Masahide Ueda

Photocontrolled aggregation of colloidal silica

Spirobenzopyran having a decamethylene spacer has been chemisorbed on the surface of colloidal silica particles of diameter 150 nm. The silica particles covered with the photochromic units displayed normal photochromism when they were dispersed in organic solvents having refractive indices matched with that of the silica. The fact that normal photochromism was observed even in highly polar ethylene glycol, in which a low-molecular-weight spiropyran shows reversed photochromism, indicates that the highly hydrophilic character of the silica surface is converted to a relatively hydrophobic one owing to the long hydrocarbon chain. When the photochromic particles were dispersed in carbon tetrachloride, the UV irradiation induced flocculation, causing sedimentation of the colloidal silica. Subsequent visible light irradiation recovered the dispersed state, showing the photocontrol of the dispersibility and offering a novel photoresponsive system.

Photoisomerizability of an azobenzene covalently attached to silica-gel matrix

Abstract Sol-gel glass thin films were prepared from tetraethyl orthosilicate in the presence of 4-methoxy-4′-(2-hydroxyethoxy)azobenzene (MHAB), 4-methoxy-4′-(N-(3-triethoxysilylpropyl)carbamoylmethoxy)azobenzene (MTAB) and 4,4′-bis(N-(3-triethoxysilylpropyl)carbamoylmethoxy)azobenzene (DTAB). The optical absorption spectra of the three azobenzenes entrapped in the films were investigated. The trans-cis photoisomerization and the thermal reversion of the azobenzene moiety were studied in the glass films and compared with those in solutions as well as in polymethyl methacrylate (PMMA) thin films. Triethoxysilyl groups linked with the azobenzene moiety had no effect on isomerization reactions. The cis fraction of the three azobenzenes in a photostationary state in PMMA was quite similar to that in solution whereas the photoisomerization was strongly suppressed in sol-gel films due to the rigidity of the matrix. The introduction of one triethoxysilyl group has a slight effect on both photoisomerization and thermal reversion in sol-gel films. Photoisomerization of DTAB with two silylating units was depressed in a sol-sol film because both sides of the azobenzene moiety were fixed to the sol-gel glass matrix.

Photochromic behaviour of a spirobenzopyran chemisorbed on a colloidal silica surface

The thermal decolouration of the photoformed merocyanine (MC) of a nitrospirobenzopyran attached onto a colloidal silica surface through two different methylene spacers [-(CH2)n–, n= 2 or 10] has been examined in ethylene glycol, chloroform, carbon tetrachloride and cyclohexane. In ethylene glycol, the visible absorption band of the MC diminished slowly without any change in the spectral shape, and the bleaching reaction was well fitted by first-order kinetics, suggesting that the reaction proceeded in a constant micropolarity without any steric constraints. Elongation of the methylene spacer from C2 to C10 caused an increase in the thermal fading rate. In other water-immiscible solvents, on the other hand, a blue shift of the absorption maximum occurred during the thermal bleaching and the reaction was properly described by the superposition of two first-order rate processes. The blue shift and the bimodal first-order kinetics have been interpreted as a partitioning of the MC moiety between two distinctive residential sites with different polarities: an inner region with the higher polarity facing an adsorbed water layer lying on the silica surface and an outer region exposed to a organic solvent phase.

Photocontrolled dispersibility of colloidal silica by surface adsorption of a calix[4]resorcinarene having azobenzene groups

A novel calix[4]resorcinarene (4-AzCRA) having fourp-butylazobenzene residues attached to the upper rim of the cyclic framework has been prepared and subjected to adsorption on the surface of colloidal silica particles. The trans–cis photoisomerization of 4-AzCRA went almost to completion without being affected by the adsorption on the surface. UV irradiation of a homogeneous dispersion of surface-modified silica particles in cyclohexane enhanced the sedimentation owing to the increase in surface polarity, leading to the formation of porous precipitates. Subsequent visible light irradiation regenerated a dispersed state, displaying photocontrol of the reversible alternation between dispersed and aggregated states of the silica particles.

Photocontrol of dispersibility of colloidal silica

Abstract The surface of colloidal silica particles was chemisorbed with spirobenzopyran molecule having a long hydrocarbon chain. The spiropyran silica particles (SP-CS) displayed normal photochromism. When SP-CS were dispersed in carbon tetrachloride, UV irradiation induced flocculation of the particles to bring about markedly enhanced sedimentation. Subsequent visible irradiation recovered the dispersed state again.

Photochemical behaviour of tethered dianthryl compounds in sol–gel glass

Sol–gel films and monoliths were prepared by hydrolysis of tetraethyl orthosilicate (TEOS) or tetramethyl orthosilicate (TMOS) in the presence of 1,3-bis(9-anthryl)propan-1-ol (BAP), bis(9-anthrylmethyl)amine (BAA) and the intramolecular photodimer of BM (DBAA). The intramolecular photodimerization of these dianthryl compounds as photoprobes was examined in sol–gel glasses and compared with those in solutions as well as in poly(methyl methacrylate)(PMMA) films. The intramolecular photodimerization of both BAP and BAA was severely suppressed in sol–gel glasses and PMMA films to lead to exclusive photo-oxidization, indicating the steric restriction of these matrices. A marked difference in the matrix effect between the two solids was found in the reversible photodimerization of DBAA. Photogenerated anthryl units of DBAA disappeared upon exposure to 365 nm light faster in PMMA than in sol–gel silica, suggesting that the face-to-face conformation of photodissociated DBAA is more readily relaxed rather than in the inorganic matrix. This was supported by the fact that further exposure to 285 nm light gave a lower regeneration of anthryl units in the silica bulk material than in the PMMA matrices.