Complex Multistate Photophysics of a Rhodanine Photoswitch

The development of new and improved photoswitches for photonic materials and photo-pharmaceutics is an important research objective. Recently a new family of photoswitches based on the rhodanine motif was described. In this work the photophysics of a representative example of that family are investigated. The photoswitch’s photophysics are shown to be remarkably different to, and more complex than, those observed for closely related photoactive monomethines dyes, which typically relax in picoseconds by internal conversion. In the rhodanine photoswitch the allowed Franck-Condon excited state relaxes on a sub-picosecond timescale to populate a forbidden (dark) state which rapidly relaxes to a new optical region of the excited state surface. This relaxed dark excited state then relaxes further through at least two intermediate states before finally recovering the ground state in about one nanosecond. The dark states are characterised by transient infra-red spectroscopy and the entire reaction pathway modelled by quantum chemical calculations. Comparison of calculated and measured IR data suggest a triplet mediated isomerization pathway is responsible for the slow excited state dynamics. The triplet state is populated via coupling of nearly degenerate n* and * states in the dark state. This unexpected isomerization pathway has implications for the synthesis, analysis and future applications of rhodanine photoswitches.