PROTON-TRANSFER PROCESSES IN WELL-DEFINED MEDIA - EXPERIMENTAL INVESTIGATION OF PHOTOINDUCED AND THERMAL PROTON-TRANSFER PROCESSES IN SINGLE-CRYSTALS OF 2-(2,4-DINITROBENZYL)PYRIDINE DERIVATIVES

A detailed spectroscopic study of photoinduced and thermally activated proton-transfer processes for a series of different crystals of 2-(2, 4-dinitrobenzyl)pyridine derivatives has been performed. The quantitat ive analysis of ground- and excited-state activation barriers and pree xponential factors in deuterated and nondeuterated crystals shows clea rly that the observed photochromism is linked to a proton-transfer pro cess. Furthermore, it is clearly seen that the supramolecular environm ent of the transferred proton participates in the proton-transfer proc ess. These supramolecular effects control the relative rates and effic iencies of the observed proton-transfer processes in both the ground a nd excited state, yielding, at room temperature, photoproducts having lifetimes ranging between hours and weeks. At least two proton-accepti ng groups may be active in the abstraction of the proton from its rela tively stable benzylic position. Additionally, low-temperature measure ments of proton-transfer processes show that tunneling processes preva il at temperatures below 100 K only in the excited state. No evidence for tunneling could be found for ground-state processes.