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 Sstate,2SeUquickly relaxes to its Sminimum and then moves in an essentially barrierless way to a nearby S/Sconical intersection near which the Sstate is populated. Next, the Ssystem arrives at an S/T/Tintersection where a large S/Tspin-orbit coupling of 430.8 cmmakes the Tstate populated. In this state, a barrier of 6.8 kcal molwill trap2SeUfor a while. In parallel, for4SeUor24SeU, the system first relaxes to the Sminimum and then overcomes a small barrier to approach an S/Sconical intersection. Once hopping to the Sstate, there exists an extended region with very close S, T, and Tenergies. Similarly, a large S/Tspin-orbit coupling of 426.8 cmdrives the S→ Tintersystem crossing process thereby making the Tstate populated. Similarly, an energy barrier heavily suppresses electronic transition to the Sstate. 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.