Photodissociation Dynamics of Ortho-Substituted Thiophenols at 243 nm

The photoinduced S–H (D) bond fission dynamics of four ortho-substituted thiophenols, 2-fluoro, 2-chloro, 2-bromo, and 2-methoxythiophenol at a pump wavelength of 243 nm, have been investigated by velocity-map imaging and high-level electronic structure calculations. The D atom images of the deuterated ortho-substituted thiophenols show much reduced X̃/Ã branching ratios of the cofragment radicals over that of bare thiophenol. The angular distributions of the D fragment display negative anisotropies, indicating that transition dipole moments are perpendicular to the fast dissociating S–D bond axis. Initial excitation at 243 nm occurs directly to the ¹πσ* state or to the 2¹ππ* state followed by efficient coupling to the ¹πσ* state. The calculated potential energy curves for the ¹πσ* or 2¹ππ* excited states of the ortho-substituted thiophenols along the CCS–D torsion angle (ϕ) display minima at the nonplanar structures, whereas all of the states for bare thiophenol present minima at the planar geometries. This different topology of the ortho-substituted thiophenols in the excited states induces the wide spread of the reactive flux along the ϕ coordinate on the repulsive surface as it should experience significant torque with respect to ϕ during the fragmentation. This encourages the dissociating molecules to follow the adiabatic path at the conical intersection between the ground and the ¹πσ* states at extended S–D bond lengths, giving rise to decreased X̃/Ã branching ratios, demonstrating that the excited-state molecular structure dictates the nonadiabatic transition probability.