Ultrafast Intersystem Crossing Dynamics of 6-Selenoguanine in Water

[Image: see text] Rationalizing the photochemistry of nucleobases where an oxygen is replaced by a heavier atom is essential for applications that exploit near-unity triplet quantum yields. Herein, we report on the ultrafast excited-state deactivation mechanism of 6-selenoguanine (6SeGua) in water by combining nonadiabatic trajectory surface-hopping dynamics with an electrostatic embedding quantum mechanics/molecular mechanics (QM/MM) scheme. We find that the predominant relaxation mechanism after irradiation starts on the bright singlet S(2) state that converts internally to the dark S(1) state, from which the population is transferred to the triplet T(2) state via intersystem crossing and finally to the lowest T(1) state. This S(2) → S(1) → T(2) → T(1) deactivation pathway is similar to that observed for the lighter 6-thioguanine (6tGua) analogue, but counterintuitively, the T(1) lifetime of the heavier 6SeGua is shorter than that of 6tGua. This fact is explained by the smaller activation barrier to reach the T(1)/S(0) crossing point and the larger spin–orbit couplings of 6SeGua compared to 6tGua. From the dynamical simulations, we also calculate transient absorption spectra (TAS), which provide two time constants (τ(1) = 131 fs and τ(2) = 191 fs) that are in excellent agreement with the experimentally reported value (τ(exp) = 130 ± 50 fs) (Farrel et al. J. Am. Chem. Soc.2018, 140, 11214). Intersystem crossing itself is calculated to occur with a time scale of 452 ± 38 fs, highlighting that the TAS is the result of a complex average of signals coming from different nonradiative processes and not intersystem crossing alone.