Theoretical and spectroscopic investigation on ultrafast nonadiabatic photoprotective mechanism of novel ultraviolet protective compounds inspired by natural sunscreens

Abstract In this work, we explored a series of broadband absorption, photostable and environmentally friendly ultraviolet protective compounds (UVPCs) inspired by natural sunscreens in plant, sinapic acid (SA) and sinapoyl malate (SM). Furthermore, their photochemical mechanism including ultrafast nonadiabatic trans-cis photoisomerization reaction at the conical intersection (CI) position of their potential energy surfaces (PES) are demonstrated for the first time. These UVPCs have strong absorption in the UV region with different positions of the maximum absorption peaks. The maximum absorption peaks of some molecules are in the UVA or UVB region, while some molecules show significant photo-absorption in both UVA and UVB regions. Femtosecond transient absorption (FTA) spectroscopic studies demonstrated that all UVPCs can undergo photoisomerization in the excited state from trans to cis isomer after UVA photo-absorption. Moreover, there is an interesting CI position between the first singlet excited state and ground state PES calculated by the time-dependent density functional theory (TDDFT) method via the isomerization coordinate for all UVPCs. As a result, the absorbed UV energy of our UVPCs can be rapidly relaxed through the nonadiabatic photoisomerization reaction pathway from the excited state to the ground state at the CI position. In addition, some of our UVPCs could be photoexcited to the second singlet excited state after UVB photo-absorption, and then relax to the lower singlet excited state via nonradiative internal conversion (IC) progress followed by the photoisomerization reaction. This work presents a new direction towards the design of broadband absorption, photostable and environmentally friendly UVPCs and commercial sunscreens.