Sum Frequency Generation Spectroscopic Study on Photoinduced Isomerization of Poly(vinyl alcohol) Containing Azobenzene Side Chain at the Air-Water Interface

Sum frequency generation (SFG) spectra were measured for the monolayers of a poly(vinyl alcohol) derivative (6Az10-PVA, see Figure 1A) containing azobenzene side chains at the air−water interface. The surface pressure dependence of the spectra was analyzed to elucidate structural changes of the monolayers associated with the trans−cis isomerization of the azobenzene group. The SFG spectra in the 3600−3150 cm-1 region observed for the trans-6Az10-PVA monolayer, where the OH stretching bands of water molecules associated with the PVA backbone are observed, suggest that a dissolution process of the backbone from the water surface into a bulk water takes place upon increasing the surface pressure from 20 to 45 mN/m. The spectra in the CH stretching vibration region of the alkyl group side chain indicates that the group in the cis-6Az10-PVA monolayer exists in a more irregular state containing gauche defects compared to those of the trans monolayer, although the content of the gauche defects is appreciably red...