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 photo dissociation dynamics of three para-substituted thiophenols (p-YPhSH; Y = CH3, F, and MeO). UV excitation in the wavelength range 305 > lambda(phot) > 240 nm results in S H bond fission and formation of p-YPhS radicals in their ground ((X) over tilde B-2(1)) and first excited ((A) over tilde B-2(2)) electronic states; the (X) over tilde/(A) over tilde state product branching ratio, Gamma, varies with para-Upsilon substituent and excitation wavelength. Excitation at lambda(phot) < 265 nm results in direct population of the dissociative 1(1)pi sigma* potential energy surface (PES). Gamma falls across the series p-CH3PhSH > p-FPhSH > p-MeOPhSH. Branching is ultimately determined at the conical intersection (CI) formed by the 1(1)pi sigma* and ground (S-0) 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 S-0 molecules prior to photoexcitation. Excitation at lambda(phot) > 265 nm populates quasi-bound levels of the respective 1(1)pi pi* states, which predissociate rapidly by tunneling under the lower diabats of the 1(1)pi pi*/1(1)pi sigma* CI at short RS-H. Less extreme (X) over tilde/(A) over tilde product branching ratios are measured, implicating intramolecular vibrational redistribution within the photoexcited 1(1)pi pi* molecules prior to their sampling the region of the 1(1)pi sigma*/S-0 CI.