Academic literature on the topic 'Photochromic Gels'

Photomechanically responsive materials are promising candidates for future smart actuator applications. The photo-responsive behaviors originate from the photoisomerization of photochromic molecules. A typical photochromic compound, azobenzene, has been studied extensively in the solution state and has played a crucial role in the photomechanical behaviors of materials such as polymers and gels, via chemical bridging with their matrix. In contrast to polymers and gels, the photomechanical attributes of molecular crystals have not progressed to the same degree, due to their rigidity and fragility. However, the past decade has witnessed an increasing number of reports of the photomechanical motion of molecular crystals, including azobenzene crystals. This paper reviews the current state-of-the-art of mechanically responsive azobenzene crystals, including the history, crystal design strategy, and future promising applications.

The structural and optical properties of the gel-forming photochromic switch 1,2-bis(2’-methyl-5’-{{((R) phenylethyl)amino}carbonyl}thienyl-3’-yl)cyclopentene are studied by means of the linear absorption and holographic grating techniques. The use of diffractive optics enables recording of holographic gratings with high long-term phase stability. The diffraction efficiency of the recorded holographic gratings approach values of 30% for low writing beam energies when diffusion is the rate determining factor. At higher writing pulse energies the competition between the diffusion and photodecomposition processes causes lower diffraction efficiencies. At irradiation doses above 10 mJ the spatial profile of the recorded gratings is strongly influenced by saturation effects. Because of the well-determined grating profile the holographic grating technique is potentially applicable for the quantitative characterisation of the diffusion process in photochromic gels.

Neste trabalho, materiais fotocrômicos foram preparados pela imobilização de um heteropolioxometalato do tipo Keggin, fosfotungstato [PW12O40]3- (HPW) em uma matriz híbrida do tipo Ormosil (do inglês Organically Modified Silicates). Estudou-se o impacto da adição dos cátions divalentes Zn2+, Mg2+, Ca2+, Sr2+ e Ba2+ sobre as propriedades dos filmes de Ormosil-Fosfotungstato. Os materiais foram preparados pelo método sol-gel e depositados na forma de filmes em diferentes substratos pela técnica de dip-coating. A formação da rede híbrida de organosilicato e a presença do [PW12O40]3- com sua estrutura de Keggin intacta nos materiais foram comprovadas pelas técnicas de espectroscopia vibracional (FTIR e Raman). Os difratogramas de raios-X das amostras de xerogéis demonstram que os materiais apresentam caráter semicristalino, sendo que um pico na região de 2θ<10° sugere a formação de unidades discretas de octasilsesquioxanos e/ou lamelas de silicato. Estudos de microscopia eletrônica de transmissão revelaram que a adição dos cátions durante a síntese induz a precipitação de nano-aglomerados constituídos de HPW e dos cátions dopantes. A análise do XANES na borda K do Zn sugere que os cátions dopantes Zn2+ estão associados aos heteropoliânions em aglomerados do sal [Zn(OH2)6]x/2Hx-3PW12O40. A imobilização destes aglomerados na rede de organosilicato durante a deposição dos filmes acarreta na preparação de filmes com maior quantidade incorporada de fosfotungstato, como demonstrado pela análise de XRF. A formação dos aglomerados é favorecida pela presença dos cátions maiores, sendo que a adição do Ba2+, que possui maior raio iônico dentre os cátions aqui estudados, permite a incorporação de quase sete vezes mais fosfotungstatp no filme de Ormosil. Essa maior incorporação dea espécie fotoativa nos filmes híbridos se reflete no aumento resposta fotocrômica dos filmes dopados de 35% (amostra dopada com Mg2+) até 685% (amostra dopada com Ba2+). Assim, a adição dos cátions, sobretudo daqueles com maior raio iônico, durante a síntese sol-gel do nanocompósito Ormosil-fosfotungstato representa uma estratégia simples e bastante reprodutível para a preparação de filmes com propriedades fotocrômicas aprimoradas. A elevada resposta fotocrômica dos filmes dopados os torna bons candidatos a serem aplicados como dosímetros colorimétricos da radiação UV.
