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

[Image: see text] 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., S(2)((1)ππ*), S(1)((1)nπ*), T(2)((3)nπ*), T(1)((3)ππ*), and S(0). In combination with MS-CASPT2 calculated linearly interpolated internal coordinate paths, the photophysical mechanism of 6SeGua has been proposed. The initially populated S(2)((1)ππ*) state decays to either S(1)((1)nπ*) or T(2)((3)nπ*) states through a three-state S(2)/S(1)/T(2) intersection point. The large S(2)/T(2) spin–orbit coupling of 435 cm(–1), according to the classical El-Sayed rule, benefits the S(2) → T(2) intersystem crossing process. The S(1)((1)nπ*) state that stems from the S(2) → S(1) internal conversion process at the S(2)/S(1)/T(2) intersection point can further jump to the T(2)((3)nπ*) state through the S(1) → T(2) 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 T(2)((3)nπ*) state, which is populated from the above S(2) → T(2) and S(1) → T(2) intersystem crossing processes, decays to the T(1)((3)ππ*) state via an internal conversion process. Because there is merely a small energy barrier of 0.11 eV separating the T(1)((3)ππ*) minimum and an energetically allowed two-state T(1)/S(0) intersection point, the T(1)((3)ππ*) state still can decay to the S(0) state quickly, which is also enhanced by a large T(1)/S(0) 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 T(1) state decay to the S(0) state compared with 6tGua. Finally, we have found that the ground-state electronic structure of 6SeGua has more apparent multireference character.