| Summary: | <i>Para</i>-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<sub>1</sub>(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<sub>1</sub>(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<sub>1</sub>(ππ*) lifetime for the s-<i>cis</i> conformer of <i>para</i>-hydroxy methylcinnamate, combined with time-dependent density functional theory to determine the major non-radiative decay route. We find the S<sub>1</sub>(ππ*) state lifetime of s-<i>cis para</i>-hydroxy methylcinnamate to be ∼2.5 picoseconds, and the major non-radiative decay route to follow the [<sup>1</sup>ππ*→<sup>1</sup>nπ*→<sup>3</sup>ππ*→S<sub>0</sub>] pathway. These results also concur with previous photodynamical studies on structurally similar molecules, such as <i>para</i>-coumaric acid and methylcinnamate.
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