In this study photohromic materials wer prepared by the entrapment of the Keggin heteropoyoxometalate phosphoungstate [PW12O40]3 (HPW) in a Ormosil (Organically Modified Silicates) hybrid matrix. We evaluated the impact of the addition of the divalent cations Zn2+, Mg2+, Ca2+, Sr2+ and Ba2+ on the properties of Ormosil-phosphotungstate films. The materials were prepared by sol-gel route and deposited as thin films on different substrates by dip coating method. The formation of the hybrid organosilicate network and the presence of the Keggin heteropolianion [PW12O40]3- were confirmed by FTIR and Raman vibrational spectroscopies. The x-ray diffractograms of the xerogels samples indicated that the materials present semi-crystalline behavior. A broad diffraction peak in the 2θ<10° region pointsto the possible formation octasilsesquioxane discrete units and/or silicate lamellae. Transmission electron microscopy studies revealed that the cations addition during the sol-gel synthesis induces the precipitation of nano-agglomerates composed of HPW and the doping cations. Zn K-edge XANES analysis suggests that the Zn2+ doping cations are associated to the heteropolianions forming [Zn(OH2)6]x/2Hx-3PW12O40 salt agglomerates. The entrapment of these agglomerates in the organosilicate network during dip-coating deposition results in the preparation of films with enhanced incorporation of phosphotungstate as demonstrated by XRF analysis. Ba2+ doping enhances in almost seven times the amount of incorporated phosphotungstate. This behavior is explained by the fact that as the solubility of polyoxometalate salts greatly decreases with the increase counter cation size, the formation of the agglomerates is favored in the presence of the larger cations (Sr2+ and Ba2+). The enhanced incorporation of the photoactive specie reflects on the phtochromic response of the doped hybrud films, wich increases in the range of 35% (Mg2+ doped sample) to 685% (Ba2+ doped sample). Therefore, the addition of the cations during sol-gel synthesis of the Ormosil-phosphotungstate nanocomposite represents a simple and reproducible strategy for the preparation of films with enhanced photochromic properties. These highly photochromic hybrid films are promising candidates for the design of practical UV-sensing devices and dosimeters.

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Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)
O processo Sol-Gel tem permido preparar materiais vítreos ou vítro-cerâmicos com novas composições com alto grau de pureza, homogêneos, materiais híbridos orgânicos e inorgânicos (OIHM) e até materiais impossíveis de serem preparados por processos convencionais. Um dos melhores sistemas para obtenção dos OIHM de sílica (ORMOSIL – Silicato Organicamente Modificado) é a reação combinada com alcoóxidos de Silício como o TEOS (Tetraetilortosilicato) e alcoóxidos funcionalizados como o GPTS (3-Glicidoxipropil-trimetoxisilano). A Prata, é aplicada em diversas áreas como por exemplo, em pinturas a base de Prata (utilizados para circuitos impressos), em filmes fotográficos, películas de vidros dentre várias outras aplicações. Dois efeitos encontrados quando uma amostra GTD-Ag (Amostra com a dopagem de AgNO3), irradiada por uma lâmpada UV e tratada a temperatura (acima de 80 °C), são os efeitos fotocrômicos e termocrômicos. Para o efeito fotocrômico, a irradiação por lâmpada UV, produz partículas de prata oxidadas (Ag+) que transformam-se em partículas de prata reduzidas (Ago), ocorrendo-se assim, o escurecimento dos filmes (redução). O clareamento das amostras GTD-Ag (oxidação), foi obtido através de um agente oxidante, o H2O2. Para observar o efeito termocrômico, as amostras GTD-Ag, são tratadas em forno convencional em temperaturas de 80 °C à 550 °C em atmosferas de N2 e ambiente. No tratamento térmico em atmosfera de N2, foi possível observar o escurecimento (redução) da amostra, a partir da temperatura de 80 °C, e em tratamento térmico à atmosfera ambiente, observamos um clareamento da amostra (oxidação), a partir da temperatura de 200 °C. Utilizando a técnica de análise térmica diferencial (DTA), foi possível verificar a óxido-redução da amostra GTD-Ag, quando tratada em atmosfera ambiente, em temperaturas superiores a 150 °C. Comportamento semelhante é observado para as amostras GTD (Amostra sem dopagem de AgNO3), em temperaturas acima de 200 °C, ocorrendo um efeito conhecido por pirólise, que no nosso caso é a degradação do grupo epóxi.
