Rhodanine to Oxorhodanine Switch Switches Switching Mechanism in a Monomethine Photoswitch

Understanding the excited-state dynamics of molecular photoswitches is key for advancing their design and optimizing their applications. Here, we characterize the excited-state chemistry of a recently reported oxorhodanine photoswitch through ultrafast spectroscopy and multireference quantum chemical calculations. Both Z and E forms undergo excited-state isomerization reactions on a sub-picosecond time scale to form a hot ground-state and the product isomer. The reaction is shown to proceed entirely within the singlet manifold, in sharp contrast to the rhodanine photoswitches, which react through the triplet state. The difference is ascribed to the nπ* state arising from the C═S bond in the rhodanines. The dominance of ultrafast relaxation in the singlet state is confirmed by multireference ab initio calculations which also show that the reaction coordinate involves torsion and pyramidialization. This reaction coordinate is consistent with the observed viscosity dependence. Calculations also indicate that the observed differences between ultrafast relaxation in the Z and E forms may arise from a shallow minimum on the excited-state of the latter.