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 S2 state that converts internally to the dark S1 state, from which the population is transferred to the triplet T2 state via intersystem crossing and finally to the lowest T1 state. This S2 → S1 → T2 → T1 deactivation pathway is similar to that observed for the lighter 6-thioguanine (6tGua) analogue, but counterintuitively, the T1 lifetime of the heavier 6SeGua is shorter than that of 6tGua. This fact is explained by the smaller activation barrier to reach the T1/S0 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 (τ1 = 131 fs and τ2 = 191 fs) that are in excellent agreement with the experimentally reported value (τexp = 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.


Section S1. Further Stationary Calculations
: Bond lengths (Å) for the ground state of 6SeGua in gas phase computed at the MP2 level using the cc-pVDZ and ANO-RCC-VDZP basis sets. The Douglas-Kroll-Hess formalism is included to account for scalar-relativistic effects when the ANO-RCC-VDZP basis set is considered.
These calculations were performed at the MP2/ANO-RCC-VDZP ground state optimized structure.     Figure S1: Electronic absorption spectrum of 6SeGua in water with/without SOC contribution. This spectrum is emulated by sampling 500 snapshots from the QM/MM MD in the ground state. At each snapshot, a total of 15 singlets and 14 triplets states were computed at the ADC(2)/cc-pVDZ level.

Section S3. Validation of the Level of Theory
Here we report a gas-phase comparison between ADC(2) and MS-CASPT2 vertical excitation energies, critical points and deactivation pathways as obtained from LIIC scans.

Subsection S3.1. Vertical Excitation Energies
6SeGua gas phase vertical excitation energies at both levels of theory are displayed in Table   1 of the main manuscript. All methods predict a low-lying 1 (nSeπ5*) excited state (the S1 at the equilibrium geometry), with similar energies computed with ADC(2) (2.76 eV), MS-CASPT2(14,12) (2.76 eV) , and MS-CASPT2(12,10) (2.83 eV) methods. This 1 (nSeπ5*) state consist of a single electronic excitation from the lone pair localized on the Se atom (nSe) to the π5 antibonding orbital. This state is similar in 2-selenouracil (2SeUra) 1 (nSeπ2*) [S1], although in the 6SeGua it is at the MS-CASPT2 level 0.5 eV lower in energy. As for 2SeUra, the S1 1 (nSeπ5*) electronic state is dark and does not contribute to the absorption spectrum. S6 The S2 state at the Franck-Condon (FC) region is the 1 (πSeπ5*), computed at 3.45 eV with the best MS-CASPT2(14,12) method, in agreement with the experimental value [S2]. The excitation energies computed with ADC(2) (3.61 eV) and TD-B3LYP (3.60 eV) agree better with that computed at the MS-CASPT2(12,10)/cc-pVDZ level of theory, that is, with a smaller active space. This state is associated with the highest oscillator strength, which confirms the S2 1 (πSeπ5*) as the bright state.
The S3 is a 1 (πSeπ6*) state, computed at 4.64 eV with MS-CASPT2(12,10) and ADC (2) levels of theory, and 4.34 eV at the MS-CASPT2(14,12) level. The associated oscillator strength is small (~ 0.05). Given its energetic separation with the lower-lying states and small oscillator strength, its contribution to the relaxation mechanisms after excitation to the first absorption band can be neglected.
The two lowest triplet states are the T1 3 (πSeπ5*) and T2 3 (nSeπ5*) states. All levels of theory predict the triplet states in the same energetic region (T1: 2.4 -2.6 eV; T2: 2.6 -2.9 eV). Both triplet states are also excitations from the selenium orbitals, as observed for the singlet states. Excitations from other orbitals give rise to triplet states with a larger energy gap (≥ 3.8 eV) (see Table S3), so that we can focus on two lowest triplet states. The same pattern is observed for 2SeUra [S1].
Summarizing the performance of the three levels of theory in describing the 6SeGua lowest lying excited states: the energetic order of the excited state is the same and the relative energies in the FC region agree. The S2 state is ~ 0.9 eV above the S1 state at the MS-CASPT2(12,10)/cc-pVDZ and ADC(2)/cc-pVDZ levels of theory, being more stabilized at the MS-CASPT2(14,12)/cc-pVDZ level; the S3 state is computed to be at about 0.9 -1.0 eV higher than the S2 state in the three cases; the T2 state is placed a bit closer in energy to the T1 state (0.08 eV) at the ADC(2)/cc-pVDZ level of theory, while the gap between the triplet states computed with the MS-CASPT2 method is larger (0.28 -0.24 eV).