The sol-gel process has allowed prepare vitreous or vitro-ceramic materials with novel compositions with high purity, homogeneous, organic and inorganic hybrid materials (OIHM) and impossible materials to be prepared by conventional methods. One of the better systems for obtaining OIHM silica (ORMOSIL - Silicate Organically Modified), is the combined reaction of silicon alkoxides such as TEOS (Tetraethylorthosilicate), and functionalised alkoxides such as GPTS (3- Glycidoxypropyl-trimethoxysilane). The Silver is applied in various areas such as Silver paintings (used for printed circuits) on photographic films, glass films among many other applications. Two effects found when a GTD-Ag sample (Sample with AgNO3 doping) irradiated by a UV lamp and treated at temperatures (above 80 °C), are photochromic and thermochromic effects. For the photochromic effect, the UV irradiation lamp produces oxidized silver particles (Ag+) which becomes reduced particles silver (Agº), occurring to thereby darkening of the films (reduction). The samples of the bleaching GTD-Ag (oxidation) were obtained by an oxidizing agent H2O2. To observe the thermochromic effect, the GTD-Ag samples are treated in a conventional oven at temperatures of 80 °C to 550 °C in N2 atmosphere and ambient. In the heat treatment in N 2 atmosphere, was observed to darkening (reduction) of the sample from the 80 °C temperature and treatments to the ambient atmosphere, we observed a whitening of the sample (oxidation) at low temperature 200 °C. Using the differential thermal analysis (DTA), we found the redox sample GTD-Ag, when treated in atmosphere ambient at temperatures above 150 °C. Similar behavior is observed for the GTD samples (Sample undoped AgNO3) at temperatures above 200 °C, occurring an effect known by pyrolysis which in our case is the degradation of the epoxy group.

Filmes híbridos fotoativos de Ormosil (silicatos organicamente modificados) contendo o polioxometalato de Keggin ácido fosfotungstico (H3PW12O40) e cátions Ag+ foram preparados por meio da rota sol-gel e deposição de filme por dip-coating. A caracterização por FTIR, Raman, XRF e DRX evidenciou a formação da matriz de organossilicato e o aprisionamento dos poliânions PW12O403- e cátions Ag+. O comportamento fotocrômico dos filmes de Ormosil-fosfotungstato baseiam-se na redução fotoinduzida do fosfotungstato por exposição à radiação UV, o que leva à formação de compostos coloridos de valência mista conhecidos como heteropolyblues. No entanto, a presença de cátions Ag+ altera completamente o comportamento fotocrômico destes filmes híbridos. Os poliânions de fosfotungstato fotorreduzidos são agentes redutores eficientes na redução dos cátions Ag+, resultando na formação in situ de nanopartículas de prata metálica na matriz híbrida como evidenciado pela intensa banda plasmônica centrada em 390 nm no espectro eletrônico da amostra. A caracterização por Uv-vis e Espectroscopia de Absorção de Raios X próxima a Borda de Absorção (XANES) mostrou que a foto-redução pode ser revertida por tratamento térmico o que caracteriza um comportamento fotocrômico reversível. O comportamento cinético da amostra foi analisado propondo mecanismos para as reações de redução e oxidação da prata, responsáveis pela alteração de coloração do material. O aprisionamento de compostos fotoativos em matrizes híbridas orgânico-inorgânico via sol-gel é uma estratégia interessante para a preparação de materiais fotocrômicos com potenciais aplicações tecnológicas. A reversibilidade fotocrômica destes materiais os torna bons candidatos para aplicação em sistemas de memória óptica, filtros óticos de passa-banda estreita, dispositivos de ótica não linear, sensores de radiação UV e revestimentos bactericidas.
Sol gel entrapment of photoactive compounds in organic-inorganic hybrid matrices is an interesting strategy for the preparation of photochromic materials with potential technological applications. In this study, we report the preparation of photoactive Ormosil (organically modified silicates) hybrid films containing the Keggin polyoxometalate phosphotungstate (H3PW12O40) and Ag+ cations by sol-gel synthesis and dip coating deposition. The materials have been characterized by infrared absorption spectroscopy (FTIR), Raman spectroscopy and X-ray Fluorescence (XRF), thus evidencing the formation of hybrid organosilicate matrix and the entrapment of the PW12O40-3 polyanions and Ag+ cations. The photochromic behavior of the Ormosil-phosphotungstate films is based on photoinduced reduction of phosphotungstate upon exposure to UV radiation, which leads to the formation of colored mixed valence compounds known as heteropolyblues. However, the presence of Ag+ cations completely changes the photochromic behavior of these hybrid films. The photoreduced phosphotungstate polyanions rapidly promote the reduction of Ag+ cations, resulting in the in situ formation of silver metal nanoparticles (Ag0) inside the hybrid matrix as evidenced by the intense plasmonic band centered at 390nm as shown by electronic, UV-vis, spectroscopy and its yellowish color. The Uv-vis and X-ray Near Edge Absorption Spectroscopy (XANES) characterization shown that this process is reversible and the yellowish color characteristic of the metallic silver nanoparticles disappears once the hybrid films are heated at 353 K. Thus, the coloration is achieved upon UV exposure and discoloration occurs upon simple heating. Therefore, the described photoactive properties, the prepared hybrid films have potential application in optical memory systems, single bandpass optical filters, non-linear optical devices, UV radiation sensors and bactericidal coatings.

