Accessing Transient Isomers in the Photoreaction of Metastable-State Photoacid

The photoreaction of a metastable-state photoacid (mPAH) generally involves multiple isomers with various connected pathways of photoinduced structural changes during a single reaction cycle. However, only a limited number of isomers have been identified experimentally so far owing to the inherent complexity in combination with the presence of various competing electronic and vibrational processes, as well as the constantly varying interactions between mPAH isomers and solvent molecules. Here, an optical spectroscopic study on a benzimidazole-based mPAH, a novel photoacid using benzimidazole as the structural moiety with the active proton, is reported. Through measurements of linear absorption and steady-state fluorescence in neat solvents and binary mixtures, a pronounced effect of neat water and its binary mixture with glycerol is discovered on the photoreaction of this benzimidazole-mPAH, manifested by the remarkably distinct spectral responses to irradiation from that observed for an organic solution under identical conditions. Measurements of time- and frequency-resolved fluorescence emission further enable to access transient isomers and the associated spectral characteristics from other competing electronic excited-state relaxation processes. Spectral deconvolution analysis and time-dependent density functional theory calculations are applied to separate distinct spectral components and access their potential origin.