Curious Case of 2-Selenouracil: Efficient Population of Triplet States and Yet Photostable

Excited-state MS-CASPT2 and ADC(2) quantum chemical calculations and nonadiabatic dynamics simulations show that 2-selenouracil is able to both efficiently populate and depopulate reactive triplet states in an ultrashort time scale. Thus, the heavier homologue of 2-thiouracil unites the ultrafast, high-yield intersystem crossing of 2-thiouracil with the short excited-state lifetime and photostability of the parent nucleobase uracil-two properties that have been traditionally thought to be diametrically opposed. Remarkably, while the S₂ → S₁ → T₂ → T₁ deactivation dynamics of 2-selenouracil is analogous to that of 2-thiouracil, the calculations show that the triplet lifetime of 2-selenouracil should decrease by up to 3 orders of magnitude in comparison to that 2-thiouracil, possibly down to the few-picosecond time scale. The main reasons for this decrease are the lack of a second T₁ minimum, the enhanced spin-orbit coupling, and the reduction of the energy barrier to access the T₁/ S₀ crossing-in particular in aqueous solution-compared to 2-thiouracil. Such unusual photophysical properties, together with its significant red-shifted absorption spectrum, could make 2-selenouracil a useful specialized photosensitizer for photodynamical therapy.