Les travaux précédents du laboratoire Hubert Curien ont permis d’élaborer des films photosensibles Ag : TiO2 sur verre, support de marquage de motifs actualisables ou permanents. Une adaptation de ces travaux aux supports plastiques et papiers est ici proposée afin d’élargir les domaines d’application potentiels au marquage sécurité des produits par exemple. Des techniques d’élaboration de films Ag : TiO2 compatibles avec les substrats considérés ont donc été développées. Deux voies ont été envisagées. La première utilise la chimie du Sol-Gel combinée à la méthode EISA et des procédés de dépôt tel le spin-coating, le jet d’encre ou la flexographie pour former un film mésoporeux de TiO2 sur les supports. Des traitements basés sur une extraction par solvant ou un recuit infrarouge ont été imaginés afin de libérer la porosité du film sans dégradation du support. Pour réaliser les films sur papier, un sel d’argent est ajouté au Sol avant son dépôt. Dans les cas des films élaborés sur plastique, l’argent est incorporé par imprégnation du matériau dans une solution de sel d’argent. La deuxième voie d’élaboration propose de formuler une encre aqueuse jet d’encre de nanoparticules de TiO2 et d’ions argent, en adaptant la composition d’une suspension commerciale de TiO2 aux exigences du jet d’encre. Après dépôt, l’encre est séchée par recuit infrarouge. Le comportement photochromique sous expositions lumineuses UV et visible des différents films permet leur coloration et décoloration de façon réversible. Les films Sol-Gel déposés sur plastique peuvent également être support de photo-inscriptions permanentes générées par irradiation par une lumière visible d’une certaine intensité
Previous research conducted at laboratory Hubert Curien led to the development of photosensitive Ag: TiO2 films on glass as support for updatable or permanent patterns. An adaptation of this work to plastic and paper substrates is proposed here to broaden the possible application areas such as goods secure labeling, for example. Fabrication techniques of Ag: TiO2 films compatible with flexible substrates have been developed, using two different paths. The first uses the combination of Sol-Gel chemistry with the EISA method. Deposition processes such as spin coating, inkjet or flexographic printing are used to form a mesoporous film of TiO2 on substrates. Treatments based on solvent extraction or infrared annealing have been devised to release the porosity of the film without damaging the supports. Silver salt is either introduced into the titania pores by soaking the films into a silver salt solution or added to the sol before its coating. The second developed option proposes formulating an aqueous ink jet ink made of TiO2 nanoparticles and silver ions by adapting the composition of a commercial suspension of TiO2 with the requirements of the ink jet process. After printing, the ink is dried by infrared annealing. The photochromic behavior under UV and visible light exposures of fabricated films allows to get coloring and bleaching reversibly. Sol-Gel films coated on plastic can also be a support for permanent colored patterns realized by irradiation with a visible light of certain intensity

Ce travail de thèse porte sur l'élaboration et la caractérisation optique et structurale de films de silice ou de titane mésostructurés porteurs de nanoparticules d'argent. Il s'intéresse également aux propriétés photochromiques des films à base de titane. Pour ce travail, des films mésoporeux de SiO2, et de TiO2, amorphe ou anatase, ont été synthétisés par voie sol-gel et caractérisés. Après leur imprégnation par des ions métalliques, trois protocoles de réduction ont été utilisés. La croissance thermique favorise la coalescence des nanoparticules; des bâtonnets d'argent sont obtenus dans les films de silice hexagonaux 2D alors que des agglomérats se forment en surface et dans l'épaisseur des films de TiO2 présentant des pores sphériques. La réduction chimique permet une croissance rapide des nanoparticules d'argent dans les pores; des bâtonnets et sphères de taille et forme contrôlées sont obtenus dans les films de SiO2 et de TiO2, respectivement. La croissance optique des nanoparticules d'argent par photoréduction est aussi un procédé rapide; des nanoparticules avec une plus large distribution de taille que leurs similaires chimiques sont obtenues dans les films de TiO2, mais seulement quelques nanoparticules dans la matrice SiO2 en raison de sa faible réponse à l'UV. L'étude du comportement photochromique des nanoparticules d'argent déposées dans des films de TiO2 a démontré l'influence de la répartition initiale des nanoparticules et de l'intensité incidente sur la couleur du film. Un blanchiment du film optique ou chimique est observé après insolation avec une lumière monochromatique puissante, et un comportement multiphotochromique pour de faibles intensités lumineuses.

