyl derivatives of benzanilides has been explored and the involvement of CF3 group in the hydrogen bonding has been detected. The evidence for the participation of CF3 group in hydrogen bond has been confirmed by number of experiments, such as, the detection of through space couplings, viz., 1hJFH, 1hJFN, and 2hJFF , where the spin polarization between the interacting spins is transmitted through hydrogen bond, the temperature and solvent dependent studies, variation in the 1JNH and two dimensional heteronuclear correlation experiments. In an interesting example of a molecule containing two CF3 groups situated on two phenyl rings of benzanilide, the simultaneous participation of fluorines of two CF3 groups in hydrogen bond has been detected. The confirmatory evidence for such an interaction, where hydrogen bond mediated couplings are not reflected in the NMR spectrum, has been derived by 19F−19F NOESY. Significant deviations in the strengths of 1JNH, in addition to variable temperature, and the solvent induced perturbation studies yielded additional evidence. The NMR results are corroborated by both DFT calculations and MD simulations, where the quantitative information on different ways of involvement of fluorine in two and three centered hydrogen bonds, their percentage of occurrences, and geometries have been obtained. The hydrogen bond interaction energies have also been calculated. The study revealed the rare observation and the first example of the C-F…H-N hydrogen bond in solution state in the molecules containing CF3 groups. Chapter 6 focuses on the mixture analysis using the diffusion ordered spectroscopy (DOSY). High Resolution-DOSY works when the NMR spectrum is well resolved and the diffusion coefficients of the combinatorial mixtures are substantially different from each other. DOSY technique fails when the mixture contains the molecules of nearly identical weights and similar hydrodynamic radii. Thus, the positional isomers, enantiomers consequent to their nearly identical rates of diffusion, are not differentiated. Some of these problems can be overcome by Matrix-Assisted Diffusion Order Spectroscopy (MAD-spectroscopy), where an external reagent acts as a matrix and aids in their diffusion edited separation, provided the molecules embedded in it possess differential binding abilities with the matrix. Such different binding properties of the matrix are the basis for resolution of many isomeric species. In the present study three different novel auxiliaries, micelles-reverse micelles, crown ether and cyclodextrin are introduced for the resolution of positional isomers, double bonded isomers, viz., fumaric acid and maleic acid and also enantiomers. Accordingly, the results of each of these studies are discussed in three different sections.

The thesis entitled “Exploring Diverse Facets of Small Molecules by NMR Spectroscopy” consists of six chapters. The main theme of the thesis is to exploit one and two dimensional NMR methodologies for understanding the diverse facets of small organic molecules, such as, weak intra- and inter- molecular interactions, chiral discrimination, quantification of enantiomeric excess and assignment of absolute configuration. Several new pulse sequences have also been designed to solve specific chemical problems, in addition to extensive utility of existing one and two dimensional NMR experiments. The results obtained on different problems, are discussed under six chapters in the thesis. The brief summary of each of these chapters is given below. Chapter 1 begins with the discussion on the importance of small molecules and their various facets, the analytical techniques available in the literature to study them. The role of NMR spectroscopy as powerful analytical technique to understand the diverse facets of organic molecules and their importance is set out in brief. A short introduction to the basic principles of NMR, the interaction parameters, the commonly employed one and two dimensional homo- and herero- nuclear NMR experiments are also given. The basic introduction to product operators essential for understanding the spin dynamics in the developed pulse sequences is given. The application of diffusion ordered spectroscopy (DOSY), the general problems encountered in the analysis of combinatorial mixtures and the matrix assisted method in circumventing such problems are discussed. Chapter 2 focuses on the chiral discrimination and the measurement of enatiomeric excess. The NMR approach to discriminate enantiomers using chiral auxiliaries such as, solvating agents, derivatizing agents, lanthanide shift reagents, the choice of such auxiliaries and the limitations are discussed in detail. The in-depth discussion on the new protocols developed using both the solvating and derivatizing agents for enantiomeric discrimination of chiral amines, hydroxy acids and diacids are discussed. The new three-component protocols that serve as chiral derivatizing agents for the discrimination of primary amines, diacids and hydroxy acids are discussed. Also the role of organic base such as DMAP in the chiral discrimination is explored for discrimination of acids using BINOL as a chiral solvating agent. Accordingly the discussion is classified into two sections. In the first section the protocol developed utilizing an enantiopure mandelic acid, a primary amine substrate and 2-formylphenylboronic acid that is ideally suited for testing the enantiopurity of chiral primary amines is discussed. The broad applicability of the protocols for testing enantiopurity has been demonstrated on number of chiral molecules using 1H and 19F NMR. The second section contains the results on the new concept developed for discrimination of hydroxy acids. The strategy involves the formation of three component protocol using chiral hydroxy acid, R-alphamethylbenzylamine and 2-formylphenylboronic acid for 1H-NMR discrimination of diacids. The section also includes the utility of ternary ion-pair complex for the discrimination of acids. The ternary ion-pair not only permitted the testing of enantiopurity of chiral acids, but is also found useful for the measurement of enantiomeric excess. Chapter 3 discusses the utilization of the developed three-component protocols for the assignment of absolute configurations of molecules of different functionality. The protocols for the assignments of absolute configuration of primary amines using 2-formylphenylboronic acid and mandelic acid yielded the substantial chemical shift differences between diastereomers. The consistent trend in the direction of change of chemical shifts of