Spectroscopy and photodissociation dynamics on nonplanar excited state of ortho-substituted thiophenol and thioanisole

Abstract

Structure-dynamics relationship is studied by inducing nonplanar excited state structure through ortho-substitution to the thioanisole and thiophenol. The structure in the first electronically excited state of the ortho-substituted molecules are studied by the resonant two photon ionization (R2PI) and slow electron velocity map imaging (SEVI) spectroscopy. The photodissociation reaction was investigated by velocity map imaging (VMI) technique. The analysis on R2PI and SEVI spectra allow us to understand $S_1$ vibrations and structure in terms of those in $S_0$ or $D_0$ state. It was revealed that $S_1$ state of 2-fluorothioanisole and 2-methoxythiophenol has a nonplanar geometry unlike $S_0$ and $D_0$ states, and thioanisole and thiophenol. In $S-CH_3$ photodissociation reaction of 2-fluorothioanisole, the total translational energy distributions derived from $CH_3$ fragment imaging show a predominant adiabatic dynamics at the two conical intersections, as a result of a nonplanar structure. Also, they indicate two adiabatic channels with different translational energies. The nonplanar structure facilitates the $1^1\pi\pi^\ast/\pi\sigma^\ast$ coupling at the low internal energy. The factors affecting the increase of the high energy channel are discussed. Investigation on 2-methoxythiophenol reveals structural change both upon UV excitation and S-H(D) dissociation. The total translational energy distribution at $S_1$ origin shows an extremely high nonadiabatic transition probability over thiophenol derivatives. This indicates that a structural change back to planar occurs which is supported by calculation. The mechanism of reaction with excitation energy is discussed. Lastly, the photodissociation dynamics is investigated using VMI at 243 nm and high-level electronic structure calculations for four ortho-substituted thiophenols (TPs)

2-fluro, 2-chloro, 2-bromo, and 2-methoxythiophenols. The D images of ortho-substituted TPs show the reduced $\tilde{X}/\tilde{A}$ branching ratios compared to that of TP. The potential energy curves along the SH torsional angle of $1$^1\pi\sigma^\ast (or 2^1\pi\pi^\ast)$ states show that nonplanar excited states of the ortho-substituted thiophenols cause the adiabatic dynamics.