NONRADIATIVE DECAY DYNAMICS OF METHYL-4-HYDROXYCINNAMATE AND ITS MONOHYDRATED COMPLEX REVEALED BY PICOSECOND PUMP-PROBE SPECTROSCOPY
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Date
2012
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Publisher
Ohio State University
Abstract
The lifetimes of methyl 4-hydroxycinnamate (OMpCA) and its mono-hydrated complex (OMpCA-H$_2$O) in the S$_1$ state have been measured by picosecond pump-probe spectroscopy in a supersonic beam. For OMpCA, the lifetime of the S$_1$ - S$_0$ origin is 8 - 9 ps. On the other hand, the lifetime of OMpCA-H$_2$O complex at the origin is 930 ps, which is 100 times longer than that. Furthermore, in the complex the S$_1$ lifetime shows rapid decrease at an energy of ~200 cm$^{-1}$ above the origin and becomes as short as 9 ps at ~500 cm$^{-1}$. Theoretical calculations with symmetry-adapted cluster-configuration interaction (SAC-CI) method suggest that in OMpCA, the trans - cis isomerization occurs smoothly without a barrier on the S$_1$surface, while in OMpCA-H$_2$O complex, there exists a barrier along the isomerization coordinate. The calculated barrier height of OMpCA-H$_2$O is in good agreement with that estimated from the lifetime measurements.
Description
Author Institution: Department of Chemistry, Graduate School of Science, Hiroshima University, Higashi-Hiroshima 739-8526, Japan; Institute for Molecular Science, 38 Myodaiji, Okazaki 444-8585, Japan