Theoretical insights into the photophysics of an unnatural base Z: A MS‐CASPT2 investigation

Abstract

We have employed the highly accurate multistate complete active space second‐order perturbation theory (MS‐CASPT2) method to investigate the photoinduced excited state relaxation properties of one unnatural base, namely Z. Upon excitation to the S2 state of Z, the internal conversion to the S1 state would be dominant. From the S1 state, two intersystem crossing paths leading to the T2 and T1 states and one internal conversion path to the S0 state are possible. However, considering the large barrier to access the S1/S0 conical intersection and the strong spin‐orbit coupling between S1 and T2 states (>40 cm−1), the intersystem crossing to the triplet manifolds is predicted to be more preferred. Arriving at the T2 state, the internal conversion to the T1 state and the intersystem crossing back to the S1 state are both possible considering the S1/T2/T1 three‐state intersection near the T2 minimum. Upon arrival at the T1 state, the deactivation to S0 can be efficient after overcoming a small barrier to access T1/S0 crossing point, where the spin‐orbit coupling (SOC) is as large as 39.7 cm−1. Our present work not only provides in‐depth insights into the photoinduced process of unnatural base Z, but can also help the future design of novel unnatural bases with better photostability.