Curious Case of 2-Selenouracil: Efficient Population of Triplet States

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Spectroscopy and Excited States

The Curious Case of 2-Selenouracil: Efficient Population of Triplet States and Yet Photostable Sebastian Mai, Anna-Patricia Wolf, and Leticia Gonzalez J. Chem. Theory Comput., Just Accepted Manuscript • DOI: 10.1021/acs.jctc.9b00208 • Publication Date (Web): 30 Apr 2019 Downloaded from http://pubs.acs.org on May 1, 2019

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Journal of Chemical Theory and Computation

The Curious Case of 2-Selenouracil: Efficient Population of Triplet States and Yet Photostable Sebastian Mai,∗ Anna-Patricia Wolf, and Leticia González∗ Institute of Theoretical Chemistry, University of Vienna, Währinger Straße 17, 1090 Vienna, Austria E-mail: [email protected]; [email protected]

Abstract 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 though 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 three 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 spinorbit 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

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properties, together with its significant red-shifted absorption spectrum, could make 2-selenouracil a useful specialized photosensitizer for photodynamical therapy.

1

Introduction

The genetic information in DNA and RNA is encoded by the sequence of the canonical nucleobases adenine, guanine, cytosine, thymine (only in DNA), and uracil (only in RNA). However, besides these canonical nucleobases, nature employs a large set of nucleobase analogues for various functions. 1,2 Modified nucleobases can also be exploited as fluorescence markers, 3,4 or as antiviral, 5 anticancer, 6,7 immuno-suppression, 8 or other 9 drugs. Their broad applicability of nucleobase analogues stems from their structural similarity to the canonical nucleobases 10 in combination with some properties (e.g., photoreactivity) that are clearly different in analogues compared to the canonical bases. An important example of nucleobase modification is given by uracil and its substituted analogues 2-thiouracil (2tUra) and 2-selenouracil (2seUra). Both 2tUra and 2seUra can be found in the tRNA 11–13 of various microorganisms. 14 Here, 2seUra—mostly present as 5methylaminomethyl-2seUra—seems to be involved in codon-anticodon interactions, 15,16 and appears to help protecting tRNA from oxidative stress. 17 Furthermore, both 2tUra and 2seUra exhibit thyroid peroxidase inhibitor activity, with 6-n-propyl-2tUra being a clinically approved drug 9 and 6-n-propyl-2seUra showing an even higher activity. 18 Furthermore, 2seUra can effectively also inhibit Type I iodothyronine deiodinase. 19,20 Besides their biological applications, the photophysical and photochemical properties of 2tUra and 2seUra have attracted considerable attention. Thio-substituted nucleobases are known to exhibit a photoreactivity that is very different from their parent compounds. 21,22 After excitation by UV radiation, the canonical nucleobases rapidly deactivate back to the electronic ground state, 23,24 a mechanism that makes these bases photostable. On the contrary, thio-nucleobases quickly undergo intersystem crossing (ISC) that produces reactive,

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diradicaloid triplet states. These triplet states can cause severe damage to biological tissue, 25 which can either be exploited in photodynamic therapy 26 or might become an unwanted side effect. Hence, understanding the photoreactivity of nucleobase analogues is critical to design their biological and medicinal applications. Following this motivation, the last years have seen a rise in research about photoreactivity of nucleobase analogues. For 2tUra, experimental studies using transient absorption 27–29 and time-resolved photoelectron spectroscopy 30–32 reported sub-picosecond ISC (τISC between 200 and 800 fs depending on excitation energy and environment 31,32 ) from the initial bright state via a dark intermediate state. The lifetime of the formed triplet state is 50–200 ps in the gas phase and 70–300 ns in solution. 31 These experiments were interpreted through static quantum chemistry calculations, 33–36 nonadiabatic dynamics simulations, 37,38 and the simulation of (time-resolved) photoelectron spectra. 32,39 The computations showed that the relaxation of the initially excited 1 πS π ∗ state (S2 ) first leads to the lower 1 nS π ∗ state (S1 ), from where ISC to the 3 πS π ∗ state occurs (T1 ). The absence of accessible S1 /S0 conical intersections, the smallness of the singlet-triplet energy gaps, and the non-negligible spin-orbit couplings (SOCs) were advocated to explain the high ISC rate and quantum yield. 22 The fact that the triplet lifetime is 1000 times longer in solution than in vacuum was rationalized by the presence of two T1 minima that are differently affected by the environment. 31,36 The abundance of results for 2tUra is contrasted with the scarcity of studies on the photophysics of 2seUra. To the best of our knowledge, neither time-resolved spectroscopy nor theoretical calculations have been carried out on 2seUra to date. One computational study on substituted thymidines 40 has reported vertical excitation energies and Franck-Condon SOC matrix elements for the related 2-selenothymidine, but no relaxation pathways. Quite interesting is a recent transient absorption study on 6-selenoguanine 41 which reports that seleno-substitution retains ultrafast ISC to the triplet states and accelerates ISC back to the ground state. The present work is the first computational study on the excited-state relaxation dynam-

