Strikingly Different Photodissociation Dynamics of Thiophenol-d1 and 2-Fluorothiophenol-d1

Abstract

The S-D bond dissociation dynamics of thiophenol-d1 and 2-fluorothiophenol-d1 are investigated by using the velocity map ion imaging technique. At many wavelengths λphot in the range of 285- 235nm, both thiophenol-d1 and 2-fluorothiophenol-d1 are dissociated to give the ground(X) and first excited electronic (A) states of cofragments, C6H5S· and C6H4FS·, respectively. Below the conical intersection(CI-1) between diabatic 1ππ*(S1) and 1nπσ*(S2) states, the excitation energies are selected based on the R2PI spectra which are taken to reveal well-resolved excited (S1) vibronic states only in the case of the fluorine substituted thiophenol. With increasing the excitation energy, the excited cofragments are dominantly formed compared to the other state in the 2-fluorothiophenol-d1 dissociation while it is not for the thiophenol-d1. At the wavelength shorter than 250nm, the X state yield of thiophenol-d1 is larger than of 2-fluorothiophenol-d1. These are relevant to the different extent of molecular planarity at the CI-2 between the diabatic 1ππ(S0) and S2 states. Namely, the role of two CIs is revealed thoroughly comparing the valuable observables resulted from the D atom elimination dynamics. For the thiophenol-d1, the dissociation mainly occurs through the torsional mode mediated tunneling along the S-D bond stretch coordinate providing the same nature of two product channels. Meanwhile, it is found out that the A state C6H4FS· product in 2-fluorothiophenol-d1 dissociation has been originated differently from X state product as well as the one for the thiophenol-d1. Intramolecular hydrogen bonding might be responsible for this, giving other reaction pathways along the hydrogen bond coordinate, which imply that the photodissociation product can be controlled by the hydrogen bond strength. The ab initio calculation results are given to support the experimental findings. Meanwhile, the optical excitation at 255< λphot < 260nm, definitely observed three peaks in both molecules have been assigned to the X and A electronic states of cofragments after initially populated to the S1 or S2 states. Finally, it is the hope that further work in experiment and theory will make us to understand conical intersection and tunneling dynamics besides the hydrogen bonded reaction, more in detail.