Influence of Light Polarization on Photoswitching of Fulgimide Monolayers on Surfaces

The photo isomerization characteristic parameters (photo-stationary states (PSS), cross section quantum yield) of indolyl fulgimide compounds dispersed in solution are quantitatively compared to those of the same compounds covalently immobilized as a monolayer on a functionalized Si(111) surfaces (see Advanced Materials 25, 416 (2013)). The photo isomerization kinetics of fulgimide compounds in solution is monitored using UV-Vis spectroscopy and that of monolayers by in situ calibrated FTIR spectroscopy. While the isomeric composition at photo stationary states are very similar in solution and at surfaces, a strong influence of the incident light polarization is found on photo isomerization cross section  (). Accounting for the local excitation electromagnetic field indicates that the fulgimide groups adopt a preferential tilt angle with respect to the surface normal and that their transition dipole is almost parallel to the surface plane for either C or E/Z isomers. DFT 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.