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 trans and cis. On one hand, trans-7HQ cannot undergo proton transfer within its excited-state lifetime because the internal rotation of the hydroxyl group to form cis-7HQ hardly occurs. On the other hand, some cis-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 cis-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.