Direct studies of proton tunneling in hydrogen bonded mixed molecular crystals by optical excitation

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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 burning measurements are not determined by the fluorescence lifetime of thioindigo. The additional contribution to the linewidth can be attributed to dephasing arising from site interconversion processes in both the ground and excited state system. A comparison of hole burning and fluorescence lifetime measurements provides a value for the proton tunneling rate in the ground state interconversion (equilibrium) process, k(H)tunneling =(3.5±1.3)×108 s−1. These results show that the hole burning phenomenon does not arise directly from these tunneling processes occurring in neighboring host dimers.

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