The thesis entitled “Design, Synthesis and Properties of Novel Oligo-Phenylenevinylene based Supramolecular Photochromic Gels and Soft Composites with Nanomaterials” deals with soft materials derived from low molecular mass photochromic gels and nanomaterials. Chapter 1 gives a general introduction and overview of the low molecular mass gel (LMMG). It briefly delves into the history of research in physical gel field, design of different types of photochromic gelator molecules, their interesting self-assembly patterns, potential applications of these gelator molecules as well as challenges to design of new gelator molecules. A comprehensive discussion on the synthesis and numerous applications of smart nanocomposites derived from LMMGs and nanomaterials were discussed. It also encompasses the relatively recent area of two component gel system to conveniently bypass the cumbersome synthetic protocol. Interesting photophysical properties of these photochromic LMMGs were discussed towards their light-harvesting properties and aggregation induced white-light emission. Chapter 2 describes the synthesis and self-assembly properties of all-trans-tri(p-phenylenevinylene) (TPV) based molecules possessing different terminal groups, e.g. oxime, hydrazone, phenylhydrazone and semicarbazone (Chart 2A.1). Various spectroscopic and microscopic studies show the aggregation pattern of the self-assemblies promoted by hydrogen bonding, aromatic π-stacking and van der Waals interactions among the individual TPV units (Figure 2A.1). The melting temperatures of the gels and viscoelastic behaviour indicate that the presence of more hydrogen-bonding donors in the periphery of the gelator molecules makes the gel thermally and mechanically more robust. However, in the presence of more numbers of hydrogen-bonding donor/acceptors at the periphery of TPVs such as with semicarbazone a precipitation as opposed to gelation was observed. Thus, the choice of the end functional groups and the number of hydrogen-bonding motifs in the TPV backbone holds the key and modulates the effective length of the chromophore, resulting in interesting optical properties Chapter 3A demonstrates successful incorporation of pristine and long-chain functionalized single-walled carbon nanotubes (SWNTs) in supramolecular organogels of 1 (Chart 2A.1) to give rise to new nanocomposites with interesting mechanical, thermal and electrical properties (Figure 3A.1). The SWNT promoted aggregation of 1 leads to quenching of the absorption and emission intensity of 1, increases the sol-to-gel transition temperature and increases the viscoelasticity of the composite gels. The composites were semiconducting in nature and showed enhanced electrical conductivity compared to that of 1 alone. Upon irradiation with a near IR laser at 1064 nm for 5 min, it was possible to selectively induce a gel-to-sol phase transition of the nanocomposites, while irradiation for even 30 min of the native organogel under identical conditions did not cause any gel-to-sol conversion Chapter 3B describes incorporation of multi-walled boron nitride nanotubes (BNNTs) and various functionalized BNNTs by Lewis bases such as trioctylamine (TOA), tributylamine (TBA), and triphenylphosphine (TPP) in the toluene gel of 1 (Chart 2A.1). Functionalized BNNTs were synthesized first and incorporation into the gel showed evidence of wrapping of the gelator molecules on to the BNNT surface presumably brought about by π-π stacking and van der Waals interactions (Figure 3B.1). This leads to the formation of densely packed and directionally aligned fibrous networks. Such “reinforced” aggregation of the gelator molecules in presence of doped BNNTs led to an increase in the sol-to-gel transition temperature and the solidification temperature of the gel-nanocomposites as revealed from differential scanning calorimetry. Rheological investigations of the gel-nanocomposites indicate that the flow properties of the resulting materials become resistant to applied stress upon incorporation of even a very low wt% of BNNTs. Finally, the increase in thermal conductivity of the nanocomposite compared to the gelator alone was observed for the temperature range of 0-60 oC which may make these composites potentially useful in various applications depending on the choice and the amount of BNNT loading in the composite. Chapter 3C presents first successful incorporation of graphene, and long aliphatic chain (n-dodecyl, n-hexadecyl) functionalized graphene in physical organogels formed by the aromatic oligo-phenylenevinylene (OPV) based gelator 1 (Chart 2A.1) and (non-aromatic) amino acid derived gelator. The large aromatic surfaces of gelator 1 serve as a host matrix for the incorporation of graphene and other nanocarbons (fullerene, SWNT). Such carbon nanomaterials (CNMs) exerted variable effects on the gelator through non-covalent interactions, due to their differences in shapes. Various microscopic images confirm the formation of densely wrapped fibrous networks for