Triplet-state acid-base reactions of 1-methyl-7-oxyquinolinium in water

Upon absorption of a photon, the 1-methyl-7-oxyquinolinium zwitterion in neutral water undergoes a proton-transfer 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 micros 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 micros)(-1), although its reverse reaction of protonation competes with a rate constant of (37 micros)(-1).