the discriminated proton(s) gave significant evidence for employing them as parameters for the assignment of spatial configuration of primary amines. Another protocol using 2-formylphenylboronic acid, hydroxy acids and enantiopure alphamethylbenzylamine permitted their configurational assignment. In the second section a novel solvating agent, obtained by the formation of an ion-pair complex among enantiopure BINOL, DMAP and chiral hydroxy acid for the assignment of the spatial configuration of hydroxy acids is discussed. Chapter 4 focuses on the development of novel NMR methodologies, and also the utility of existing two-dimensional experiments for addressing certain challenging problems. This chapter has been divided into three sections. In Section-I the utilization of well-known homonuclear 2D-J-resolved methodology for unravelling the overlapped NMR spectra of enantiomers, an application for chiral discrimination and the measurement of enantiomeric excess is discussed. The utilization of the chiral auxiliaries, such as, chiral derivatizing agents, chiral solvating agents and lanthanide shift reagents permits enantiodiscrimination and the measurement of excess of one form over the other. Nevertheless many a times one encounters severe problems due to small chemical shift difference, overlap of resonances, complex multiplicity pattern because of the presence of number of interacting spins, and enormous line broadening due to paramagnetic nature of the metal complex. This section is focused on combating such problems utilizing 2D-J-1JNH resolved spectroscopy where a 450 tilting of the spectrum in the F2 dimension, yielded the pure shift NMR spectrum. The method circumvents several problems involved in chiral discrimination and allows the accurate measurement of enantiomeric excess. In Section-II, the development of novel NMR experimental methodology cited in the literature as C-HetSERF and its application for the study of symmetric molecules, such as, double bonded cis- and trans- isomers, and extraction of magnitudes and signs of long range homo- and hetero- nuclear scalar couplings among chemically equivalent protons in polycylic aromatic hydrocarbons is discussed. The extensive utility of the new pulse sequence has been demonstrated on number of symmetric molecules, where the conventional one dimensional experiment fails to yield spectral parameters. In section III, yet another novel pulse sequence called RES-TOCSY developed for unravelling of the overlapped NMR spectrum of enantiomers and the measurement of enantiomeric contents, has been utilized for the accurate measurement of magnitudes and signs of 1H-19F couplings in fluorine containing molecules. The method has distinct advantages as the strengths of the couplings and their relative signs could be extracted on diverse situations, such as, couplings smaller than line widths, the spectrum where the coupling fine structures are absent. Chapter 5 covers the study of nature of intra- and inter- molecular hydrogen bond in amide and its derivative. The chapter is accordingly divided into two sections. In the first section the study of acid and amide hydrogen bonding is discussed and the hydrogen bonded interactions are probed by extensive utility of 1H, 13C and 15N-NMR. The temperature perturbation experiments, measurements of the variation in the couplings, monitoring of diffusion coefficients and the association constants, detection of through space correlation have given unambiguous evidence for the hydrogen bond formation. The results were also supported by DFT calculations. Similar interaction in the solid state has also been derived by obtaining the crystal structure of complex phenylacetic acid with benzamide. In the second section of the chapter the hydrogen bond interaction of organic fluorine in trifluoromethyl derivatives of benzanilides has been explored and the involvement of CF3 group in the hydrogen bonding has been detected. The evidence for the participation of CF3 group in hydrogen bond has been confirmed by number of experiments, such as, the detection of through space couplings, viz., 1hJFH, 1hJFN, and 2hJFF , where the spin polarization between the interacting spins is transmitted through hydrogen bond, the temperature and solvent dependent studies, variation in the 1JNH and two dimensional heteronuclear correlation experiments. In an interesting example of a molecule containing two CF3 groups situated on two phenyl rings of benzanilide, the simultaneous participation of fluorines of two CF3 groups in hydrogen bond has been detected. The confirmatory evidence for such an interaction, where hydrogen bond mediated couplings are not reflected in the NMR spectrum, has been derived by 19F−19F NOESY. Significant deviations in the strengths of 1JNH, in addition to variable temperature, and the solvent induced perturbation studies yielded additional evidence. The NMR results are corroborated by both DFT calculations and MD simulations, where the quantitative information on different ways of involvement of fluorine in two and three centered hydrogen bonds, their percentage of occurrences, and geometries have been obtained. The hydrogen bond interaction energies have also been calculated. The study revealed the rare observation and the first example of the C-F…H-N hydrogen bond in solution state in the molecules containing CF3 groups. Chapter 6 focuses on the mixture analysis using the diffusion ordered spectroscopy (DOSY). High Resolution-DOSY works when the NMR spectrum is well resolved and the diffusion coefficients of the combinatorial mixtures are substantially different from each other. DOSY technique fails when the mixture contains the molecules of nearly identical weights and similar hydrodynamic radii. Thus, the positional isomers, enantiomers consequent to their nearly identical rates of diffusion, are not differentiated. Some of these problems can be overcome by Matrix-Assisted Diffusion Order Spectroscopy (MAD-spectroscopy), where an external reagent acts as a matrix and aids in their diffusion edited separation, provided the molecules embedded in it possess differential binding abilities with the matrix. Such different binding properties of the matrix are the basis for resolution of many isomeric species. In the present study three different novel auxiliaries, micelles-reverse micelles, crown ether and cyclodextrin are introduced for the resolution of positional isomers, double bonded isomers, viz., fumaric acid and maleic acid and also enantiomers. Accordingly, the results of each of these studies are discussed in three different sections.

