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  • Title: Multistep Intersystem Crossing Pathways in Cinnamate-Based UV-B Sunscreens.
    Author: Yamazaki K, Miyazaki Y, Harabuchi Y, Taketsugu T, Maeda S, Inokuchi Y, Kinoshita SN, Sumida M, Onitsuka Y, Kohguchi H, Ehara M, Ebata T.
    Journal: J Phys Chem Lett; 2016 Oct 06; 7(19):4001-4007. PubMed ID: 27657500.
    Abstract:
    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 S1 (1ππ*) state to the lowest triplet T1 (3ππ*) state via two competing pathways through the T2 state in the time scale of 100 ps: (a) stepwise ISC followed after the internal conversion (IC) from S1 to the dark 1nπ* state; (b) direct ISC from the S1 to T2 states. These picosecond multistep ISCs result in the torsion of C═C double bond by ∼95° in the T1 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.
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