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.