the resulting nanocomposites upon incorporation of CNMs. Variable temperature UV-vis and fluorescence spectra reveal CNMs mediated aggregation of the gelator molecules in solution and the presence of supramolecular interaction was evident from Raman spectroscopy. This ‘reinforced’ aggregation of the gelator molecules on doped CNMs was reflected in significantly enhanced thermal, mechanical and electrical properties of the nanocomposites. Rheological investigations of gels containing small amount of CNMs indicated that the flow of nanocomposites became resistant to applied stress at a very low wt-% of CNM incorporation (0.83 wt-%). An interesting synergistic behaviour was observed in case of the composite gel of OPV based LMOG containing a mixture of EG and SWNT when compared with other mixtures of CNMs in all combinations with EG. These studies are therefore of great contemporary interest as they provide molecular level control into the preparation of novel nanocomposites of LMOG and nanocarbons. Chapter 4 describes the synthesis of two low molecular mass organogelators based on tri-p-phenylenevinylene derivatives one of which could be designated as “acceptor” while the other one as “donor” (Figure 4.1). These were prepared specifically to show the inter-gelator interactions at the molecular level between each other through the donor-acceptor type of assembly to achieve control over their macroscopic properties. Intermolecular H-bonding, π-stacking and van der Waals forces are operational for both the individual and the mixture leading them to the gel formation in chosen organic solvents. Due to the photochromic nature of this class of molecules, they exhibited interesting photophysical properties. An efficient energy transfer was demonstrated from the mixture of donor-acceptor assemblies in solution. An array of four chromophores was built up by including two known dyes i.e. anthracene and rhodamine 6G for the energy transfer studies. Interestingly a cascade energy transfer was observed in the assembly of four chromophores in the series. This allowed building up of a wide range of light harvesting process, excitation at one end of which produces an emission at the other end of the assembly. Chapter 5A discusses the synthesis of new dicationic chromophoric phenylenedivinylene bis-N-alkyl pyridinium salts to study their hydrogelation behaviour through π-stacking and van der Waals interactions (Figure 5A.1). A crucial hydrophilic-hydrophobic balance in aqueous medium controls the gelation when a specific length of the aliphatic chain (n-octyl, 2) is appended on the both ends of the central aromatic core. The hydrogels showed considerably high gel-melting temperature and more viscoelastic solid-like properties with increasing concentrations of the gelator 2. Microscopic studies exhibited concentration dependent mixed fiber-coil morphology above its gelation concentration and only fibrillar networks below the gelation concentration (Figure 5A.1). Variable temperature, UV-visible and fluorescence spectroscopy showed aggregation induced emission switches for the self-assemblies promoted by addition of various salts (either cations or anions) in diluted solutions. Aggregation induced white-light emission could be achieved in aqueous medium either by tuning the concentration of the added salt or by varying the temperature of the mixture. Cyclic-voltametric studies indicate a reversible one-electron redox behavior for the chromophore which is also diffusion-controlled in nature. Lamellar type arrangement in the self-assembly was evident from the X-ray diffraction analysis. Gradual downfield shift in the proton signals of the 1H-NMR spectra upon heating suggest aromatic π-stacking and van der Waals interactions are operational among the gelator molecules and a balance with the electrostatic interactions lead to the physical gelation in water Chapter 5B presents supramolecular π-gel formation by phenylenedivinylene bis-N-alkyl pyridinium salts appended with terminal aliphatic hydrocarbon chains of different lengths (Chart 5A.1) in specific ratios of aliphatic alcohols and water mixture. The temperature- and the ratio-variation in the ethanol/water mixture showed the aggregation pattern of the self-assemblies promoted by electrostatic, aromatic π-stacking and van der Waals interactions among the individual gelators as observed under UV-visible and fluorescence spectroscopy. With increase in the number of carbon atoms in the aliphatic chain, greater gel-melting temperature, increased viscoelastic solid-like behavior and decreased fiber diameter was observed among the gelators. However, presence of excess hydrophobic moiety at the periphery, a precipitation as opposed to gelation was observed. Cyclic-voltametric studies show a one-electron reversible redox behavior for the chromophore and the redox potential decreases with increasing the aliphatic chain length. A diffusion-controlled redox behavior was observed for shorter aliphatic chains but the longer chains