Letteratura scientifica selezionata sul tema "Exciton Coupling Chirality"

Triarylmethylium-o,o-dimers adopt a twisted geometry so that two diarylmethyliums are stacked in a slipped manner. Thus, chiral auxiliaries on the aryl groups induce a preference in the axial chirality of the central biphenyl unit. Strong circular dichroism is attained through exciton coupling, which can be used for additional spectral output in their electrochromic behavior. Diastereoselectivity based on π–π stacking exhibits unique solvent effects, thus endowing multifunctional response properties.

The circular dichroism (CD) exciton chirality method (ECM) is a very popular approach for assigning the absolute configuration (AC) of natural products, thanks to its immediacy and ease of application. The sign of an exciton couplet (two electronic CD bands with opposite sign and similar intensity) can be directly correlated with the molecular stereochemistry, including the AC. However, a correct application of the ECM necessitates several prerequisites: knowledge of the molecular conformation; knowledge of transition moment direction; and preeminence of the exciton coupling mechanism with respect to other sources of CD signals. In recent years, by using quantum-chemical CD calculations, we have demonstrated that some previous applications of ECM were wrong or based on incorrect assumptions. In a recent publication of this journal (Mar. Drugs, 2017, 15(4), 121), the ECM was employed to assign the AC of a marine metabolite, laucysteinamide A. This is a further case of incorrect application of the method, where none of the aforementioned prerequisites is fully met. Using this example, we will discuss the criteria required for a correct application of the ECM.

Abstract Assemblies of single-walled carbon nanotubes with a specific chiral structure are promising future optofunctional materials because of their strong light–matter coupling arising from sharp optical resonances of quasi-one-dimensional excitons. Their strong optical resonances, which lie in the infrared-to-visible wavelength region, can be selected by their chiralities, and this selectivity promises a wide range of applications including photonic and thermo-optic devices. However, the broadband complex optical spectra of single-chirality carbon nanotube assemblies are scarce in the literature, which has prevented researchers and engineers from designing devices using them. Here, we experimentally determine broadband complex refractive index spectra of single-chirality carbon nanotube assemblies. Free-standing carbon nanotube membranes and those placed on sapphire substrates were fabricated via filtration of the nanotube solution prepared by the separation method using gel chromatography. Transmission and reflection spectra were measured in the mid-infrared to visible wavelength region, and the complex refractive indices of nanotube assemblies were determined as a function of photon energy. The real and imaginary parts of the refractive indices of the nanotube membrane with a bulk density of 1 g cm−3 at the first subband exciton resonance were determined to be approximately 2.7–3.6 and 1.3i–2.4i, respectively. We propose an empirical formula that phenomenologically describes the complex refractive index spectra of various single-chirality nanotube membranes, which can facilitate the design of photonic devices using carbon nanotubes as the material.

Optical rotatory dispersion (ORD) is a beautiful analytical technique for the study of chiral molecules and polymers. In this study, ORD was applied successfully to follow the degree of polycondensation of l-(+)-lactic acid toward the formation of poly(lactic acid) oligomers (PLAO) and high molecular weight poly(l-lactic acid) (PLLA) in a simple esterification reaction equipment. PLLA is a biodegradable polymer obtainable from renewable raw materials. The racemization of the intrinsically isotactic PLLA through thermal treatment can be easily followed through the use of ORD spectroscopy. Organic or molecular electronics is a hot topic dealing with the combination of π-conjugated organic compounds and polymers with specific properties (e.g., chirality) which can be exploited to construct optoelectronic devices, such as organic light-emitting diodes (OLEDs), organic photovoltaic (OPV) high efficiency cells, switchable chirality devices, organic field-effect transistors (OFETs), and so on. ORD spectroscopy was applied to study either the gigantic optical rotation of PLLA films, as well as to detect successfully the excitonic coupling, occurring in thin solid PLLA green film loaded with a combination of two dyes: SY96 (a pyrazolone dye) and PB16 (the metal-free phthalocyanine pigment). The latter compound PLLA loaded with SY96 and PB16 shows a really gigantic optical activity in addition to typical ORD signal due to exciton coupling and may be considered as a simple and easily accessible model composite of a chiral polymer matrix combined with π-conjugated dyes for molecular electronics studies.

