“…The πσ*-mediated photochemistry of the heteroaromatic molecular system − has long been spotlighted not only because it gives the mechanistic clue for the ultrafast excited state relaxation of biological building blocks carrying the genetic code but also because it is extremely helpful to elucidate the nonadiabatic transition dynamics in the vicinity of the conical intersections. − The Born–Oppenheimer approximation breaks down when the conical intersections are encountered along the passage of the reactive flux, giving the nonstraightforward dynamic outputs of reaction rates, product yields, or branching ratios. − From the dynamic point of view, the πσ*-mediated chemistry of the heteroaromatic system has provided the ideal model for investigating the predissociation dynamics (Herzberg type-I or -II) − and/or tunneling dynamics. In the predissociation event (e.g., the S–CH 3 bond dissociation of the S 1 thioanisole), − the reactive flux placed in the proximity of the conical intersection either nonadiabatically funnels through the narrowly defined conical intersection or sticks to the adiabatic potential energy surfaces to explore the phase space for riding on the minimum energy reaction path. For the tunneling case, the light atom (H or D) escapes from the S 1 potential well via tunneling through the barrier which is dynamically shaped by the upper-lying conical intersection. − Notably, the tunneling dynamics of the S 1 state has recently been thoroughly investigated in a state-specific way for a number of heteroaromatic molecular systems such as phenol, ,− substituted phenols, − o -cresol, , thiophenol, , 2-fluorothiophenol, ,, 2-methoxythiophenol, − or 2-chlorothiophenol. ,…”