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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 t h e hydroxyl group to the endocyclic N atom. This is accomplished by t h e action of t h e 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 ESlPT process has been found. The overall rate constant for t h e ESIPT process is found to be controlled by the rotational diffusion of the proton crane over an energy barrier, E, , with a height of 5.6 kJ mol-', independent of solvent and viscosity. There is a deuterium isotope effect on t h e 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 a s an efficient dissipative mode in the radiationless decay of K*. The ground-state reverse tautomerization has been studied on a nanosecond timescale 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

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