make the process diffusion-limited. The electrical conductivity studies show semiconducting behavior for individual compounds and the magnitude of current increases with increasing fiber diameter (with decreasing aliphatic chain length). Chapter 6A demonstrates the synthesis of new oligo-phenylenevinylene (OPV) analogues with pyridine end-functionality (Chart 6A.1) to show efficient supramolecular organogel formation through molecular complexation with tartaric acids (TA). The salt formation between the end-pyridine and TA exhibited a significant decrease in the IR stretching frequency of the carboxylic acid. Microscopic studies showed a nucleation induced growth of the fibers that essentially led to larger aggregate formation. A circular dichroism study demonstrated an opposite sense of chirality in the complexes for two optically active TA (L and D). The expression of chiral transcription in the achiral OPVs was manifested under atomic force microscopy which showed a specific handedness in the fibers for the complex with particular optically active TA. Fluorescence spectroscopic studies exhibited a remarkable red-shift of the emission maxima due to the J-type aggregation leading to the gel formation. In a particular condition, energy transfer from aggregated donor to aggregated acceptor was observed in the gel phase. A liquid crystalline behavior was observed under polarized optical microscopy as well. Chapter 6B describes selective Hg2+ sensors which have been achieved separately under ‘naked eye’ and fluorimetric method for two-coordination assisted conjugated pyridine-end oligo-phenylenevinylene moieties (Chart 6A.1, 1 and 2). A drastic visual color change was exhibited based on the conjugation length of such chromophores. The visual color change was more prominent in the chromophore containing five aromatic rings in a conjugation compared to only three aromatic rings. However, breakdown of the conjugation length in the chromophore unit through incorporation of semicarbazide moiety (isoniazid) (Chart 6B.1) leads to a lesser degree of change either visually or spectroscopically. Coincidently, the isoniazid moiety provides an extra motif for their anion sensing properties through the deprotonation of ‘N-H’ group. Thus a selective CN- sensor was achieved. The presence of H-bonding donor (-NH-) and acceptor (-CO-) group in the semicarbazone segment and the long n-hexadecyl chains induced a physical gel formation. Addition of Hg2+ or CN- to the gel leads to the gel-to-sol transition and further addition in a reverse order could induce a reversible gel formation. Effect of addition of Hg2+ and CN- to the gel was probed by UV-vis and 1H-NMR spectroscopy which showed significant spectral shifts in favor of their interactions.

The aim of this work was to test and tailor new sol gel derived hybrid polymer coatings for the incorporation of photochromic spirooxazines and chromenes. The development and optimisation of work was performed via two different routes (dye and matrix ones), that led to photochromic multi-layer coating systems with coloration depth and photostabilities comparable to commercially available products. Hybrid sol-gel derived polymers were found to be suitable host materials for photochromic dyes. Matrix properties and the type of entrapment heavily influence the photochromic activity, as well as the degradation rate and the kinetics of incorporated dyes. Dyes incorporated within more polar and rigid matrices were found to show slower kinetics and higher coloration but associated with faster photodegradation. On the other hand, hosts with less polar sites, low residual water concentration and low rigidity are preferable in terms of photostability. Significant differences were found for physically incorporated and covalently grafted chromophores. Using silylated dyes that can participate in the sol-gel process, the photodegradation rate of the whole system can be decreased as compared to the physically entrapped systems. The higher photostability and slower kinetics for covalently bonded photochromes is probably due to sterical hindrance. Addition of proper stabilisers increases the photostability: The employment of UV light stabilisers, excited state quenchers and HALS was found to be beneficial but not sufficient. Besides the presence of stabilisers, also the reduction of oxygen migration into the coating (by a hard top coat and an inorganic anti reflective coating) strongly increases stability of photochromes. Finally, it was found that the separation of photochromes within two (or more) different layers leads to a further improvement of the coloration and fatigue behaviour of the whole coating stack, presumably by preventing the contact of dye molecules with excited states of other molecules or their degradation products. These latter findings are considered to pave the way for stable photochromic coatings based on hybrid polymers. Future development should be directed towards more photostable yellow and red switching dyes. The results of the present investigations should help to choose the most suitable molecular environments for the tested photochromes in terms of photostability, kinetics and activity, which is considered relevant with respect to potential applications, in particular in the ophthalmic sector. Furthermore, the interesting combination of properties of this type of materials offers a large potential with regard to many applications, such as coatings for sunglasses, radiation protectors, filters, sunroofs, reversible markings, printing applications and smart textiles
