Controlling electronic product branching at conical intersections in the UV photolysis of para-substituted thiophenols

H (Rydberg) atom photofragment translation spectroscopy and high-level ab initio electronic structure calculations are used to explore the photodissociation dynamics of three para-substituted thiophenols (p-YPhSH; Y = CH3, F, and MeO). UV excitation in the wavelength range 305 > λphot > 240 nm results in S–H bond fission and formation of p-YPhS radicals in their ground (X̃2B1) and first excited (Ã2B2) electronic states; the X̃/Ã state product branching ratio, Γ, varies with para-Y substituent and excitation wavelength. Excitation at λphot < 265 nm results in direct population of the dissociative 11πσ* potential energy surface (PES). Γ falls across the series p-CH3PhSH > p-FPhSH > p-MeOPhSH. Branching is ultimately determined at the conical intersection (CI) formed by the 11πσ* and ground (S0) PESs at extended RS–H bond length but is sensitively dependent on the orientation of the S–H bond (relative to the ring plane) in the S0 molecules prior to photoexcitation. Excitation at λphot > 265 nm populates quasi-bound levels of the respective 11ππ* states, which predissociate rapidly by tunneling under the lower diabats of the 11ππ*/11πσ* CI at short RS–H. Less extreme X̃/Ã product branching ratios are measured, implicating intramolecular vibrational redistribution within the photoexcited 11ππ* molecules prior to their sampling the region of the 11πσ*/S0 CI.