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.

Herein, the multi-dimensional nature of the conical intersection seam has been experimentally revealed in the photodissociation reaction of thioanisole-d3 (C6H5SCD3) excited on S1, giving C6H5S·(Ã or X̃]) +·CD3 products. The translational energy distribution of the nascent·CD3 fragment, reflecting the relative yields of the C6H5S·(Ã) and C6H5S·(X̃) products, was measured at each S1 vibronic band using the velocity map ion imaging technique. Direct access of the reactant flux to the conical intersection seam leads to the increase of the nonadiabatic transition probability resulting in sharp resonances in the X̃/ÃC6H5S·product branching ratio at several distinct S1 vibronic bands. The nature of the S1 vibronic bands associated with such dynamic resonances was clarified by the mass-analyzed threshold ionization spectroscopy. The bound state embedded in continuum generated by the conical intersection is observed as a distinct dynamic resonance, revealing the nature of the nuclear motion responsible for the nonadiabatic coupling of two potential energy surfaces at the conical intersection. The multi-dimensional facets of the conical intersection seam in terms of its detailed structure and dynamic role are discussed with the aid of theoretical calculations.

Conical intersection seam comprised of crossing surfaces of two lowest excited states of dimethyl sulfide (DMS) has been directly accessed by the one-photon excitation from the ground equilibrium state. Since the S-C bond rupture takes place promptly, the molecular structure on the excited state effectively belongs to C(S) symmetry. Namely, excited states of 1(1)B1 and 1(1)A2 in C(2)V become 1(1)A'' and 2(1)A'' states in C(S), respectively, and the optical transition from the ground equilibrium state to the dissociating molecule at the conical intersection seam is symmetry-allowed to facilitate the nonadiabatic transition on the 2(1)A'' state, leading eventually to the CH3S + CH3 products. The dynamic study of DMS, in this sense, gives the great opportunity to unravel the vibronic structure of the conical intersection seam by the conventional one-photon excitation method. In this work, utilizing the photofragment excitation (PHOFEX) spectroscopic method, the vibronic structures of DMS and its isotope analogs (CD3SCD3, CH3SCD3) at the conical intersection seam have been revealed, providing accurate lifetimes and detailed dynamics associated with individual vibronic transitions. The lifetime of the excited DMS is estimated to be ~100 fs, indicating that the dissociation is complete within one single oscillation in the conical intersection region. It is also found that the symmetric CSC stretching mode is strongly coupled to the reaction coordinate, as manifested by our experimental finding that the fragmentation yield of the S-CD3 bond is enhanced compared to that of the S-CH3 bond in the CH3SCD3 dissociation reaction only when the CSC symmetric stretching vibrational mode is excited at the conical intersection region. This work demonstrates that the better understanding of the excited state could make the bond-selective chemistry into reality.

Effect of chemical substitution at the para-position of the thiophenoxyl radical has been theoretically investigated in terms of energetics, structures, charge densities and orbital shapes for the ground and first electronically excited states. It is found that the adiabatic energy gap increases when $CH_3$ or F is substituted at the para-position. This change is attributed to the stabilization of the ground state of thiophenoxyl radical through the electron-donating effect of F or $CH_3$ group as the charge or spin of the singly-occupied molecular orbital is delocalized over the entire molecule especially in the ground state whereas in the excited state it is rather localized on sulfur and little affected by chemical substitutions. Quantitative comparison of predictions based on four different quantum-mechanical calculation methods is presented.