In this thesis we have developed electronic and spin model Hamiltonians to understand magnetism in molecule based magnets like photomagnets, high-nuclearity transition metal complexes and single molecule magnets. In chapter 1, we provide an overview of molecular magnets. Here, we present a survey on the literature available on molecule based magnets. The chapter throws light on various phenomena found in molecular magnetic systems that range in dimensions from 3D down to molecular dimension. This is followed by a brief introduction to high-nuclearity transition metal complexes and single molecule magnets (SMMs). In the last two sections of this chapter, we discuss Light Induced Excited Spin State Trapping (LIESST) and photomagnetism in some molecular systems. Chapter 2 discusses various theoretical models that have been developed for magnetism. We begin with an introduction to the spin Hamiltonian and the origin of direct and kinetic exchange in simple systems and extend it to larger systems. Then we introduce the concept of superexchange proposed by Goodenough and Kanamori, followed by introduction to anisotropic Dzyalashinskii-Moria (DM) exchange and Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions. We also discuss molecular magnetic anisotropy, long-range magnetic interactions and higher order exchange interactions. These are effective model Hamiltonians that do not provide microscopic origin of magnetism, hence electronic model Hamiltonians need to be invoked. We introduce electronic model Hamiltonians like Huckel, Hubbard and Pariser-Parr-Popple (PPP) models and then present numerical techniques like valencebond (VB) and constant MS techniques that are used to exactly solve these model Hamiltonians. We present a many-body electronic model involving the active orbitals on the transition metal ions for photomagnetism in MoCu6 cluster, in chapter 3. The model is exactly solved using a valence bond approach. The ground state solution of the model is highly degenerate and is spanned by five S=0 states, nine S=1 states, five S=2 states and one S=3 state. The orbital occupancies in all these states correspond to six Cu(II) ions and one diamagnetic Mo(IV ) ion. The optically excited chargetransfer (CT) state in each spin sector occurs at nearly the same excitation energy of 2.993 eV for physically reasonable parameter values. We find that the excitation cross sections in different spin manifolds are similar in magnitude. The lifetime of the S=3 excited states is expected to be the largest as the number of states below that energy is very sparse in this spin sector compared to other spin sectors. This shows that photomagnetism is not due to preferential excitation to the S = 3 state. The inputs from the electronic model allows us to develop a kinetic model. In this model, photomagnetism is attributed to a long lived S=3 charge transfer excited state for which there appears to be sufficient experimental evidence. Based on this postulate, we model photomagnetism by including internal conversions and intersystem crossings. The key feature of the model is the assumption of existence of two kinds of S=3 states; one of which has no direct pathway for internal conversion and the other characterized by slow kinetics for internal conversion to the lowenergy states. The trapped S=3 state can decay via a thermally activated barrier to the other S = 3 state. The experimental XMT vs. T variation for two different irradiation times are fitted using Arrhenius dependence of the rate constants in the model. Conventional superexchange rules predict ferromagnetic exchange interaction between Ni(II) and M (M = MoV ,WV , NbIV ). Recent experiments show that in some systems this superexchange is antiferromagnetic. To understand this feature, in chapter 4 we develop a microscopic model for Ni(II) - M systems and solve it exactly using a valence bond approach. We identify direct exchange coupling, splitting of the magnetic orbitals and interorbital electron repulsions, on the M site as the parameters which control the ground state spin of various clusters of the Ni(II) - M system. We present quantum phase diagrams which delineate the high-spin and low-spin ground states in the parameter space. We fit the spin gap to a spin Hamiltonian and extract the effective exchange constant within the experimentally observed range, for reasonable parameter values. We also find a region in the parameter space where an intermediate spin state is the ground state. These results indicate that the spin spectrum of the microscopic model cannot be reproduced by a simple Heisenberg exchange Hamiltonian. The electronic model for A − B systems has been employed to reproduce the experimental magnetic data of the { NiW }2 system. In chapter 5, we present a theoretical approach to calculate the molecular magnetic anisotropy parameters, DM and EM for single molecule magnets in any eigenstate of the exchange Hamiltonian, treating the anisotropy Hamiltonian as a perturbation. Neglecting inter-site dipolar interactions, we calculate molecular magnetic anisotropy in a given total spin state from the known single-ion anisotropies of the transition metal centers. The method is applied to Mn12Ac and Fe8 in their ground and first few excited eigenstates, as an illustration. We have also studied the effect of orientation of local anisotropies on the molecular anisotropy in various eigenstates of the exchange Hamiltonian. We find that, in case of Mn12Ac, the molecular anisotropy depends strongly on the orientation of the local anisotropies and the spin of the state. The DM value of Mn12Ac is almost independent of the orientation of the local anisotropy of the core Mn(IV ) ions. In the case of Fe8, the dependence of molecular anisotropy on the spin of the state in question is weaker. We have also calculated the anisotropy constants for several sets of exchange parameters and find that in Mn12Ac the anisotropy increases with spin excitation gap while in Fe8, the anisotropy is almost independent of the gap. We have modeled the magnetic property of Nb6Ni12 cluster using a spin Hamiltonian in chapter 6. From Goodenough-Kanamori rules we should expect a ferromagnetic exchange between Nb and Ni ions. However, the magnetic studies indicate that the interaction is antiferromagnetic. We give reasons for the anomaly and fit the XMT data using an antiferromagnetic Heisenberg model. The observed XMT value at 2 K however does not correspond to ferrimagnetic ground state of Stot=9 and we invoke intermolecular interaction to explain this feature.

