Direct Observation of Conformation-Dependent Pathways in the Excited-State Proton Transfer of 7‑Hydroxyquinoline in Bulk Alcohols

The excited-state proton transfer (ESPT) of 7-hydroxyquinoline (7HQ) in bulk alcoholic solvents has been explored with variation of protic hydrogen atoms as well as alcohols. By measuring time-resolved kinetic profiles at two different excitation wavelengths, we have observed conformation-specific pathways in the ESPT of 7HQ. There are two possible rotamers of 7HQ according to the configuration of its hydroxyl group, which are <i>trans</i> and <i>cis</i>. On one hand, <i>trans</i>-7HQ cannot undergo proton transfer within its excited-state lifetime because the internal rotation of the hydroxyl group to form <i>cis</i>-7HQ hardly occurs. On the other hand, some <i>cis</i>-7HQ molecules exist as cyclic complexes of 7HQ·(alcohol)₂ at the moment of excitation, and the cyclic complexes can undergo ESPT rapidly via tunneling. However, the other <i>cis</i>-7HQ molecules should undergo solvent reorganization to form cyclic 7HQ·(alcohol)₂ complexes prior to intrinsic ESPT, and the solvent reorganization becomes the rate-determining step. In contrast to proton transfer, where intrinsic ESPT and solvent reorganization have been observed separately in time-resolved kinetic profiles, intrinsic deuteron transfer is too slow to be distinguished kinetically from solvent reorganization