Knowledge Bank

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$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ above the origin and becomes as short as 9 ps at ~500 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$. 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.