Complex Multistate Photophysics of a Rhodanine Photoswitch

Development of new and improved photoswitches for molecular photonics and photo-pharmaceutics is an increasingly important research objective. Recently a promising family of photoswitches based on the rhodanine motif was described. Here, the photophysics of a typical example are investigated by ultrafast UV and IR spectroscopy and quantum chemical calculations. Remarkably, the photophysics are very different to and more complex than those of closely related monomethine photoswitches, which relax by ultrafast internal conversion to the electronic ground state. In the rhodanine photoswitch, the allowed Franck-Condon excited state also relaxes on a sub-picosecond timescale, but the ground state is repopulated only after several hundred picoseconds. Instead, the Franck-Condon state relaxes through (at least) two intermediate states. These states are characterized by transient spectroscopy, and the reaction pathway is modeled by quantum chemical calculations. Comparison of calculated and measured IR data suggests that a triplet mediated isomerization pathway is responsible for the slow excited state dynamics. The triplet state is rapidly populated via coupling of a nearly degenerate nπ* state populated by ultrafast internal conversion from the bright ππ* state. This unexpected isomerization pathway has important implications for the synthesis, analysis, and application of rhodanine photoswitches.