Das Ziel dieser Arbeit war es, neue, über das Sol Gel Verfahren hergestellte Beschichtungsmaterialien zu entwickeln, die als Matrices für photochrome Spirooxazine und Chromene geeignet sind. Die Entwicklungs- und Optimierungsarbeit erfolgte über zwei Routen (Farbstoff- und Matrix-Rout) und führte zu komplexen photochromen Mehrschichtaufbauten, deren Einfärbungs und Photostabilitätseigenschaften mit kommerziell erhältlichen Produkten vergleichbar sind. Über das Sol-Gel Verfahren hergestellte hybride Polymere stellen geeignete Wirtsmaterialien für photochrome Farbstoffen dar. Die Eigenschaften der Matrix und die Einbauweise beeinflussen sowohl die photochrome Aktivität, als auch die Degradationsgeschwindigkeit und Kinetik der eingebauten Farbstoffe. Farbstoffe integriert in rigide Matrices höherer Polarität zeigen geringere Schaltgeschwindigkeit und stärkere Einfärbung, jedoch verknüpft mit höherer Degradationsgeschwindigkeit. Andererseits Matrixmaterialien mit eher unpolarem Charakter, niedriger Konzentration an Restwasser, und niedriger Rigidität sind zu bevorzugen, wenn gute Photostabilität erreicht werden soll. Wesentliche Unterschiede bestehen zwischen chemisch angebundenen und physikalisch eingebauten Farbstoffen. Durch den Einsatz von silylierten Farbstoffen, die am Sol Gel-Prozess partizipieren können, kann eine Verringerung der Photodegradationsgeschwindigkeit im Vergleich zu physikalisch eingebauten Farbstoffen erreicht werden. Höhere Photostabilität und langsamere Schaltkinetikt sind vermutlich auf sterische Hinderung zurückzuführen. Der Zusatz geeigneter Stabilisatoren erhöht die Photostabilität: Der Einsatz von UV Stabilisatoren, Excited State Quenchern und HALS ist vorteilhaft aber nicht ausreichend. Neben der Präsenz von Stabilisatoren, führt auch die Verringerung der Sauerstoffpermeation (durch eine harte Deckschicht und eine reflexionsmindernde PVD Beschichtung) zu einer Stabilitätserhöhung. Schließlich wurde gefunden, dass die räumliche Trennung der Farbstoffe in zwei (oder mehr) unabhängigen Schichten ebenfalls die Färbetiefe und Photoresistenz der Beschichtungen erhöhte, vermutlich dadurch, dass der Kontakt von Farbstoffmolekülen mit angeregten Zuständen anderer Moleküle oder ihren Degradationsprodukten verhindert oder zumindest eingeschränkt wird. Diese Befunde können den Weg für stabile photochrome Beschichtungen auf de Basis von Hybridpolymeren ebnen. Weiterer Optimierungsbedarf wird in den bisher erzielbaren Farbtönen gesehen. Zukünftige Entwicklungen sollten sich auf photostabilere gelb und rot schaltende Farbstoffe konzentrieren. Die Ergebnisse der vorgelegten Untersuchungen können bei der Auswahl der am besten geeigneten, molekularen Umgebung für erprobte Photochrome in Bezug auf Photostabilität, Kinetik und Aktivität helfen. Dies ist von großer Bedeutung im Hinblick auf mögliche Anwendung vor allem im ophthalmischen Bereich. Darüber hinaus bergen die interessante Eigenschaftskombinationen dieser Art von Materialien großes Potential für ein weites Feld möglicher Anwendungen (Sonnenbrillen, Strahlungsschutz, Filter, Sonnendächer, schaltbare Markierungen, Druck, Textilien)

This book chapter discusses the versatile sol–gel processing technique that has been used to synthesize the nanostructures of titanium dioxide (TiO2) and their different morphologies. The sol–gel syntheses of different nanostructures of TiO2, namely TiO2 nanoparticles, nanocrystalline thin film, nanorods, nanofibers, nanowires, nanotubes, aerogels, and opals are described. These nanostructures have been characterized by Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) whose images clearly depict the formation of the nanostructures. Some of the morphologies of nano-TiO2 such as nanorods, nanotubes, nanofibers, nanowires, have been synthesized by sol–gel process in combination with spin-coating, dip-coating, template, surfactant, diblock polymer, micelles, polystyrene. In comparison to the bulk TiO2, presence of porous and nanocrystalline morphologies has played a role in enhancing the performance in applications such as photovoltaics, photocatalysis, photocatalytic water-splitting, H2 storage, gas sensors, photochromic, opto-electronic, and electrochromic devices. The chapter concludes with challenges and practical concerns in using the sol–gel process to produce thin films of complex oxides, porous nanostructures, solid nanorods, nanotubes, which need to be addressed in future research efforts.