In this thesis we have developed electronic and spin model Hamiltonians to understand magnetism in molecule based magnets like photomagnets, high-nuclearity transition metal complexes and single molecule magnets. In chapter 1, we provide an overview of molecular magnets. Here, we present a survey on the literature available on molecule based magnets. The chapter throws light on various phenomena found in molecular magnetic systems that range in dimensions from 3D down to molecular dimension. This is followed by a brief introduction to high-nuclearity transition metal complexes and single molecule magnets (SMMs). In the last two sections of this chapter, we discuss Light Induced Excited Spin State Trapping (LIESST) and photomagnetism in some molecular systems. Chapter 2 discusses various theoretical models that have been developed for magnetism. We begin with an introduction to the spin Hamiltonian and the origin of direct and kinetic exchange in simple systems and extend it to larger systems. Then we introduce the concept of superexchange proposed by Goodenough and Kanamori, followed by introduction to anisotropic Dzyalashinskii-Moria (DM) exchange and Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions. We also discuss molecular magnetic anisotropy, long-range magnetic interactions and higher order exchange interactions. These are effective model Hamiltonians that do not provide microscopic origin of magnetism, hence electronic model Hamiltonians need to be invoked. We introduce electronic model Hamiltonians like Huckel, Hubbard and Pariser-Parr-Popple (PPP) models and then present numerical techniques like valencebond (VB) and constant MS techniques that are used to exactly solve these model Hamiltonians. We present a many-body electronic model involving the active orbitals on the transition metal ions for photomagnetism in MoCu6 cluster, in chapter 3. The model is exactly solved using a valence bond approach. The ground state solution of the model is highly degenerate and is spanned by five S=0 states, nine S=1 states, five S=2 states and one S=3 state. The orbital occupancies in all these states correspond to six Cu(II) ions and one diamagnetic Mo(IV ) ion. The optically excited chargetransfer (CT) state in each spin sector occurs at nearly the same excitation energy of 2.993 eV for physically reasonable parameter values. We find that the excitation cross sections in different spin manifolds are similar in magnitude. The lifetime of the S=3 excited states is expected to be the largest as the number of states below that energy is very sparse in this spin sector compared to other spin sectors. This shows that photomagnetism is not due to preferential excitation to the S = 3 state. The inputs from the electronic model allows us to develop a kinetic model. In this model, photomagnetism is attributed to a long lived S=3 charge transfer excited state for which there appears to be sufficient experimental evidence. Based on this postulate, we model photomagnetism by including internal conversions and intersystem crossings. The key feature of the model is the assumption of existence of two kinds of S=3 states; one of which has no direct pathway for internal conversion and the other characterized by slow kinetics for internal conversion to the lowenergy states. The trapped S=3 state can decay via a thermally activated barrier to the other S = 3 state. The experimental XMT vs. T variation for two different irradiation times are fitted using Arrhenius dependence of the rate constants in the model. Conventional superexchange rules predict ferromagnetic exchange interaction between Ni(II) and M (M = MoV ,WV , NbIV ). Recent experiments show that in some systems this superexchange is antiferromagnetic. To understand this feature, in chapter 4 we develop a microscopic model for Ni(II) - M systems and solve it exactly using a valence bond approach. We identify direct exchange coupling, splitting of the magnetic orbitals and interorbital electron repulsions, on the M site as the parameters which control the ground state spin of various clusters of the Ni(II) - M system. We present quantum phase diagrams which delineate the high-spin and low-spin ground states in the parameter space. We fit the spin gap to a spin Hamiltonian and extract the effective exchange constant within the experimentally observed range, for reasonable parameter values. We also find a region in the parameter space where an intermediate spin state is the ground state. These results indicate that the spin spectrum of the microscopic model cannot be reproduced by a simple Heisenberg exchange Hamiltonian. The electronic model for A − B systems has been employed to reproduce the experimental magnetic data of the { NiW }2 system. In chapter 5, we present a theoretical approach to calculate the molecular magnetic anisotropy parameters, DM and EM for single molecule magnets in any eigenstate of the exchange Hamiltonian, treating the anisotropy Hamiltonian as a perturbation. Neglecting inter-site dipolar interactions, we calculate molecular magnetic anisotropy in a given total spin state from the known single-ion anisotropies of the transition metal centers. The method is applied to Mn12Ac and Fe8 in their ground and first few excited eigenstates, as an illustration. We have also studied the effect of orientation of local anisotropies on the molecular anisotropy in various eigenstates of the exchange Hamiltonian. We find that, in case of Mn12Ac, the molecular anisotropy depends strongly on the orientation of the local anisotropies and the spin of the state. The DM value of Mn12Ac is almost independent of the orientation of the local anisotropy of the core Mn(IV ) ions. In the case of Fe8, the dependence of molecular anisotropy on the spin of the state in question is weaker. We have also calculated the anisotropy constants for several sets of exchange parameters and find that in Mn12Ac the anisotropy increases with spin excitation gap while in Fe8, the anisotropy is almost independent of the gap. We have modeled the magnetic property of Nb6Ni12 cluster using a spin Hamiltonian in chapter 6. From Goodenough-Kanamori rules we should expect a ferromagnetic exchange between Nb and Ni ions. However, the magnetic studies indicate that the interaction is antiferromagnetic. We give reasons for the anomaly and fit the XMT data using an antiferromagnetic Heisenberg model. The observed XMT value at 2 K however does not correspond to ferrimagnetic ground state of Stot=9 and we invoke intermolecular interaction to explain this feature.

