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 S-1 state results in internal vibrational energy redistribution within 3ps. Intersystem crossing then occurs in 30ps between S-1 and T-1 and the latter state then persists outside our probe window (>> 1ns). 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 3ps via a 11* /S-0 conical intersection. The population which does not complete the isomerisation, relaxes via vibrational energy transfer to the solvent in 9ps. A long-lived photoproduct is then observed to persist and is identified as the cis-isomer.