Investigating the Nonradiative Decay Pathway in the Excited State of Silepin Derivatives: A Study with Second-Order Multireference Perturbation Wavefunction Theory

The fluorescence quantum yield for fluorescent organic molecules is an important molecular property, and tuning it up is desired for various applications. For the computational estimation of the fluorescence quantum yield, the theoretical prediction of the nonradiative decay rate constant has become an attractive subject of study. The rate constant of thermally activated nonradiative decay is related to the activation energy in the photoreaction; thus, the accuracy and reliability of the excited-state potential energies in the quantum chemical computation are critical. In this study, we employed a second-order multireference perturbation wavefunction theory for studying the thermally activated decay via conical intersection (CI) of 1,1-dimethyldibenzo[b,f]silepin derivatives. The correlation between the computed activation energy to reach the CI geometry in the S₁ state and the experimentally determined fluorescence quantum yield implied that silepins nonradiatively decay via the CI triggered by the twisting of the central C-C bond. Geometry optimization of the transition state using multireference perturbation theory drastically reduced the estimated activation energy. Our computation gave reasonable predictions of the activation free energies of photoexcited 1,1-dimethyldibenzo[b,f]silepin. The energy profiles and geometry optimizations using proper quantum chemical methods played a critical role in reliable estimation of the rate constant and fluorescence quantum yield.

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 "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) → C[double bond, length as m-dash]C 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.

Exploring radiative and nonradiative decay paths in indole, isoindole, quinoline, and isoquinoline

Radiative and nonradiative decay paths from the first excited singlet electronic state (S₁) in four heteroaromatics, indole, isoindole, quinoline, and isoquinoline, were systematically explored.