Molecular Mechanism of the Z/E-Photoisomerization of Hemithioindigo Hemistilbene

摘要

Hemithioindigo hemistilbene (HTI) can be selectively photoisomerized from the Z-isomer into the E-isomer andvice versa. Using quantum chemical calculations, we have investigated the mechanism of the photoisomerizationin detail. Our calculations demonstrate that two Z- and E-isomers exist in the electronic ground state as well as onthe S, surface. The S, isomers are separated by small energy barriers along the dihedral twisting coordinate, butalso a conical intersection with the electronic ground state is present at about 90° twisting angle. Comparison withpreviously published experimental data reveals that most excited molecules, however, do not isomerize but insteadrelax to the equilibrium structure of the Z-isomer on the S, surface and return back to the ground state via regularfluorescence. Only a small fraction of the excited state population decays via the identified conical intersection andforms ground state E-isomers. This explains the comparably long lifetime of 38 ps of the excited HTI moleculesand the observed low quantum yield of photoswitching.