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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  n π*), T 2 ( 3  n π*), 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  n π*) or T 2 ( 3  n π*) 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  n π*) 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  n π*) 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  n π*) 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.

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