Andrographolide, the major constituent from the terrestrial plant Andrographis paniculata is a much-studied bioactive ent-labdane diterpene lactone and has become an important medicinal intermediate. Its structure as determined by X-ray crystallography has been applied in molecular docking studies to explain biological activities. Nevertheless, recently there has been a number of conflicting reports concerning the stereochemistry at the C-14 and C-10 positions affecting the absolute configuration (AC) of the compound. Since a lack of information on the molecular flexibility of the molecule can lead to misleading conclusions on biological activity, a conformational analysis of the molecule in the solution state was necessary. The conformational analysis was performed by the Spartan14 package using the Merck Molecular Force Field (MMFF). The exciton chirality method in electronic circular dichroism spectroscopy (ECM-ECD) and vibrational circular dichroism (VCD) techniques were then jointly employed to re-establish the AC of andrographolide. Theoretical calculations were performed using TD-DFT methods by using the hybrid functionals B3LYP and CAM-B3LYP combined with 6-31G(d,p) basis set. Long-range exciton coupling of 2-naphthoyl chromophores at C-14 and C-19 led to the establishment of the AC to be 3R, 4R, 5S, 9R, 10R and 14S. Comparison between the theoretical VCD data of 14-S and 14-R stereoisomer confirmed a configuration of S at C-14 position instead of R.

Under acidic conditions and at high ionic strength, the zinc cation is removed from its metal complex with 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (TPPS4) thus leading to the diacid free porphyrin, that subsequently self-organize into J-aggregates. The kinetics of the demetallation step and the successive supramolecular assembly formation have been investigated as a function of pH and ionic strength (controlled by adding ZnSO4). The demetallation kinetics obey to a rate law that is first order in [ZnTPPS4] and second order in [H+], according to literature, with k2 = 5.5 ± 0.4 M−2 s−1 at 298 K (IS = 0.6 M, ZnSO4). The aggregation process has been modeled according to an autocatalytic growth, where after the formation of a starting seed containing m porphyrin units, the rate evolves as a power of time. A complete analysis of the extinction time traces at various wavelengths allows extraction of the relevant kinetic parameters, showing that a trimer or tetramer should be involved in the rate-determining step of the aggregation. The extinction spectra of the J-aggregates evidence quite broad bands, suggesting an electronic coupling mechanism different to the usual Frenkel exciton coupling. Resonance light scattering intensity in the aggregated samples increases with increasing both [H+] and [ZnSO4]. Symmetry breaking occurs in these samples and the J-aggregates show circular dichroism spectra with unusual bands. The asymmetry g-factor decreases in its absolute value with increasing the catalytic rate kc, nulling and eventually switching the Cotton effect from negative to positive. Some inferences on the role exerted by zinc cations on the kinetics and structural features of these nanostructures have been discussed.