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ics of 2seUra. We present both static calculations—i.e., vertical excitation computations, optimization of critical points, and potential energy scans—and nonadiabatic dynamics simulations. For the sake of comparison to 2tUra, these calculations were carried out analogously to Refs. 35,38. Two fundamental questions are addressed in this work. First, how do the larger relativistic effects and the lower ionization potential of Se (compared to S) affect the potential energy surface landscape in 2seUra and its photodynamics? This question is of general interest to understand the relationship between molecular structure and excitedstate dynamics. Second, which photoreactivity can be expected for 2seUra? This issue is important to judge whether 2seUra could be either used as a medicinal drug—e.g., in place of 2tUra derivatives—without light-induced side effects, or as photosensitizer for photodynamic therapy. 29

2

Computational Details

In our investigations, we focus exclusively on the seleno-oxo tautomeric form of 2seUra (see Figure 1 top right corner). This tautomer was reported to be the most stable form of 2seUra in gas phase and in aqueous solution, 42 much in the same way as the thio-oxo form is the most stable tautomer of 2tUra. 43 The following subsections provide computational details of the electronic structure levels of theory, the static calculations, and the dynamics simulations.

2.1

Electronic structure methods

Two electronic structure methods were employed, multi-state complete active space perturbation theory of second order (MS-CASPT2) 44 and the algebraic diagrammatic construction scheme to second order for the polarization propagator (ADC(2)). 45,46

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2.1.1

MS-CASPT2

We employed MS-CASPT2 44 as the reference level of theory, as it has been shown before 35,38 to be adequate for thio-nucleobases and we expect the same for 2seUra. The MS-CASPT2 calculations are based on molecular orbitals from complete active space self-consistent field (CASSCF) 47 calculations with an active space of 12 electrons in 9 orbitals (denoted CAS(12,9)). The active space orbitals are shown in Figure 1 and include the 8 orbitals of the π system and the lone pair of the selenium atoms. For the vertical excitation calculation (see below) we also included the lone pair of oxygen (CAS(14,10)), but this orbital was removed from the active space in all optimizations and scans because it did not affect the energies notably but introduced significant convergence problems. The employed basis set is of ANO-RCC 48 type and uses a valence double-ζ plus polarization contraction (ANO-RCC-VDZP). Scalar-relativistic one-electron integrals were computed with the Douglas-Kroll-Hess formalism. 49 As Molcas cannot state-average over states of different multiplicities, we employed two independently optimized active spaces, one optimized for 4 singlet states and another one for 3 triplet states. The same number of states was also used for the multi-state CASPT2 treatment. 44 The MS-CASPT2 computations were performed with an IPEA shift 50 of zero 51 and an imaginary level shift of 0.3 a.u. in order to avoid intruder states. 52 Additionally, Cholesky decomposition 53 of the two-electron integrals was used to speed up the calculations. The MS-CASPT2(14,10) vertical excitation calculation included 9 singlets and 8 triplets and employed the larger ANO-RCC-VTZP basis set in combination with an IPEA shift of 0.25 a.u. 51 and an imaginary level shift of zero. All SA-CASSCF and MS-CASPT2 calculations were performed with Molcas 8.0. 54

2.1.2

ADC(2)

