Surface hopping dynamics reveal ultrafast triplet generation promoted by S1-T2-T1 spin-vibronic coupling in 2-mercaptobenzothiazole

We investigate the T-1 formation upon populating the optically "bright" S-2 in 2-mercaptobenzothiazole to interpret the underlying relaxation pathways associated with the experimental decay constants reported by D. Koyama and A. J. Orr-Ewing, Phys. Chem. Chem. Phys., 2016, 18, 26224-26235. Energetics, electronic populations and geometries of various stationary points of low-lying electronic states are computed using the semi-classical ab initio surface hopping dynamics simulations. Estimated decay constants of S-2-S-1 internal conversion (IC) and S-1-T-2 intersystem crossing (ISC) are in excellent agreement with the experiment. The observed ultrafast ISC is analyzed based on the S-1-T-2-T-1 spin-vibronic coupling mechanism. In contrast to the previous assignment of 6 ps to the T-2-T-1 IC, our findings enable us to attribute this decay constant to the combined events of T-2-T-1 IC followed by relaxation of vibrationally hot T-1.