Gas‐Solution Phase Transient Absorption Study of the Plant Sunscreen Derivative Methyl Sinapate

The plant sunscreen derivative methyl sinapate is studied in the gas‐ and solution‐phase as an important step in understanding the structure‐dynamics‐function relationship of plant sunscreens such as sinapoyl malate. Results from gas‐phase experiments suggest a three step deactivation process after photoexcitation. Photoexcitation to the S1 state results in internal vibrational energy redistribution within 3 ps. Intersystem crossing then occurs in 30 ps between S1 and T1 and the latter state then persists outside our probe window ( 1 ns). In solution with the weakly perturbing solvent cyclohexane, a dramatic change in the photophysical properties is observed. A trans‐cis isomerisation is observed to occur in 3 ps via a 1 /S0 conical intersection. The population which does not complete the isomerisation, relaxes via vibrational energy transfer to the solvent in 9 ps. A long‐lived photoproduct is then observed to persist and is identified as the cis‐isomer.