The second electronic structure method employed was ADC(2). 45,46 This level of theory is computationally much cheaper than MS-CASPT2—especially due to the availability of 5



O8 H H12

4

N3

5 6

H

2

N1 H

Se7

CAS(14,10) CAS(12,9)

nO

π1

π3

πO

π5

πSe

nSe

π2∗

π4∗

π6∗

Figure 1: Structure of 2seUra with atom numbering and active space orbitals. The larger CAS(14,10) is denoted by a blue frame, while the smaller CAS(12,9) is shown with a yellow frame. The indices of the orbitals refer to the atom or part of the molecule where the largest part of the orbital is localized. analytical gradients—but still provides reasonably accurate potential energy surfaces, as is shown below. The computations employed the def2-TZVP 55 basis set, except for the dynamics simulations that employed def2-SVP 55 instead. Note that only non-relativistic one-electron integrals were employed. The ADC(2) calculations, as well as the corresponding MP2 ground state computations, were carried out with Turbomole 7.0. 56 As reported elsewhere, 38 the ADC(2) implementation in Turbomole uses a very efficient resolution-of-identity (RI) implementation to speed up the calculations, 57 which is therefore also used here. The appropri6


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ateness of these settings are justified in Section 3.

2.2

Static calculations

Static calculations were employed to explore the potential energy surfaces of 2seUra. We optimized the ground state (S0 ) minimum as well as several excited-state minima (S1 , 2× S2 , T1 ) and minimum energy crossing points (MXPs) (2× S1 /S2 , S1 /T2 , T1 /T2 , T1 /S0 ). The optimized S0 minimum was used to compute vertical excitation energies. The optimizations were carried out with the external optimizer feature of ORCA 3.0.3, 58 which was fed with the appropriate gradients from either ADC(2) or MS-CASPT2. For the MXP optimizations, the gradients to follow were obtained with either a projection algorithm 59 for singlet-triplet crossings or a penalty function method 60 for conical intersections. Initial geometries were prepared from the corresponding geometries of 2tUra. 30,35 The optimizations resulted in ten critical points for ADC(2) and ten critical points for MS-CASPT2. The corresponding Cartesian coordinates are given in the Supporting Information. Subsequently, the optimized critical points were connected in an appropriate order by linear interpolation in internal coordinates (LIIC) scans. These scans were used to check for the presence or absence of barriers. Note that these scans do not provide minimum-energy paths, but only upper limits for the involved barriers. 61

2.3

Nonadiabatic dynamics simulations

In order to investigate the temporal evolution of 2seUra after excitation, we performed nonadiabatic dynamics simulations with the SHARC (surface hopping including arbitrary couplings) method. 62,63 This method is an extension of the popular Tully surface hopping method 64 able to treat ISC, and is the same method that was previously used for the simulations of 2tUra dynamics. 37,38 The dynamics simulations employed the RI-ADC(2)/def2-SVP level of theory. To obtain initial conditions for the simulations, we sampled 1000 geometries from the Wigner distri7



bution of the harmonic ground state potential, using frequencies and normal modes from an MP2/def2-SVP frequency calculation. For each of these 1000 geometries, 8 singlet states were computed with ADC(2) to generate an ensemble-broadened absorption spectrum. This spectrum was obtained as the sum over Gaussians (full width at half maximum of 0.2 eV, the smallest value that sufficiently smoothed the spectrum) centered at the excitation energies of all 8000 states, with height proportional to the oscillator strength. The initial excited state was selected stochastically in the excitation window of 3.95–4.05 eV (306–314 nm) based on the oscillator strength, 65 i.e., brighter states were more likely to be selected as initial state. However, in order to reduce computation time and to avoid convergence problems from higher states, the dynamics simulations only included 3 singlet states (S0 , S1 , S2 ) and 2 triplet states (T1 , T2 ). Hence, all 107 selected initial conditions started in the S2 . The 107 trajectories were propagated with SHARC for a total simulation time of 700 fs, using a 0.5 fs nuclear propagation step. The electronic wave function was propagated with a 0.02 fs time step (using interpolated quantities) with the local diabatization formalism. 66 Note that the electronic wave function was a linear combination of a total of 9 states (3 singlets, 2×3 triplets), as in SHARC the Ms components of the triplet states are treated separately. An energy-based decoherence scheme 67 was applied.

