Substitution effect on the nonradiative decay and trans → cis photoisomerization route: a guideline to develop efficient cinnamate-based sunscreens

Abstract

Cinnamate derivatives are very useful as UV protectors in nature and as sunscreen reagents in daily life. They convert harmful UV energy to thermal energy through effective nonradiative decay (NRD) including trans → cis photoisomerization. However, the mechanism is not simple because different photoisomeirzation routes have been observed for different substituted cinnamates. Here, we theoretically examined the substitution effects at the phenyl ring of methylcinnamate (MC), a non-substituted cinnamate, on the electronic structure and the NRD route involving trans → cis isomerization based on time-dependent density functional theory. A systematic reaction pathway search using the single-component artificial force-induced reaction method shows that the very efficient photoisomerization route of MC can be essentially described as “¹ππ* (trans) → ¹nπ* → T₁ (³ππ*) → S₀ (trans or cis)”. We found that for efficient ¹ππ* (trans) → ¹nπ* internal conversion (IC), MC should have the substituent at the appropriate position of the phenyl ring to stabilize the highest occupied π orbital. Substitution at the para position of MC slightly lowers the ¹ππ* state energy and photoisomerization occurs via a slightly less efficient “¹ππ* (trans) → ³nπ* → T₁ (³ππ*) → S₀ (trans or cis)” pathway. Substitution at the meta or ortho positions of MC significantly lowers the ¹ππ* state energy so that the energy barrier of IC (¹ππ* → ¹nπ*) becomes very high. This substitution leads to a much longer ¹ππ* state lifetime than that of MC and para-substituted MC, and a change in the dominant photoisomerization route to “¹ππ* (trans) → CC bond twisting on ¹ππ* → S₀ (trans or cis)”. As a whole, the “¹ππ* → ¹nπ*” IC observed in MC is the most important initial step for the rapid change of UV energy to thermal energy. We also found that the stabilization of the π orbital (i) minimizes the energy gap between ¹ππ* and ¹nπ* at the ¹ππ* minimum and (ii) makes the 0–0 level of ¹ππ* higher than ¹nπ* as observed in MC. These MC-like relationships between the ¹ππ* and ¹nπ* energies should be ideal to maximize the “¹ππ* → ¹nπ*” IC rate constant according to Marcus theory.