The DNA Nucleobase Thymine in Motion - Intersystem Crossing Simulated with Surface Hopping

Abstract

We report ab initio excited-state dynamics simulations on isolated thymine to investigate the mechanism of intersystem crossing, based on CASSCF potential energy surfaces and the \textsc{Sharc} surface hopping method. We show that even though $S_2\to S_1$ internal conversion is not described accurately with CASSCF, intersystem crossing can be correctly simulated. Intersystem crossing in thymine occurs from the $S_1$ (${}^{1}n{\pi }^{\ast }$) minimum, via a nearby crossing with $T_2$ (${}^3\pi {\pi }^{\ast }$). The system further relaxes via ultrafast internal conversion in the triplet manifold to the $T_1$ (${}^3\pi {\pi }^{\ast }$) state. The simulations reveal that, once the system is trapped in the ${}^{1}n{\pi }^{\ast }$ minimum, intersystem crossing might proceed with a time constant of 1~ps. Furthermore, the change of the system's electronic state is accompanied respectively by elongation/shortening of specific bonds, which could thus be used as indicators to identify which state is populated in the dynamics.

References in corpus (2)

• A general method to describe intersystem crossing dynamics in trajectory surface hopping
• A singlet and triplet excited-state dynamics study of the keto and enol tautomers of cytosine