3 3.1

Results Vertical excitations

Table 1 presents an overview of the vertical excited states, together with previously published results of 2tUra. 35,38 For 2seUra, three calculations were performed, one with MS(9,8)CASPT2(14,10)/ANO-RCC-TZVP, one with MS(4,3)-CASPT2(12,9)/ANO-RCC-VDZP, and one with ADC(2)/def2-TZVP. The ADC(2) calculation has the advantage that no bias is introduced by a limited active space. The large MS-CASPT2(14,10) calculation is taken as the reference, whereas the small MS-CASPT2(12,9) and ADC(2) settings are the levels of 8


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Table 1: Vertical excitation energies of 2seUra in eV (oscillator strength in parentheses) computed at the MS(9,8)-CASPT2(14,10)/ANO-RCC-TZVP, MS(4,3)-CASPT2(12,9)/ANORCC-DZVP, and ADC(2)/def2-TVZP levels of theory. For comparison, corresponding values for 2tUra at the MS(3,3)-CASPT2(12,9)/cc-pVDZ 35 and ADC(2)/aug-cc-pVTZ levels of theory. 38 2-Selenouracil Charactera MS-CASPT2(14,10) MS-CASPT2(12,9) E (fosc ) E (fosc ) 1n π∗ 3.46 (0.00) 3.22 (0.00) Se 2 1π π∗ 3.98 (0.62) 3.75 (0.50) Se 2 1π π∗ 4.45 (0.03) 4.12 (0.14) Se 6 1n π∗ 4.51 (0.00) — (—) Se 6 1n π∗ 5.16 (0.00) — (—) O 4 1π π∗ 5.32 (0.04) — (—) O 4 1n π∗ 6.03 (0.00) — (—) O 6 1π π∗ 6.12 (0.34) — (—) O 6 1n π∗ — (—) — (—) Se Ryd 3π π∗ 3.18 2.94 Se 2 3n π∗ 3.46 3.27 Se 2 3π π∗ 3.99 — O 6 3π π∗ — 3.76 5 6 a

2-Thiouracil ADC(2) MS-CASPT2(12,9) 35 ADC(2) 38 E (fosc ) E (fosc ) E (fosc ) 3.29 (0.00) 3.70 (0.00) 3.75 (0.00) 3.97 (0.38) — (—) 4.92 (0.11) 4.66 (0.03) 4.30 (0.11) 4.42 (0.38) 4.49 (0.00) 4.89 (0.00) 5.28 (0.00) 4.95 (0.00) — (—) — (—) 5.34 (0.04) — (—) — (—) 5.40 (0.00) — (—) 4.58 (0.00) — (—) — (—) — (—) 5.91 (0.02) — (—) 5.91 (0.02) 3.08 3.24 3.42 3.20 3.76 3.64 3.91 — 4.40 — 3.83 3.98

For the states of 2tUra, nSe and πSe are replaced by nS and πS .

theory at which the optimizations and scans were carried out. For all three methods, the lowest excited state at the Franck-Condon geometry has 1 nSe π2∗ character (see Figure 1 for the orbitals). This state is hence an excitation localized on the selenourea part of the molecule. At an energy of about 3.3 eV, this state is dark and hence does not contribute to the absorption spectrum. In this regard, the state is very similar to the 1 nS π2∗ state of 2tUra, although the 1 nSe π2∗ is about 0.5 eV lower in energy. The second excited state is the 1 πSe π2∗ state, with a vertical excitation energy of about 4.0 eV (3.8 eV with MS-CASPT2(12,9)) and an oscillator strength of 0.4–0.6. Again, this is a local excitation on the selenourea group. Interestingly, this state seems to be in contrast to 2tUra, where the thiourea-localized ππ ∗ state (1 πS π2∗ ) is darker than the charge-transfer ππ ∗ state (1 πS π6∗ ). 35 The corresponding state of 2seUra—the 1 πSe π6∗ state—shows only a smaller oscillator strength of 0.03–0.14 and is located at energies of 4.1–4.7 eV, depending on the method. Both states are located at lower energies than their 2tUra counterparts, indicating

