Ultrafast Intersystem Crossing Dynamics of 6-Selenoguanine in Water.

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.