Various heterospin oligoradicals were prepared. The intramolecular spin coupling of these radicals was studied by ESR spectroscopy. Oligonitroxides and oligophenoxys were synthesized and investigated as potential molecular magnetic materials. Heterospin biradical 3,5-di-tert-butyl-3′ -(N-tert-butyl-N-aminoxy)-4-oxybiphenyl was synthesized by Suzuki reaction. It showed reversible temperature dependent behaviors in both ESR and UV-vis spectra. Heterospin radicals 3-( N-tert-butyl-N-aminoxy)-nitrenobenzene and 4-(N-tert-butyl-N-aminoxy)-nitrenobenzene were prepared by photolysis of the corresponding azido precursors. The relationship between the intramolecular exchange coupling and the geometry of the molecular structures was studied by ESR spectroscopy. Trinitroxide 2,4,6-tris(4-N-tert-butyl- N-aminoxylphenyl)-s-trazine was synthesized by either Suzuki reaction or direct coupling of aromatic anions with cyanuric chloride. This triradical has a quartet ground state and is stable in air. The crystalline trinitroxide was studied by magnetic analysis and showed a intramolecular ferromagnetic exchange with J/kB = 9.0 K and a intermolecular antiferromagnetic exchange with J/kB = −1.45 K. Oligophenoxys 2,3,5-tris(3,5-di-tert-butyl-4-oxyphenyl)- s-triazine and 2,3-bis(3,5-di-tert-butyl-4-oxyphenyl)-5-( N,N-dimethylamino)-s-triazine were prepared to investigate the properties of s-triazine as an exchange linker. Tetraradical tetrakis(4-N-tert-butyl- N-aminoxylphenyl)silane was designed and synthesized as a potential organic magnet. This tetraradical has a tetrahedral geometry with a silicon atom at its center. Its solution shows a clean nonet in ESR spectrum at room temperature. Its crystal structure was also studied. Black precipitates were observed immediately after it was added in a solution of Mn(II)(hfac) 2 in heptane.

博士
國立臺灣大學
物理研究所
103
During development of electronics and spintronics, modifications on the surface electronic and magnetic properties have drawn broad interest. One of the most sophisticated surface-sensitive instrument, scanning tunneling microscopy (STM), has largely expanded the horizon of fundamental physics as well as nanotechnology, from observations of crystalline structures and electronic states to control of individual adatoms for artificial and novel constructions. In this dissertation, deposition of metals and organic molecules and the consequent effects on exotic surfaces are revealed by STM and spin-polarized STM (SP-STM). From morphology in topography to variance of electronic and magnetic structures in space and energy, the studies function as the cornerstone of surface engineering of heterostructure and its applications. On single ferromagnetic domains from Co nanoisland on Cu(111), investigations of manganese-phthalocyanine(MnPc)-Co and pentacene(PEN)-Co hybrid systems are achieved using STM, SP-STM, and first-principle calculations. While MnPc follows the Co spin-polarization near the Fermi level, PEN exhibits an opposite one. We conclude that different orbitals hybridizing with Co and different exchange mechanism result in the inverse spin polarization between MnPc and PEN. In addition, symmetry reduction of PEN, which clarifies the Co stacking methods, is found recovered in SP-STM spin-dependently, which brings out the significance of bonding and antibonding states in organic-ferromagnetic interfaces for designs of spin-distribution. On the other hand, spatial variations of electron and spin-polarization distributions induced by quasiparticle interference in Co islands are revealed by SP-STM. Tip manipulations allow us to compare interference patterns with and without spin information, which correspond to each other closely and are described by the particle-in-a-box model. Quasiparticle interference also reveals spin-dependent scattering processes in the Rashba spin-split surface states. STM measurements on the strong bulk Rashba crystal BiTeI resolve both the morphology of different surface terminations as well as scattering events from an interference dispersion. To put BiTeI in practical use, submonolayer Fe is deposited on it to unveil influences from magnetic impurities on Rashba spin-split states and electron scattering. Without appearance of new scattering channels and distortions in the intrinsic interference dispersion, Fe induces band shift which can be attributed to creations of Fe-substitution Bi defects. Organic molecules PTCDA (perylene-3,4,9,10-tetracarboxylic dianhydride) and PEN are also deposited and termination-selected growths are observed. Intramolecular dipoles in PTCDA induce molecule self-assembly as well as possible preference to the Te-termination. The dissertation covers issues in surface science from measurements of electronic and magnetic properties using STM to state modifications and creations via deposition of metals and organic materials. The combination of in-situ sample fabrications and delicate STM measurements with first-principles calculations resolves the complicated interactions among different elements and systems. Each of them contributes to development of spintronics from different angles, and in total, they depict a vision of future devices based on organic molecules and emergent crystals with all kinds of versatility.