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that the absorption spectrum of 2seUra is significantly red-shifted. The fourth excited singlet state is the 1 nSe π6∗ state, which has an energy of 4.5 eV and is dark. Hence, the four lowest singlet states are all excitations from selenium orbitals. Excitations from other occupied orbitals—localized on the oxygen or the ring—only appear above 5 eV. Hence, for the present investigations we will focus only on the excitations from the selenium orbitals. The two lowest triplet states have 3 πSe π2∗ and 3 nSe π2∗ characters, making them the triplet analogues of the two lowest singlet states. Their energies are about 3.1–3.4 eV. Higher triplet states only appear after a notable energy gap, again allowing us to focus on the lowest triplet states. When comparing the three levels of theory, a generally good agreement is found. The order of the excited states is mostly reproduced, although ADC(2) reorders the close-lying 1

nSe π6∗ and 1 πSe π6∗ states. Due to the smaller number of states included and the neglect

of the nO orbital, the MS-CASPT2(12,9) calculation cannot describe most of the higher states. The MS-CASPT2(12,9) also produces a red-shift of about 0.2 eV for all excited states, but the energy gaps between the states are in satisfactory agreement with the large MS-CASPT2(14,10) calculation. The ADC(2) and MS-CASPT2 results are also in good agreement with previously reported vertical excitation energies at the M06/6-31+G* level of theory for 2-selenothymidine 40 (S1 : 3.43 eV, S2 : 3.97 eV, T1 : 2.91 eV, T2 : 3.28 eV). We thus conclude that the computationally efficient ADC(2) and MS-CASPT2(12,9) levels of theory provide sufficiently accurate results compared to the MS-CASPT2(14,10) level of theory.

3.2

Excited state minima and crossing points

The optimized critical points of 2seUra are depicted in Figure 2. Important geometry parameters and relative energies are given in Table 2 for ADC(2) and in Table 3 for MS-CASPT2. The ground state geometry of 2seUra is nearly planar with both the MP2 and MSCASPT2 methods, as shown in Figure 2a. The aromatic ring has virtually the same bond 10


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(a) planar

(b1) C2 -pyramid.

(b2) C2 -pyramid.

(c) boat

(d) C6 -pyramid.

(e) C2 -perpendic.

S0 min

S1 min (ADC) (1 nSe π2∗ ) S1 /S2pyr MXP (ADC) (1 nSe π2∗ /1 πSe π2∗ )

S1 min (1 nSe π2∗ ) (PT2) S2pyr min (1 πSe π2∗ ) T1 min (3 πSe π2∗ ) S1 /S2pyr MXP (PT2) (1 nSe π2∗ /1 πSe π2∗ ) S1 /T2 MXP (1 nSe π2∗ /3 πSe π2∗ ) T1 /T2 MXP (3 πSe π2∗ /3 nSe π2∗ )

S2boat min (1 πSe π6∗ )

S1 /S2boat MXP (1 nSe π2∗ /1 πSe π6∗ )

S0 /T1 MXP (S0 /3 πSe π2∗ )

Figure 2: Prototypical geometries of the critical points of 2seUra, optimized with ADC(2)/def2-TZVP and MS-CASPT2(12,9)/ANO-RCC-VDZP. The large labels directly below the geometries indicate the main geometric features. The smaller, framed labels indicate which optimized critical points are similar to the shown prototypical geometries, e.g., the MS-CASPT2 S1 minimum or the T1 /T2 MXP have geometries similar to (b2). The labels “ADC” and “PT2” indicate that a critical point is similar to different prototypes depending on the level of theory. See Tables 2 and 3 for corresponding geometrical parameters, and the Supporting Information for Cartesian coordinates. lengths and angles as in 2tUra. 35 However, the C-Se bond is significantly longer (1.79 Å) than the C-S bond in 2tUra (1.67 Å). In contrast to the ground state geometry, none of the excited-state critical points are planar. Hence, we quantify the extent of ring deformation—called ring puckering—by the Cremer-Pople parameter 68 Q (amplitude) and the classification scheme of Boeyens. 69 The latter scheme allows identifying the type of ring conformation, e.g., envelope (E), boat (B), screw-boat (S), or twist-boat (T ). The indices given in the Boeyens symbols, e.g., B3,6 , indicate which atoms (atom numbering in Figure 1) are displaced away from the ring plane. The displacement of the selenium atom from the ring plane is described by the pyramidalization angle of the Se-CN2 group, defined as 90◦ minus the angle between the C-Se bond and the normal vector of the CN2 plane. 35 Both ADC(2) and MS-CASPT2 predict a minimum of the 1 nSe π2∗ state, which is the S1 both at the Franck-Condon point and at the minimum geometry. The most striking feature of

