Photoisomerization of hemithioindigo compounds: Combining solvent- and substituent- effects into an advanced reaction model

Time resolved absorption and emission experiments are combined with quantum chemical calculations to obtain a quantitative understanding of the light-induced Z to E isomerization of the hemithioindigo photoswitch. Substitution and solvent polarity change the Z to E reaction time by three orders of magnitude from 9 ps for the para-methoxy substituted 5-methyl hemithioindigo in the unpolar cyclohexane to 9 ns for the para-cyano substituted molecule in di-chloromethane. A comparison with quantum chemical calculations reveals the role of the solvent polarity on the reaction speed for distinct substitution patterns of the stilbene moiety. The dipole moments of the different hemithioindigo photoswitches strongly vary on the excited state potential energy surface. Energetic stabilization of the minimum and simultaneous destabilization in the transition region increase the effective reaction barrier for polar solvents, thus strongly decelerating the reaction.