This dissertation presents the investigation of magnetic exchange and anisotropy in novel metal-organic complexes containing minimum number of magnetic ions. Such complexes can serve as a model system to understand the exciting magnetic phenomena in such class of materials and also can put forward as candidates for the so called molecular nanomagnets. A direct assessment of the effective magnetic moment and the effective interaction between the metal ions in the complex can be done using magnetization measurements. Here the magnetization studies are performed as a function of temperature and field using a SQUID magnetometer. Yet another powerful tool to characterize and determine the spin levels, the ESR spectroscopic methods, has also been exploited. The study of the dynamical properties of this class of materials was relevant to understand the relaxation mechanism in the low temperatures. For this a new ac susceptometer has been built in house which was another main objective of this dissertation work. The design, fabrication, calibration and automation done on this device is presented in this thesis. The device has been tested using the known molecular magnet Mn12 acetate, and the antiferromagnet Dy2PdSi3. The present work is mainly focused on the magnetic properties of Mn, Ni and Fe based organometallic complexes. The studied Mn dimer with different acceptor and donor ligands exhibit the fine tuning of the electron density at the core of molecular complex by variation in ligands. This in turn shows that the change in peripheral ligands can control the magnetism of the molecule. The influence of the change in Ni-S-Ni bond angle in the magnetic exchange interaction is studied in a Ni(2) dimer and a Ni(2) trimer complex. The Ni dimer complex shows a ferromagnetic interaction (J = -42K) whereas trimer shows an antiferromagnetic interaction (J = 140K). Another Ni based complex bridged via phosphorous has been studied which shows the existence of glassy nature at low temperature. Also a polymeric chain compound based on Fe is studied and presented. All these phosphorous or sulphur bridged complexes are novel materials and these are the first data on these complexes.

The 207Pb and 119Sn NMR chemical shift were used to study the effect of temperature on Ph3MCl (M= Pb and Sn) adducts in the presence of 10% excess pyridine. The 207Pb and 119Sn chemical shift indicate a slow exchange at low temperatures below -90 0C and a significant exchange at higher temperatures above 10 0C. A plot of temperature against 207Pb or 119Sn chemical shift showed a curve with gentle slope at lower and a steep slope at higher temperatures. A good linear correlation (coefficient. of 0.95) between Hammett substituent constant and 207Pb or 119Sn chemical shift of para-substituted derivatives of Ph3MCl.py* (py* = NMe2, OMe, Me, Ph, H, Br, COPh and COMe; at -90 0C in CD2Cl2/CH2Cl2) was found. Both 207Pb and 119Sn chemical shift ranges are characteristic of five coordinate systems resolving into trigonal bipyramidal geometry as shown by X-ray crystal structures. New complexes of the type [CpFe(CO)(SnPh3)L] (L = PPh3, PBu3, PCy3, PMe3, P(NMe2)3, PMePh2, PMe2Ph, P(p-FC6H5)3, P(p-OMeC6H4)3, P(p-tolyl)3, P(OMe)3, and P(OPh)3 were synthesized by ultraviolet irradiation of [CpFe(CO)2(SnPh3)] and the appropriate phosphine or phosphite ligand. 57Fe NMR studies of the complexes showed an increasing linear relationship with Tolman’s steric parameter, whereas with Tolman’s electronic parameter the 57Fe chemical shift showed a decrease. The X-ray crystallographic profile of the selected new piano stool type complexes shows a significant correlation to the NMR data (solution state), i.e. Fe-Sn, Fe-P bond length and Sn-Fe-P bond angle against chemical shifts of 207Pb and 119Sn. Disubstituted complexes of the type [CpFe(SnPh3)L2] (L = PMe3, PMe2Ph, P(OMe)3 and P(OPh)3 were synthesized under similar conditions as monosubstituted compounds. The correlation trends between the NMR data and X-ray crystallographic profiles are similar to those found for monocarbonylated complexes. Tungsten phosphine complexes of the type [W(CO)5(PR3)] (prepared from [W(CO)6] under thermal conditions) and [W(CO)4(NCMe)(PR3)] (prepared from [W(CO)5(PR3)] by use of Me3NO-promoted decarbonylation) were synthesized and characterized by, among other methods X-ray diffraction techniques (R = Ph, p-tolyl, p-OMeC6H4, p-FC6H4, p-CF3C6H4, and NMe2). The tungsten complexes [W(CO)4(NCMe)(PR3)] react with [(dppp)Pt{C≡C-C5H4N}2] at room temperature to form new complexes of the type [(dppe)Pt{C≡C-C5H4N-W(CO)4(PR3)}2] which were characterized unambiguously by NMR spectroscopy. There is a fair correlation between 195Pt and 183W NMR chemical shifts and Tolman’s electronic parameter which indicates a fair influence by the substituents of the phosphorus atom on both metal centres. Tungsten complexes of the type [W(CO)4(NCMe)(L)] (L= PPh3, P(p-FC6H4)3, P(p-OMeC6H4)3, P(p-tolyl)3, P(p-CF3C6H4)3, PMePh2, and PPh2(C6F5) react with [(PPh3)2Rh(H)2(pytca)] (pytca = 2-(4-pyridyl)thiazole-4-carboxylate) to form new complexes of the type [(PPh3)2Rh(H)2(pytca)- W(CO)4(L)] under mild conditions. These complexes were characterized principally by NMR spectroscopy and X-Ray crystallography (L = P(p-tolyl)3). Crystallographic evidence was found for π-π-π interactions involving two phenyl rings, one of the two phosphines bonded to rhodium atom, one of the three phosphines bonded to tungsten and the pyridyl ring of the thiazole corboxylate group. A second π-π interaction is found between a thiazole and a phenyl ring of the phosphine ligand bonded to the rhodium atom. A fair correlation was found between the rhodium and tungsten chemical shift measured from this series of complexes as a result of varied paraphenyl substituent of phosphine ligand bonded to the tungsten atom. This therefore implies the possible existence of electronic communication between the two bridged metal centres.

