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

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 "(1)pi pi* (trans) -> (1)n pi* -> T-1 ((3)pi pi*) -> S-0 (trans or cis)". We found that for efficient (1)pi pi* (trans) -> (1)n pi* internal conversion (IC), MC should have the substituent at the appropriate position of the phenyl ring to stabilize the highest occupied pi orbital. Substitution at the para position of MC slightly lowers the (1)pi pi* state energy and photoisomerization occurs via a slightly less efficient "(1)pi pi* (trans) -> (3)n pi* -> T-1 ((3)pi pi*) -> S-0 (trans or cis)" pathway. Substitution at the meta or ortho positions of MC significantly lowers the (1)pi pi* state energy so that the energy barrier of IC ((1)pi pi* -> (1)n pi*) becomes very high. This substitution leads to a much longer (1)pi pi* state lifetime than that of MC and para-substituted MC, and a change in the dominant photoisomerization route to "(1)pi pi* (trans) -> C = C bond twisting on (1)pi pi* -> S-0 (trans or cis)". As a whole, the "(1)pi pi* -> (1)n pi*" 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 pi orbital (i) minimizes the energy gap between (1)pi pi* and (1)n pi* at the (1)pi pi* minimum and (ii) makes the 0-0 level of (1)pi pi* higher than (1)n pi* as observed in MC. These MC-like relationships between the (1)pi pi* and (1)n pi* energies should be ideal to maximize the "(1)pi pi* -> (1)n pi*" IC rate constant according to Marcus theory.