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

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.