Excited-state proton-relay dynamics of 7-hydroxyquinoline controlled by solvent reorganization in room temperature ionic liquids

Abstract

The excited-state triple proton relay of 7-hydroxyquinoline (7HQ) along a hydrogen-bonded methanol chain in room temperature ionic liquids (RTILs) has been investigated using picosecond time-resolved fluorescence spectroscopy. The rate constant of the proton relay in a methanol-added RTIL is found to be slower by an order of magnitude than that in bulk methanol and to have unity in its kinetic isotope effect. These suggest that the excited-state tautomerization dynamics of 7HQ in methanol-added RTILs is mainly controlled by the solvent reorganization dynamics to form a cyclically hydrogen-bonded complex of 7HQ·(CH₃OH)₂ upon absorption of a photon due to high viscosity values of RTILs. Because the cyclic complex of 7HQ·(CH₃OH)₂ at the ground state is unstable in RTILs, the collision-induced slow formation of the cyclic complex should take place upon excitation prior to undergoing subsequent intrinsic proton transfer rapidly.