Selenium substitution effects on excited-state properties and photophysics of uracil: a MS-CASPT2 study.

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., S, S, S, T, and T) are explored. On the basis of these results, their photophysical mechanisms are proposed. Upon photoirradiation to the bright S state, 2SeU quickly relaxes to its S minimum and then moves in an essentially barrierless way to a nearby S/S conical intersection near which the S state is populated. Next, the S system arrives at an S/T/T intersection where a large S/T spin-orbit coupling of 430.8 cm makes the T state populated. In this state, a barrier of 6.8 kcal mol will trap 2SeU for a while. In parallel, for 4SeU or 24SeU, the system first relaxes to the S minimum and then overcomes a small barrier to approach an S/S conical intersection. Once hopping to the S state, there exists an extended region with very close S, T, and T energies. Similarly, a large S/T spin-orbit coupling of 426.8 cm drives the S→ T intersystem crossing process thereby making the T state populated. Similarly, an energy barrier heavily suppresses electronic transition to the S 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.