Experimental and Computational Analysis of Para -Hydroxy Methylcinnamate following Photoexcitation.

Para -hydroxy methylcinnamate is part of the cinnamate family of molecules. Experimental and computational studies have suggested conflicting non-radiative decay routes after photoexcitation to its S 1 (ππ*) state. One non-radiative decay route involves intersystem crossing mediated by an optically dark singlet state, whilst the other involves direct intersystem crossing to a triplet state. Furthermore, irrespective of the decay mechanism, the lifetime of the initially populated S 1 (ππ*) state is yet to be accurately measured. In this study, we use time-resolved ion-yield and photoelectron spectroscopies to precisely determine the S 1 (ππ*) lifetime for the s- cis conformer of para -hydroxy methylcinnamate, combined with time-dependent density functional theory to determine the major non-radiative decay route. We find the S 1 (ππ*) state lifetime of s- cis para -hydroxy methylcinnamate to be ∼2.5 picoseconds, and the major non-radiative decay route to follow the [ 1 ππ*→ 1 nπ*→ 3 ππ*→S 0 ] pathway. These results also concur with previous photodynamical studies on structurally similar molecules, such as para -coumaric acid and methylcinnamate.