De nouveaux dérivés hélicéniques ont été synthétisés dans le but d'investiguer leur réponse photophysique et chiroptique ainsi que leur capacité à filtrer le spin à travers différentes techniques. La première partie de cette thèse offre une introduction de la chiralité et des hélicènes par les méthodes de synthèse et leur comportement chiroptique. Une attention particulière est accordée à la définition de couplage excitonique et à l'effet CISS. Des techniques de mesure et des exemples illustratifs de la littérature sont présentés. Le deuxième chapitre illustre la synthèse et la caractérisation des premiers derivés hélicène-porphyrine, en mettant l'accent sur le rôle de l'hélicène dans le couplage excitonique. Tout d'abord, nous avons illustré deux nouvelles familles de dérivés hélicène-porphyrine, H6Pr1-3 et PrZnPhH6, préparés à partir d'un noyau hélicène commun et d'unités de porphyrine. Les premiers systèmes H6Pr1-3 ont été entièrement caractérisés, et leurs propriétés photophysiques et chiroptiques ont été étudiées expérimentalement et par calculs théoriques (grâce à nos collaborateurs de l'Université de Buffalo), confirmant la présence et la contribution des hélicènes dans le couplage excitonique, grâce à la très forte réponse CD bisignée dans la bande Soret ainsi qu'aux bandes Q légèrement actives en ECD. De manière intéressante, ces systèmes ont également révélé une activité CPL, ce qui est encore très rare dans les porphyrines chirales. Ce travail a été publié dans Chemical Communications. Le deuxième type de système hélicène-porphyrine a présenté une chiralité excitonique similaire, mais avec une activité chiroptique globalement moins intense (à la fois en ECD et en CPL). Le phénomène du couplage excitonique est ensuite approfondi dans le troisième chapitre au travers de la description de différents systèmes hélicène-porphyrine. Une amélioration notable a été obtenue avec la famille de molécules MPrH6 : le 2,5-bis-éthynylcarbo[6]hélicène est directement lié aux porphyrines, évitant ainsi le linker et réduisant la distance entre les chromophores. Par conséquent, l'étude des propriétés a été étendue à l'influence du métal, et plusieurs dérivés ont été préparés. Grâce aux résultats prometteurs, ils ont été sélectionnés comme candidats préliminaires pour des mesures CISS par mc-AFM, en collaboration avec le groupe de Naaman à l'Institut Weizmann de Science, et une polarisation de spin aussi élevée que 50 % a été obtenue pour le complexe de zinc. Le dernier système, MPrOMeH6, qui comprend une liaison C-C simple entre l'unité chirale et le chromophore, a été conçu pour optimiser la distance entre les centres métalliques dans la porphyrine et exploiter les interactions métal-métal, en particulier dans les centres magnétiquement actifs comme le dérivé V(IV)O. Dans le dernier chapitre, nous explorons une deuxième technique pour évaluer la capacité de filtrage du spin dans les hélicènes décorés par des unités redox et greffés sur différentes surfaces. Grâce à la combinaison de la carbohélicène avec des chromophores et différentes fonctions de greffage, nous sommes en mesure d'obtenir des matériaux prometteurs qui présentent des propriétés chiroptiques intéressantes et une capacité de filtrage de spin
During this thesis work, new helical molecular structures were prepared, their chiroptical responses, in particular the Exciton Coupling phenomenon, were studied and an intriguing property recently discovered, the Chiral-Induced Spin Selectivity, was investigated through different techniques. First, we illustrated two novel families of helicene-porphyrin conjugates, H6Pr1-3 and PrZnPhH6, prepared starting from one common helicenic core and porphyrin units. The first systems H6Pr1-3 were fully characterized and their photophysical and chiroptical properties were investigated experimentally and through theoretical calculations (thanks to our collaborators at University at Buffalo), unambiguously confirming the presence and effectiveness of EC chirality in helicenes through the very strong bisignate CD response in the Soret band along with slightly ECD-active Q-bands. Interestingly, these systems also revealed CPL activity which is still very rare in chiral porphyrins. This work was published in Chemical Communications. The second type of helicene-porphyrin conjugate displayed similar exciton chirality but with overall less intense chiroptical activities (both in ECD and in CPL). These results led us to further investigations on the Exciton Coupling of these systems: the role of the interchromophoric distance and of the conjugation were explored by changing the spacer between the chiral unit and the chromophores, as well as the metallic centers. A noteworthy enhancement was achieved with the family of molecules MPrH6: the 2,5-bis-ethynyl carbo[6]helicene is directly linked to the porphyrins avoiding the spacer, so that the interchromophoric distance was reduced. Therefore, the investigation of the properties was extended to the influence of the metal and several derivatives were prepared. Thanks to the promising results these derivatives were selected as preliminary candidates for CISS measurements through mc-AFM, in collaboration with Naaman’s group at Weizmann Institute of Science and spin polarization as high as 50% was obtained for the zinc complex. The last system MPrOMeH6, which includes a direct C-C bond between the chiral unit and the chromophore, was designed to optimize the distance between the metallic centers in the porphyrin core and to exploit the metal-metal interactions, specifically in magnetically-active centres like the V(IV)O derivative, that will be studied further through EPR spectroscopy. Overall, these results nicely illustrate how molecular engineering using functionalization with strongly polarizable units in close proximity enables the fine-tuning of the chiroptical signatures. This strategy also allows the access to CPL active porphyrin derivatives and opens up new opportunities for spintronics applications. In the last chapter we described the investigation of the spin filtering ability of helicene-based Self-Assembled Monolayer through the formation of a chemical bond with the surface: the helicenic unit was decorated with a redox moiety and a grafting unit, in order to obtain a proper orientation on the surface and to explore how the electron transfer reaction depends on the chirality through Spin Dependent Electrochemistry. More specifically, the bis-ethynyl hexahelicene has been functionalized with a conjugated ferrocene unit as redox probe on one side and i) a carboxylic acid (anchoring group for TiO2) for H6FeCOOH; ii) a thioacetate moiety (for Au surface) for H6FeSAc and ii) with a terminal alkyne (for Si-H surface). Promising preliminary results have been obtained for the H6FeCOOH and H6FeSAc derivatives but further characterizations are needed to explore the organization of the compounds on surface, the measurements are ongoing in collaboration with Naaman’s group. Thanks to the combination of the carbohelicene with chromophores and different grafting functions we are able to obtain promising materials with intriguing chiroptical properties and spin filtering ability

We show that vibronic coupling of exciton states in helical porphyrin aggregates leads to ultrafast evolution of the exciton angular momentum from chiral to achiral through time-dependent depolarization ratios measurements.

There is significant amount of research to analyze the thermal, electrical and other physical properties of carbon nanotubes (CNTs). However, the energy transport mechanism at the contact of two tubes is still not well understood. This study investigates the interfacial thermal interaction between two carbon nanotubes using molecular dynamics simulation and wavelet methods. We place the tubes in a crossed configuration and pass a high temperature pulse along one of the CNTs while keeping other ends fixed, and analyze the interaction of this pulse with other nanotube. We apply this technique for nanotubes of chirality in the range of (5,0) to (10,0) to observe the response of tubes with changing diameter. This thermal pulse analysis shows that the coupling between the two tubes is very weak and may be dominated by the slow moving phonon modes with high energy. We perform a wavelet analysis of thermal pulse propagation along a CNT and its impact on another CNT in cross contact. Wavelet transformations of the heat pulse show how different phonon modes are excited and how they evolve and propagate along the tube axis depending on its chirality.