The goal of this thesis was to characterize and quantify changes to Canadian Arctic organic matter (OM) induced by a physical disruption to the permafrost active layer by employing molecular-level techniques such as biomarker extraction and NMR to help elucidate its contribution to carbon turnover and global climate change. The initial biomarker characterization study determined that the extractable plant lipids were unaltered originating from the deposition of new vascular material or permafrost melt where a high alteration of lignin-derived OM was observed suggesting a long residence time in the ecosystem. Analysis of samples where there was a new and historical physical disruption to the permafrost landscape showed an initial increase in bacterial biomass biomarkers, and was corroborated with increased bacterial protein contributions and peptidoglycan signals in the NMR spectra. It is hypothesized that this increase in bacterial biomass resulted in a faster rate of degradation, possibly leading to OM priming.

In this dissertation, the intermolecular interactions between soil organic matter (SOM) and organofluorine compounds have been studied at the molecular-level using Nuclear Magnetic Resonance (NMR) spectroscopy. NMR probes the local magnetic environment surrounding atomic nuclei, and is uniquely capable as an analytical tool to probe molecular environments in complex disordered materials, such as soils. Several NMR techniques were employed in this work, including Pulse Field Gradient (PFG)-NMR based diffusion measurements, solid-state cross-polarization (CP), saturation transfer difference (STD) spectroscopy, and reverse-heteronuclear saturation transfer difference (RHSTD) spectroscopy. Using organofluorine compounds as molecular probes, xenobiotic interactions with SOM were studied. Using 1H{19F} RHSTD, the interaction sites in humic acid for organofluorine compounds were identified by direct molecular-level methods. Protein and lignin were identified as major binding sites, with different preferences exhibited for these sites by dissimilar organofluorine compounds: aromatic organofluorine compounds display varied preference for aromatic humic acid sites while perfluorooctanoic acid exhibits near total selectivity for protein-derived binding sites. The mechanisms underlying these preferences were probed in the solution state. Using crucial knowledge from the humic acid studies, a detailed molecular-level investigation of xenobiotic interactions in an intact and unmodified whole soil was made possible. A direct and in situ elucidation of the components in soil organic matter that interact with small organofluorine xenobiotic molecules has been presented, allowing, for the first time, resolution of multiple interactions occurring for xenobiotics simultaneously at different sites within a whole soil.

Characterization of phosphorus (P) in organic amendments is essential for environmentally sustainable fertilization of agricultural soils. The sequential chemical extraction (SCE) technique commonly used for P characterization does not provide any direct molecular information about P species. Studies were conducted to characterize P species in organic amendments and amended soils at a molecular level. The SCE was used to fractionate P in organic amendments including biosolids, hog, dairy and beef cattle manures, and poultry litter. The extracts were analyzed for total P and P species using inductively coupled plasma - optical emission spectroscopy (ICP-OES) and solution 31P nuclear magnetic resonance (NMR) spectroscopy, respectively. The relative proportions of P species in intact organic amendments and residues after each extraction, and calcareous soils amended with organic amendments and monoammonium phosphate (MAP) were estimated using the synchrotron-based P 1s X-ray absorption near edge structure (XANES) spectroscopy. The solution 31P NMR provided a detailed characterization of organic P in the non-labile NaOH and HCl fractions of organic amendments, but was limited in characterizing the labile fractions of most of these organic amendments due to their proneness to alkaline hydrolysis. The XANES analysis, however, identified the actual chemical species constituting the labile P that was only characterized as inorganic P or orthophosphates by sequential extraction and solution 31P NMR. In the amended Vertisolic and Chernozemic soils, XANES analysis estimated ‘soluble and adsorbed P’ as the dominant P species. For the Vertisolic soil, both the unamended and soil amended with biosolids and MAP contained hydroxyapatite (HAP). In addition, soil amended with biosolids, hog and dairy manures contained β-tricalcium phosphate (TRICAL), a more soluble CaP than HAP. TRICAL was found in all amended soils except in that amended with hog manure, while HAP was present in appreciable amount only in the control. Overall, the combination of techniques used in these studies improved the understanding of P species in organic amendments and amended soils that would not have been possible with any individual technique. Technological advances in P analysis should therefore be combined with conventional chemical extraction techniques to determine the fate of P in the environment.

