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

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 (HB) spectroscopy and density functional theory (DFT) calculations revealed the presence of syn and anti isomers, where the methoxy (−OCH 3 ) 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 ππ*)) state occurs at a rate constant of ∼10 10 –10 11 s –1, attributed to internal conversion (IC) to the 1 nπ* state. Nanosecond UV–deep UV (DUV) pump–probe spectroscopy identified a transient triplet (T 1 ( 3 ππ*)) 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 ππ*) → 1 nπ* → T 1 ( 3 ππ*) → S 0 ( cis )".