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

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(¹ nπ*), T₂(³ nπ*), 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(¹ nπ*) or T₂(³ nπ*) 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(¹ nπ*) state that stems from the S₂ → S₁ internal conversion process at the S₂/S₁/T₂ intersection point can further jump to the T₂(³ nπ*) 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(³ nπ*) 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.