QM/MM Studies on the Photophysical Mechanism of a Truncated Octocrylene Model.

Abstract

Methyl 2-cyano-3,3-diphenylacrylate (MCDPA) shares the same molecular skeleton with octocrylene (OCR) that is one of the most common molecules used in commercially available sunscreens. However, its excited-state relaxation mechanism is unclear. Herein, we have used the QM(CASPT2//CASSCF)/MM method to explore spectroscopic properties, geometric and electronic structures, relevant conical intersections and crossing points, and excited-state relaxation paths of MCDPA in methanol solution. We found that in the Franck-Condon (FC) region, the V(1ππ*) state is energetically lower than the V'(1ππ*) state only by 2.8 kcal/mol and is assigned to experimentally observed maximum absorption band. From these two initially populated singlet states, there exist three nonradiative relaxation paths to repopulate the S0 state. In the first one, when the V(1ππ*) state is populated in the FC region, the system diabatically evolves along the V(1ππ*) state into its minimum where the internal conversion to S0 occurs. In the second one, the V'(1ππ*) state is populated in the FC region and the system adiabatically overcomes a barrier of ca. 3.0 kcal/mol to approach the V(1ππ*) minimum eventually leading to a V(1ππ*)-to-S0 internal conversion. In the third one, the V'(1ππ*) state first hops via the intersystem crossing to the T2 state, which then decays through the internal conversion to the T1 state. The T1 state is finally converted to the S0 state via the T1/S0 crossing point. Our present work contributes to understanding the photophysics of OCR and its variants.