The phenomena of excited-state intramolecular proton transfer (phototautomerization) are distinguished in molecular mechanism according to four classes. Intrinsic intramolecular proton transfer (e.g. 3-hydroxyflavone) involves ultra-rapid proton transfer across an internal H-bond. Concerted biprotonic transfer (e.g. 7-azaindole dimers and solvates, benzanilide dimers) involves cooperative double proton transfer in a cyclical complex. Static and dynamic catalysis of proton transfer (e.g. lumichrome, adenine, guanine) involves strong catalysis in doubly H-bonded acetic acid complexes, or a proton transport by H-bonded pyridine. Proton-relay transfer (e.g. 7-hydroxyquinoline, 3-hydroxyxanthone) is suggested as involving multiproton-bridged solvates, which may serve as experimental models for proton relays and proton pumps in biological systems.

Potential functions for proton-transfer spectroscopy are discussed and the perturbations of these potentials by specific solvation and by spectroscopic solvent-cage effects are discussed on the basis of a Born–Oppenheimer spectroscopic solvent-cage model.