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)pi pi*) state to the lowest triplet T-1 ((3)pi pi*) 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, to the dark 1n pi* 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 similar to 95 degrees in the T-1 state, whose measured adiabatic energy and lifetime are 16577 cm(-1) and similar to 20 ns, respectively, for p-MMC. These results suggest that the ISC processes play an indispensable role in the photoprotecting sunscreens in natural plants.