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(pi pi*) 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(pi pi*) 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(pi pi*) 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(pi pi*) state lifetime of s-cis para-hydroxy methylcinnamate to be similar to 2.5 picoseconds, and the major non-radiative decay route to follow the [(1)pi pi*->(1)n pi*->(3)pi pi*-> S-0] pathway. These results also concur with previous photodynamical studies on structurally similar molecules, such as para-coumaric acid and methylcinnamate.