Structural Control of Photoinduced Dynamics in 4H-Imidazole-Ruthenium Dyes

The photoinduced dynamics of a series of terpyridine 4H-imidazole-ruthenium complexes, which constitute a new family of panchromatic dyes, is investigated. The dynamics involves two excited states localized within the 4H-imidazole sphere. Upon MLCT excitation, an excited state is populated, which is localized on the central part of the 4H-imidazole ligand caused by its nonplanar conformation. The population of the second excited state is connected to a planarization of the 4H-imidazole ligand as revealed by viscosity-dependent measurements, and the excess electronic charge on the ligand is shifted to the terminal rings. The impact on the photoinduced dynamics of the electronic character of the substituent at the terminal rings and the protonation state of the 4H-imidazole ligand is studied. Significant changes in the lifetime of the excitation and alterations of the decay mechanism are observed depending on the interplay of the electronic character of the substituent and ligand protonation. In a NMe2 substituted complex, the character of the substituent is changed from electron donating to electron withdrawing upon stepwise protonation, resulting in pH switchable decay mechanism.