Structure and dynamic role of conical intersections in the πσ*-mediated photodissociation reactions

Conical intersection as a dynamic funnel in nonadiabatic transition dictates many important chemical reaction outputs such as reaction rates, yields, and energy disposals especially for chemical reactions taking place on electronically excited states. Therefore, the energetics and topology of conical intersections have been subjected to intensive theoretical and experimental studies for decades as these things are the keys to understanding and controlling nonadiabatic transitions which are ubiquitous in nature. In this article, we focus on πσ*-mediated photodissociation reactions of thiophenols and thioanisoles. Interestingly, for these chemical systems, the nonadiabatic transition probability can be precisely measured as a function of the excitation energy, giving a great opportunity for spectroscopic characterization of the multi-dimensional conical intersection seam that governs the nonadiabatic transition dynamics of polyatomic molecules. The passage of the reactive flux in the proximity of the conical intersection gives rise to dynamic resonances corresponding to dramatic state-specific increases of the nonadiabatic transition probability. Accordingly, it is found that the electronic and nuclear configurations of the reactive flux and their evolution, coupled to the conical intersection seam, are critical in nonadiabatic transition dynamics. Nonadiabaticity is found to be extremely sensitive to the conformational molecular structure, and this has been demonstrated in the photodissociation dynamics of the chemical derivatives of thiophenol. Intramolecular vibrational redistribution, which is nontrivial in surmounting the reaction barrier, is found to wash out state-specific dynamic resonances, implying the importance of the dynamic interplay between vibrational energy flow and nonadiabatic transition. The experimental results on conical intersection dynamics presented in this review provide many interesting and important issues to be pursued in the near future by both theoreticians and experimentalists.