Rate-limiting action of a proton crane in long-range intramolecular proton transfer

Abstract

The photo-induced enol–keto tautomerization in the compounds 7-hydroxy,8-(N-morpholinomethyl)quinoline, HMMQ, and 7-hydroxy, 8-(N,N-dimethylaminomethyl)quinoline, HDMQ, involves the long-distance intramolecular transfer of the proton from the hydroxyl group to the endocyclic N atom. This is accomplished by the action of the amine in the mobile substituent attached to the aromatic ring system which is referred to as the proton crane. The excited-state intramolecular proton transfer, ESIPT, proceeds via two sequential adiabatic proton-transfer steps, as revealed by time-resolved fluorescence spectroscopy. The isoviscous and isothermal behaviour of the time dependence of the fluorescence of solutions in a series of n-alkanols and alkanenitriles has been studied on a picosecond time-scale. No deuterium kinetic isotope effect on the ESIPT process has been found. The overall rate constant for the ESIPT process is found to be controlled by the rotational diffusion of the proton crane over an energy barrier, E_a, with a height of 5.6 kJ mol^–1, independent of solvent and viscosity. There is a deuterium isotope effect on the decay of the electronically excited keto tautomer, K*. This suggests that motion along the N—H bond between the proton and endocyclic N atom acts as an efficient dissipative mode in the radiationless decay of K*. The ground-state reverse tautomerization has been studied on a nanosecond time-scale by transient absorption spectroscopy. This process turns out to be controlled by the rotational diffusion of the crane over a barrier with a height of < 16 kJ mol^–1.