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–cisisomerization 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 S1–S0transition; UV–UV hole-burning (HB) spectroscopy and density functional theory (DFT) calculations revealed the presence of synand antiisomers, 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 (S1(1ππ*)) state occurs at a rate constant of ∼1010–1011s–1, attributed to internal conversion (IC) to the 1nπ* state. Nanosecond UV–deep UV (DUV) pump–probe spectroscopy identified a transient triplet (T1(3ππ*)) state, whose energy (from S0) and lifetime are 18 400 cm–1and 20 ns, respectively. These results demonstrate that the photoisomerization of 2EH4MC includes multistep internal conversions and intersystem crossings, described as "S1(trans, 1ππ*) → 1nπ* → T1(3ππ*) → S0(cis)".