PHOTOCHEMICAL AND THERMAL-ISOMERIZATION OF AZOBENZENE DERIVATIVES IN SOL-GEL BULK MATERIALS

Marked differences in properties of sol-gel silica between bulk materials and thin films was revealed by using azobenzenes as reactive probe molecules. Three types of sol-gel bulk materials and sol-gel films doped with azobenzene chromophores were prepared by hydrolysis of tetraethyl orthosilicate (TEOS) in the presence of 4-methoxy-4'-(2-hydroxyethoxy)azobenzene (MHAB), 4-methoxy-4'-[N-(3- (triethoxysilyl)propyl)carbamoylmethoxy azobenzene (MTAB), and 4,4'-bis[N-(3 -(triethoxysilyl)propyl)carbamoylmethoxy azobenzene (DTAB), respectively. Azobenzenes were initially protonated by hydrochloric acid as a hydrolytic catalyst, and the deprotonation took place during the drying process to form sol-gel glasses in a bulk system much more slowly than in a film-forming process. The transcis photoisomerization of the azobenzenes in the sol-gel bulk materials were traced spectroscopically to compare with those in sol-gel silica films prepared from the same solutions and in poly(methyl methacrylate) (PMMA). The results implied that the pores in the sol-gel bulks are larger than those in the sol-gel films. The cis-to-trans thermal isomerization of MHAB in a sol-gel bulk obeyed first-order kinetics in a rate similar to that of the solution chemistry, whereas the isomerization in a glass film deviated from the first kinetics, indicating that the rate retardation due to steric factors plays no essential role in the bulk matrix. The steric constraint due to the matrix decreased in the following order: sol-gel films > PMMA > sol-gel bulks.