Electronic State and Photophysics of 2-Ethylhexyl-4-methoxycinnamate as UV-B Sunscreen under Jet-Cooled Condition

Abstract

The title compound, 2-ethylhexyl-4-methoxycinnamate (2EH4MC), is known as a typical ingredient of sunscreen cosmetics that effectively converts the absorbed UV-B light to thermal energy. This energy conversion process includes the nonradiative decay (NRD): trans-cis isomerization and finally going back to the original structure with a release of thermal energy. In this study, we performed UV spectroscopy for jet-cooled 2EH4MC to investigate the electronic/geometrical structures as well as the NRD mechanism. Laser-induced-fluorescence (LIF) spectroscopy gave the well-resolved vibronic structure of the S-1-S-0 transition; UV-UV hole-burning (HE) spectroscopy and density functional theory (DFT) calculations revealed the presence of syn and anti isomers, where the methoxy (-OCH3) groups orient in opposite directions to each other. Picosecond UV-UV pump-probe spectroscopy revealed the NRD process from the excited singlet (S-1((1)pi pi*)) state occurs at a rate constant of similar to 10(10)-10(11) s(-1), attributed to internal conversion (IC) to the (1)n pi* state. Nanosecond UV-deep UV (DUV) pumpprobe spectroscopy identified a transient triplet (T-1 ((3)pi pi*)) state, whose energy (from S-0) and lifetime are 18 400 cm(-1) and 20 ns, respectively. These results demonstrate that the photoisomerization of 2EH4MC includes multistep internal conversions and intersystem crossings, described as "S-1 (trans, (1)pi pi*) -> (1)n pi* -> T-1 ((3)pi pi* -> S-0 (cis)".