Triplet-State Acid-Base Reactions of 1-Methyl-7-oxyquinolinium in Water

Abstract

Upon absorption of a photon, the l-methyl-7-oxyquinolinium zwitterion in neutral water undergoes a protontransfer cycle involving protonation at the lowest excited triplet state and redeprotonation at the ground state. On one hand, the proton transfer of a water molecule to the triplet-state zwitterion takes place on the time scale of 28 us with an activation energy of 12.6 kJ mol-1. On the other hand, the ground-state redeprotonation occurs with a rate constant of (13 us)-1, although its reverse reaction of protonation competes with a rate constant of (37 us)-1.