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.