Multistep Intersystem Crossing Pathways in Cinnamate-Based UV-B Sunscreens

The nonradiative decay pathways of jet-cooled para -methoxy methylcinnamate ( p -MMC) and para -methoxy ethylcinnamate ( p -MEC) have been investigated by picosecond pump–probe and nanosecond UV-Deep UV pump–probe spectroscopy. The possible relaxation pathways were calculated by the (time-dependent) density functional theory. We found that p -MMC and p -MEC at low excess energy undergo multistep intersystem crossing (ISC) from the bright S 1 ( 1 ππ *) state to the lowest triplet T 1 ( 3 ππ *) state via two competing pathways through the T 2 state in the time scale of 100 ps: (a) stepwise ISC followed after the internal conversion (IC) from S 1 to the dark 1 nπ* state; (b) direct ISC from the S 1 to T 2 states. These picosecond multistep ISCs result in the torsion of C═C double bond by ∼95° in the T 1 state, whose measured adiabatic energy and lifetime are 16577 cm –1 and ∼20 ns, respectively, for p -MMC. These results suggest that the ISC processes play an indispensable role in the photoprotecting sunscreens in natural plants.