Nonradiative decay dynamics of methyl-4-hydroxycinnamate and its hydrated complex revealed by picosecond pump–probe spectroscopy

Abstract

The lifetimes of methyl 4-hydroxycinnamate (OMpCA) and its mono-hydrated complex (OMpCA–H₂O) in the S₁ state have been measured by picosecond pump–probe spectroscopy in a supersonic beam. For OMpCA, the lifetime of the S₁–S₀ origin is 8–9 ps. On the other hand, the lifetime of the OMpCA–H₂O complex at the origin is 930 ps, which is ∼100 times longer than that of OMpCA. Furthermore, in the complex the S₁ lifetime shows rapid decrease at an energy of ∼200 cm⁻¹ above the origin and finally becomes as short as 9 ps at ∼500 cm⁻¹. Theoretical calculations with a symmetry-adapted cluster-configuration interaction (SAC-CI) method suggest that the observed lifetime behavior of the two species is described by nonradiative decay dynamics involving trans → cis isomerization. That is both OMpCA and OMpCA–H₂O in the S₁ state decay due to the trans → cis isomerization, and the large difference of the lifetimes between them is due to the difference of the isomerization potential energy curve. In OMpCA, the trans → cis isomerization occurs smoothly without a barrier on the S₁ surface, while in the OMpCA–H₂O complex, there exists a barrier along the isomerization coordinate. The calculated barrier height of OMpCA–H₂O is in good agreement with that observed experimentally.