Radionuclides, 129I and 239,240Pu, are major products or by-products of nuclear fission and among the top risk drivers for waste disposal at the Savannah River Sites (SRS) and Rocky Flats Environmental Technology Sites (RFETS), respectively, due to their perceived mobility in the environment, excessive inventory, toxicity, and long half-life. The objective of this study is to investigate the role of natural organic matter in retarding or facilitating the migration of 129I and 239,240Pu in the Department of Energy (DOE) sites. Measurements of 127I and 129I in humic acids (HAs) and fulvic acids (FAs) obtained by five successive alkaline, two glycerol and one citric acid-alkaline extractions, demonstrated that these extractable humic substances (HS) together account for 54-56 percent and 46 percent of the total 127I and 129I in the soil, respectively. The variations among 127I and 129I concentrations, isotopic ratios (129I/127I), chemical properties of all these humic substances indicated iodine was bound to a small-size aromatic subunit (~10 kDa), while the large-size subunit (~90 kDa), which likely linked the small-size unit through some weak chemical forces, determined the relative mobility of iodine bound to organic matter. Soil resuspension experiments simulating surface runoff or stormflow and erosion events were conducted with soils collected from SRS. Results showed that 72-77 percent of the newly-introduced I- or IO3- were irreversibly sequestered into the organic-rich soil, while the rest was transformed into colloidal and dissolved organo-iodine by the soil. The resulting iodine remobilization contradicts the conventional view that considers only I- or IO3- as the mobile forms. Quantitative structure analysis by 13C DPMAS NMR and solution state 1H NMR on these humic substances indicate that iodine is closely related to the aromatic regions containing esterified products of phenolic and fomic acid or other aliphatic carboxylic acids, amide functionalities, quinone-like structure activated by electron-donating groups (e.g., NH2) or hemicelluloses-lignin-like complex with phenyl-glycosidic linkage. The micro-molecular environment, such as the hydrophobic aliphatic periphery hindering the active aromatic cores and the hydrophilic polysaccharides favoring its accessibility towards hydrophilic iodine species, play another key role in the interactions between iodine and SOM. NMR spectra of the colloidal organic Pu carrier which can potentially be released from the soil during the surface runoff or stormflow showed Pu was transported, at sub-pM concentrations, by a cutin-derived soil degradation products containing siderophore-like moieties and virtually all mobile Pu.

Optical modulation of the physical properties of materials is important for future development of optical memories and switches, optoelectronics, and smart surfaces. Incorporation of an optically bistable photochromic compound into an electronically bifunctional material is a promising strategy for a development of photoswitchable materials. Photochromic spirooxazine ligands undergo light-induced ring-opening and closure between the closed-spirooxazine (SO) and open-photomerocynanine (PMC) forms. The structural reorganization leads to accompanying changes in electronic structure which can lead to a change in the oxidation/reduction potentials and spin state of a bound metal center. Changes in the ligand field about a metal center in turn can lead to “non-classical” photoinduced magnetic (PIM) effects. The “non-classical” PIM effect is an effect that occurs through ligand-centered processes via the metal center, rather than direct excitation at the metal center. The structural change of the photochromic compounds also results in a change in the frontier orbital energies and donor-acceptor character, which may lead to optically-gated charge-transfer and energy-transfer processes. In this dissertation, the structural factors that govern thermal relaxation of spirooxazines, as optical control units, was investigated toward controlling the photostationary states of this important class of photochromes. The electronic structure of the PMC form of azahomoadamantyl-based spirooxazines was found to control the thermal coloration/decoloration rates of photochromic spirooxazines. A significant charge-separated character of the PMC form was correlated with the slow thermal coloration/decoloration rates in spirooxazines. This concept was then extended to an investigation of the effect of Lewis-acidic metal complexation. Solution study of the charge-separated character of the PMC form via metal complexation of the photochromic spirooxazines supported the correlation between the charge-separated character of the PMC form and the rate of the thermal coloration/decoloration. The studies provide a potential pathway for modulating PMC thermal relaxation rates through optimization of the structure of the spirooxazines and metal complexation. The studies were then extended to an investigation of the photomodulation of charge-transfer processes in cobalt multinuclear clusters by photoisomerization of photochromic spirooxazines. Incorporation of optically bistable phenanthroline-spirooxazine ligands into a magnetically bistable cobalt-dioxolene valence tautomeric cluster resulted in large magnetic moments in the solid and solution states. This study suggests that the redox-isomeric behavior of the cobalt dioxolenes can be coupled to isomerization of the photochromic ligand in the solution state when the π-acceptor ability of the photochromic ligands align with the direction of charge transfer of the cobalt dioxolene components. The potential of these cobalt multinuclear clusters to enhance the relaxivity of water in MRI for biological imaging was investigated. A cobalt tetranuclear cluster was prepared and found to exhibit high magnetic moments in solution at room temperature, and large relaxivities relative to commercially available gadolinium based MRI contrast agents. Lastly, the photomodulation of ionic doping of graphene organic field-effect transistors (OFETs) by photochromic spirooxazines was investigated. The electron donor or acceptor nature of the photochromic isomers modulates the direction and magnitude of ionic doping of graphene, and in turn the gate voltages of graphene OFETs, leading to optical modulation of OFET gate voltages for data processing and memory technologies.
Graduate
2020-02-08