ChemInform Abstract: DIRECT STUDIES OF PROTON TUNNELING IN HYDROGEN BONDED MIXED MOLECULAR CRYSTALS BY OPTICAL EXCITATION

Abstract

Absorption, excitation, and fluorescence spectra have been observed for thioindigo in benzoic acid host crystals at 1.4–4.2 K. The spectra reveal sites which arise from different tautometic forms of neighboring host dimer molecules. These sites exist in thermal equilibrium and interconvert during the excited state lifetime of thioindigo. The site interconversion process arises from a change in the tautomer configuration of a neighboring benzoic acid dimer due to proton tunneling. Fluorescence lifetime measurements for these sites give a direct measure of the proton tunneling rates in the excited state system at liquid helium temperatures k(H)tunneling =(4.45±0.10)×108 s−1. The deuteron tunneling rate is considerably slower in the acid deuterated host crystal k(D)tunneling =(4.9±1.5)×106 s−1. The acid protonated host crystal exhibits reversible hole burning phenomena for all thioindigo sites with recovery of the original line profile on the time scale of 1 min. The homogeneous linewidths obtained from hole...