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Table 2: Energies, geometry parameters, and SOCs for excited-state minima and crossing points of 2seUra at the ADC(2)/def2-TZVP level of theory. Q is the puckering amplitude, 68 and “Boeyens” refers to the configuration of the ring 69 (E: envelope, B: boat, S: screw-boat, T : twist-boat). Sub-/superscripts of r (bond length), a (bond angle), p (pyramidalization angle), and “Boeyens” refer to the atom numbering given in Figure 1. S0 min S1 min S2pyr min S2boat min† T1 min S1 /S2pyr S1 /S2boat S1 /T2 T2 /T1 T1 /S0 1π π∗ 3π π∗ 1n π∗/ 1n π∗/ 1n π∗/ 3n π∗/ 3π π∗/ cs 1 nSe π2∗ 1 πSe π2∗ Se 6 Se 2 Se 2 Se 2 Se 2 Se 2 Se 2 1π π∗ 1π π∗ 3π π∗ 3π π∗ cs Se 2 Se 6 Se 2 Se 2 Fig. 2 (a) (b1) (b2) (c) (b2) (b1) (d) (b2) (b2) (e) E (eV) 0.00 2.86 3.15 3.60 2.64 3.19 3.74 2.90 2.70 3.23 r1,2 (Å) 1.37 1.40 1.39 1.32 1.40 1.38 1.30 1.40 1.40 1.43 r2,3 (Å) 1.36 1.41 1.39 1.35 1.40 1.38 1.30 1.39 1.40 1.42 r3,4 (Å) 1.41 1.40 1.40 1.48 1.39 1.41 1.52 1.40 1.40 1.40 r4,5 (Å) 1.45 1.45 1.45 1.41 1.46 1.45 1.39 1.46 1.45 1.45 r5,6 (Å) 1.35 1.36 1.36 1.41 1.36 1.36 1.43 1.35 1.36 1.35 r6,1 (Å) 1.37 1.36 1.36 1.38 1.36 1.36 1.46 1.37 1.36 1.36 r2,7 (Å) 1.79 1.91 1.97 1.91 1.91 1.99 1.92 1.90 1.92 1.99 a1,2,7 (◦ ) 122.4 116.4 110.7 120.8 108.0 107.2 117.1 111.7 115.1 102.0 a3,4,8 (◦ ) 120.2 120.5 120.9 115.3 121.2 121.2 114.2 120.8 120.5 120.1 p7,2,1,3 (◦ ) 0.0 37.6 48.6 -5.3 41.4 56.2 -1.2 31.4 38.6 66.6 p12,6,5,1 (◦ ) 0.0 0.8 0.2 5.1 -0.5 0.8 32.1 -0.7 -0.3 -0.6 Q (Å) 0.00 0.26 0.18 0.08 0.01 0.20 0.09 0.17 0.04 0.08 5S 3S 6T Boeyens planar E2 E2 B3,6 planar E2 E2 6 2 2 −1 SOC (cm ) — — — — — — — 650 — 0 † The “S boat min” is not a true minimum at ADC(2) level of theory, but is listed here for 2 completeness. ADC(2)

this geometry is the large pyramidalization angle of the selenourea group of about 38◦ . This pyramidalization is mainly due to the electron in the π2∗ orbital, which effectively changes the C2 atom from sp2 to sp3 hybridization and thus induces non-planarity. The geometry differs slightly between the two methods—with ADC(2), the selenium is in the molecular plane and the C2 atom is puckered strongly (Figure 2b1), whereas with MS-CASPT2 the ring is planar and the selenium is moved out of the ring plane to keep the pyramidalization angle unchanged (Figure 2b2). This is reminiscent of the situation in 2tUra, 37 where MS-CASPT2 predicts the planar ring plus displaced sulfur geometry at the minimum, but the C2 -puckered ring with in-plane sulfur is only 0.03 eV higher in energy. Hence, the two different-looking S1 minima from ADC(2) and MS-CASPT2 are simply a consequence of the very flat S1 potential. Another interesting finding is that most bond lengths in the S1 minimum are very 12


