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., S(ππ*), S( π*), T( π*), T(ππ*), and S. In combination with MS-CASPT2 calculated linearly interpolated internal coordinate paths, the photophysical mechanism of 6SeGua has been proposed. The initially populated S(ππ*) state decays to either S( π*) or T( π*) states through a three-state S/S/T intersection point. The large S/T spin-orbit coupling of 435 cm, according to the classical El-Sayed rule, benefits the S → T intersystem crossing process. The S( π*) state that stems from the S → S internal conversion process at the S/S/T intersection point can further jump to the T( π*) state through the S → T 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 and is expected to occur relatively fast. Finally, the T( π*) state, which is populated from the above S → T and S → T intersystem crossing processes, decays to the T(ππ*) state via an internal conversion process. Because there is merely a small energy barrier of 0.11 eV separating the T(ππ*) minimum and an energetically allowed two-state T/S intersection point, the T(ππ*) state still can decay to the S state quickly, which is also enhanced by a large T/S spin-orbit coupling of 252 cm. Our proposed mechanism explains experimentally observed ultrafast intersystem crossing processes in 6SeGua and its 835-fold acceleration of the T state decay to the S state compared with 6tGua. Finally, we have found that the ground-state electronic structure of 6SeGua has more apparent multireference character.