Surface hopping dynamics reveal ultrafast triplet generation promoted by S1–T2–T1 spin-vibronic coupling in 2-mercaptobenzothiazole†
Abstract
We investigate the T1 formation upon populating the optically “bright” S2 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 S2–S1 internal conversion (IC) and S1–T2 intersystem crossing (ISC) are in excellent agreement with the experiment. The observed ultrafast ISC is analyzed based on the S1–T2–T1 spin-vibronic coupling mechanism. In contrast to the previous assignment of 6 ps to the T2–T1 IC, our findings enable us to attribute this decay constant to the combined events of T2–T1 IC followed by relaxation of vibrationally hot T1.