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

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(ππ*) state is energetically lower than the V'(ππ*) 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 S state. In the first one, when the V(ππ*) state is populated in the FC region, the system diabatically evolves along the V(ππ*) state into its minimum where the internal conversion to S occurs. In the second one, the V'(ππ*) state is populated in the FC region and the system adiabatically overcomes a barrier of ca. 3.0 kcal/mol to approach the V(ππ*) minimum eventually leading to a V(ππ*)-to-S internal conversion. In the third one, the V'(ππ*) state first hops via the intersystem crossing to the T state, which then decays through the internal conversion to the T state. The T state is finally converted to the S state via the T/S crossing point. Our present work contributes to understanding the photophysics of OCR and its variants.