The photophysics of selenium-substituted nucleobases has attracted recent experimental attention because they could serve as potential photosensitizers in photodynamic therapy. Herein, we present a comprehensive MS-CASPT2 study on the spectroscopic and excited-state properties, and photophysics of 2-selenouracil (2SeU), 4-selenouracil (4SeU), and 2,4-selenouracil (24SeU). Relevant minima, conical intersections, crossing points, and excited-state relaxation paths in the lowest five electronic states (i.e., S₀, S₁, S₂, T₂, and T₁) are explored. On the basis of these results, their photophysical mechanisms are proposed. Upon photoirradiation to the bright S₂ state, 2SeU quickly relaxes to its S₂ minimum and then moves in an essentially barrierless way to a nearby S₂/S₁ conical intersection near which the S₁ state is populated. Next, the S₁ system arrives at an S₁/T₂/T₁ intersection where a large S₁/T₁ spin-orbit coupling of 430.8 cm^-1 makes the T₁ state populated. In this state, a barrier of 6.8 kcal mol^-1 will trap 2SeU for a while. In parallel, for 4SeU or 24SeU, the system first relaxes to the S₂ minimum and then overcomes a small barrier to approach an S₂/S₁ conical intersection. Once hopping to the S₁ state, there exists an extended region with very close S₁, T₂, and T₁ energies. Similarly, a large S₁/T₁ spin-orbit coupling of 426.8 cm^-1 drives the S₁→ T₁ intersystem crossing process thereby making the T₁ state populated. Similarly, an energy barrier heavily suppresses electronic transition to the S₀ state. The present work manifests that different selenium substitutions on uracil can lead to a certain extent of different vertical and adiabatic excitation energies, excited-state properties, and relaxation pathways. These insights could help understand the photophysics of selenium-substituted nucleobases.

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