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 S 2 state of Z, the internal conversion to the S 1 state would be dominant. From the S 1 state, two intersystem crossing paths leading to the T 2 and T 1 states and one internal conversion path to the S 0 state are possible. However, considering the large barrier to access the S 1 /S 0 conical intersection and the strong spin‐orbit coupling between S 1 and T 2 states (>40 cm −1 ), the intersystem crossing to the triplet manifolds is predicted to be more preferred. Arriving at the T 2 state, the internal conversion to the T 1 state and the intersystem crossing back to the S 1 state are both possible considering the S 1 /T 2 /T 1 three‐state intersection near the T 2 minimum. Upon arrival at the T 1 state, the deactivation to S 0 can be efficient after overcoming a small barrier to access T 1 /S 0 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.