MS-CASPT2 Studies on the Photophysics of Selenium-Substituted Guanine Nucleobase

Abstract The MS-CASPT2 method has been employed to optimize minimum-energy structures of 6-selenoguanine (6SeGua) and related two- and three-state intersection structures in and between the lowest five electronic states, i.e., S2(1ππ*), S1(1nπ*), T2(3nπ*), T1(3ππ*), and S0. In combination with MS-CASPT2 calculated linearly interpolated internal coordinate paths, the photophysical mechanism of 6SeGua has been proposed. The initially populated S2(1ππ*) state decays to either S1(1nπ*) or T2(3nπ*) states through a three-state S2/S1/T2 intersection point. The large S2/T2 spin–orbit coupling of 435 cm–1, according to the classical El-Sayed rule, benefits the S2 → T2 intersystem crossing process. The S1(1nπ*) state that stems from the S2 → S1 internal conversion process at the S2/S1/T2 intersection point can further jump to the T2(3nπ*) state through the S1 → T2 intersystem crossing process. This process does not comply with the El-Sayed rule, but it is still related to a comparatively large spin–orbit coupling of 39 cm–1 and is expected to occur relatively fast. Finally, the T2(3nπ*) state, which is populated from the above S2 → T2 and S1 → T2 intersystem crossing processes, decays to the T1(3ππ*) state via an internal conversion process. Because there is merely a small energy barrier of 0.11 eV separating the T1(3ππ*) minimum and an energetically allowed two-state T1/S0 intersection point, the T1(3ππ*) state still can decay to the S0 state quickly, which is also enhanced by a large T1/S0 spin–orbit coupling of 252 cm–1. Our proposed mechanism explains experimentally observed ultrafast intersystem crossing processes in 6SeGua and its 835-fold acceleration of the T1 state decay to the S0 state compared with 6tGua. Finally, we have found that the ground-state electronic structure of 6SeGua has more apparent multireference character.