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

TL;DR

This study uses ab initio simulations with surface hopping to explore thymine's excited-state dynamics, revealing the intersystem crossing mechanism and associated bond changes, despite some limitations of the CASSCF method.

Contribution

First simulation of thymine's intersystem crossing mechanism using surface hopping with CASSCF, highlighting the process from singlet to triplet states and bond indicators.

Findings

Intersystem crossing occurs from the S1 (nπ*) minimum via T2 (ππ*) crossing.

The system relaxes to T1 (ππ*) state through ultrafast internal conversion.

Intersystem crossing may happen within approximately 1 ps.

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 $S_2 \rightarrow S_1$ internal conversion is not described accurately with CASSCF, intersystem crossing can be correctly simulated. Intersystem crossing in thymine occurs from the $S_1$ ($^1n\pi^*$) minimum, via a nearby crossing with $T_2$ ($^3\pi\pi^*$). The system further relaxes via ultrafast internal conversion in the triplet manifold to the $T_1$ ($^3\pi\pi^*$) state. The simulations reveal that, once the system is trapped in the $^1n\pi^*$ 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.