Influence of Light Polarization on Photoswitching of Fulgimide Monolayers on Surfaces

The photoisomerization characteristic parameters [photostationary states (PSSs), cross section sigma, and quantum yield eta] of indolyl fulgimide compounds dispersed in solution are quantitatively compared to those of the same compounds covalently immobilized as a monolayer on functionalized Si(111) surfaces (see Adv. Mater. 2013, 25, 416). The photoisomerization kinetics of fulgimide compounds in solution is monitored using UV-vis spectroscopy and that of monolayers is monitored by in situ calibrated Fourier transform infrared spectroscopy. Although the isomeric composition at PSSs is very similar in solution and at surfaces, a strong influence of the incident light polarization is found on photoisomerization cross section sigma (sigma(s) << sigma(p), < sigma(sol)). Accounting for the local excitation electromagnetic field indicates that the fulgimide groups adopt a preferential tilt angle with respect to the surface normal and their transition dipole is almost parallel to the surface plane for either C or E/Z isomers. Density functional theory indicates that this orientation corresponds to photochromic groups that are lying in contact with the underlying monolayer. This interaction plausibly explains why the quantum yield of photoisomerization is found nearly twice lower at the surface than in the solution.