Synthesis of ordered photoresponsive azobenzene–siloxane hybrids by self-assembly†
In this study, photoresponsive azobenzene–siloxane hybrids with lamellar structures were prepared by self-assembly using two types of alkoxysilane precursors, 4-[3-(triethoxysilyl)propoxy]azobenzene (P1) and 4-[3-(diethoxymethylsilyl)propoxy]azobenzene (P2). The films H1 and H2 were prepared by spin-coating hydrolyzed solutions of P1 and P2, respectively, on a glass substrate followed by heating to induce polycondensation. X-ray diffraction patterns revealed that H1 and H2 have lamellar structures with different d-spacings (3.20 nm and 2.37 nm, respectively), suggesting that the arrangements of the azobenzene moieties are different. These samples show slight but reversible changes in the d-spacings under photo-irradiation. Under UV irradiation, H1 shows a slight decrease in d-spacing, while H2 shows a slight increase. Such changes were caused by trans–cis isomerization of a part of the azobenzene moieties in the films, as confirmed by UV-vis absorption spectroscopy. These processes were reversible, with the d-spacings recovering their original values under visible light irradiation. Furthermore, P1 and P2 were co-hydrolyzed and polycondensed with tetraethoxysilane to give lamellar films (H1′ and H2′) showing a higher degree of trans–cis photoisomerization of the azobenzene moieties. Both H1′ and H2′ show increase in the d-spacings after soaking in various organic solvents. Possible structural models have been proposed to explain these photoresponsive properties of the azobenzene–siloxane nanohybrids, which will find potential applications as smart sensors and adsorbents in future.