Subsection S3.2. Excited-State Minima and Crossing Points
Gas phase electronic states geometries and minimum energy crossing points were optimized at the MS-CASPT2(12,10)/cc-pVDZ and ADC(2)/cc-pVDZ levels of theory. Representative S7 optimized structures are displayed in Figure S2 and relevant conformational parameters and relative energies are collected in Table S6. The optimized structures were classified according to the six-membered ring parameters proposed by Cremer-Pople [S3] (Q, amplitude) and Boeyens [S4] for ring conformations (envelope (E), boat (B), screw-boat (S), twist-boat (T), half-chair (H), etc). The indices in the symbols represent the atoms moving out of the ring plane. Figure S2: 6SeGua gas phase optimized geometries and minima energy crossing points computed at ADC(2)/cc-pVDZ and MS-CASPT2(12,10)/cc-pVDZ levels of theory. The geometries were gathered according to Cremer-Pople and Boeyens parameters, The "ADC2" and "PT2" labels indicate that a given critical point was classified differently depending on the level of theory.
The optimized ground state geometry is nearly planar at both levels of theory, except for the hydrogen atoms of the amino group, which are outside the molecular plane by 48°. The first singlet excited state S1 1 (nSeπ5*) has a minimum located adiabatically around 2.5 eV above the ground state minimum. In both levels, the optimized structure is classified to be in the same Boeyens group ( 1 H6), but the pyramidalization on the selenium (MS-CASPT2: 6.7° and ADC (2): 24.2°) and H12 (MS-CASPT2: 18.5° and ADC (2): 25.5°) atoms is slightly different.
The minimum on the S2 1 (πSeπ5*) PES is placed adiabatically 3.0 eV above the ground state. The most important difference with respect to the ground state minimum is the elongated C=Se bound (~0.2 Å) and the pyramidalization of the C6 atom, moving the selenium atom out of plane by about 50° at the MS-CASPT2 (42° degree at ADC (2)).
For the T1 3 (πSeπ5*) minimum, the optimized structures resemble much each other at both levels of theory. The largest discrepancy is observed for the T2 3 (nSeπ5*), for which the sixmembered ring itself is planar with the selenium (~ 32°) and H12 atoms (~ 19°) out of the plan, S8 while the ADC(2) corresponding optimized geometry is fully planar.  Besides minima, we also optimized minimum energy crossing points (MECP) with both levels of theory. The search for a MECP between the S1 and S2 states indicate that at this point there is a three-state near-degeneracy encompassing the S1, S2, and T2 states; nonetheless, we still label this point as (S1/S2)CI, because in this region the nonadiabatic couplings between the S1 and S2 states are likely to be more relevant for the photophysics that the singlet-triplet couplings as we will comment below. Thus, we have not explicitly optimized a three-state degenerate structure. At both levels, this (S1/S2)CI geometry is S9 classified as envelope at C2 position (E2), with the amino group hydrogen atoms almost perpendicular to the molecular plane.
A conical intersection between the two lowest lying triplet states was also optimized and shows a pyramidalization of the C6 atoms of about 19° at MS-CASPT2 and 35° at ADC(2).
The (S1/T2)ISC structure resembles the (S1)min geometry, whereas the (T1/S0)ISC structure presents the largest pyramidalization angle of the selenium atom among all optimized geometries.

Subsection S3.3. Excited-State Deactivation Pathways
Here we investigate the gas phase photochemical pathways using with the linear interpolation in internal coordinates (LIIC) methodology. Note that MS-CASPT2 scans are smooth and do not exhibit the "wiggle" artifacts sometimes reported for MS-CASPT2 calculations [S6]. PATH I connects the FC and S2 1 (πSeπ5*)min regions. As it can be noticed, the S2 state evolves barrierless from the FC region to its minimum. The next step goes from the (S2)min to the (S1/S2)CI region, on which one observes an energetic barrier computed at the MS-CASPT2/cc-pVDZ level to be about 0.41 eV (11.7 kcal/mol), a little bit lower than that computed with the ADC(2) method (0.62 eV; 14.2 kcal/mol). In a recent work [S5] the barrier was estimated to be ~10.2 kcal/mol with MS-CASPT2 (using geometries optimized at CASSCF level). It is noteworthy that this barrier is completely absent in water [S7], which will lead to a faster population transfer to the S1 state in solution. The next photochemical step, labelled PATH II, connects the (three-state) neardenegeracy (S1/S2)CI and the (S1)min structures. This minimum is reached from the (S1/S2)CI region by an energy of 0.75 eV and 0.67 eV with MS-CASPT2 and ADC(2) methods, respectively. In the vicinity of this minimum, the triplet state may be easily accessed by a crossing between the S1 and T2 states. This crossing point is a bit higher in comparison with the (S1)min (0.01 eV with MS-CASPT2 and 0.13 eV with ADC (2)). Both methods predict a smaller SOC on this region in relation to the (S1/S2)CI crossing point (284, and 207 cm -1 with S11 MS-CASPT2 and ADC(2) methods, respectively). Note that there is a mix of contributions coming from * and * transitions at the crossing regions, enabling a large SOC. From here, the system may follow towards the minimum of the T2 state.
The last path (PATH III) starts on the (T2)min, from which the system can evolve towards a conical intersection with the T1 state. Subsequently, the system is lead to a minimum on the T1 hypersurface releasing an energy of 0.03 eV with ADC(2) and 0.24 eV with MS-CASPT2.