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Table 3: Energies, geometry parameters, and SOCs for excited-state minima and crossing points of 2seUra at the MS-CASPT2(12,9)/ANO-RCC-VDZP level of theory. Q is the puckering amplitude, 68 and “Boeyens” refers to the configuration of the ring 69 (E: envelope, B: boat, S: screw-boat, T : twist-boat). Sub-/superscripts of r (bond length), a (bond angle), p (pyramidalization angle), and “Boeyens” refer to the atom numbering given in Figure 1. MS-CASPT2 S0 min S1 min S2pyr min S2boat min T1 min S1 /S2pyr S1 /S2boat cs 1 nSe π2∗ 1 πSe π2∗ 1 πSe π6∗ 3 πSe π2∗ 1 nSe π2∗ / 1 nSe π2∗ / 1π π∗ 1π π∗ Se 2 Se 6 Fig. 2 (a) (b2) (b2) (c) (b2) (b2) (d) E (eV) 0.00 2.88 3.18 3.32 2.55 3.23 3.32 r1,2 (Å) 1.38 1.40 1.37 1.32 1.39 1.36 1.33 r2,3 (Å) 1.37 1.40 1.37 1.34 1.40 1.36 1.32 r3,4 (Å) 1.41 1.40 1.41 1.51 1.40 1.42 1.52 r4,5 (Å) 1.45 1.46 1.45 1.40 1.46 1.45 1.40 r5,6 (Å) 1.36 1.36 1.36 1.42 1.36 1.36 1.42 r6,1 (Å) 1.38 1.37 1.38 1.42 1.36 1.38 1.45 r2,7 (Å) 1.82 1.99 2.12 1.93 2.00 2.20 1.92 a1,2,7 (◦ ) 122.4 115.4 110.0 120.6 110.3 106.5 119.1 a3,4,8 (◦ ) 119.8 120.4 120.6 114.6 120.9 120.6 114.6 p7,2,1,3 (◦ ) -0.1 38.0 48.5 -1.8 45.1 55.7 0.0 p12,6,5,1 (◦ ) 0.0 -0.9 -0.2 -0.2 19.2 0.2 24.8 Q (Å) 0.03 0.11 0.05 0.11 0.05 0.07 0.06 1S Boeyens planar 3 S2 E2 B3,6 E E6 2 2 −1 SOC (cm ) — — — — — — —

S1 /T2 T2 /T1 T1 /S0 1n π∗/ 3n π∗/ 3π π∗/ Se 2 Se 2 Se 2 3π π∗ 3π π∗ cs Se 2 Se 2 (b2) 2.93 1.40 1.39 1.40 1.46 1.36 1.38 1.96 105.7 120.9 39.8 -1.1 0.14 E2 650

(b2) 2.67 1.39 1.38 1.40 1.46 1.36 1.38 2.05 114.6 120.7 40.9 -1.4 0.16 E2 —

(e) 2.78 1.43 1.40 1.39 1.46 1.35 1.36 1.99 94.7 120.7 58.1 -1.1 0.10 3S 2 660

similar to the S0 minimum, indicating that no bond length alteration takes place. The only exception is the C-Se bond, which becomes 0.1–0.2 Å longer in the excited state. The MS-CASPT2 optimizations found two minima on the S2 adiabatic surface, one corresponding to 1 πSe π2∗ and the other one to 1 πSe π6∗ . At ADC(2) level, only the 1 πSe π2∗ minimum is a true minimum, whereas the optimization of the 1 πSe π6∗ energy follows a very small gradient until eventually converging to the 1 πSe π2∗ minimum (for a side-by-side discussion of both method, Table 2 includes a geometry from the small-gradient region). Of these two points, the 1 πSe π2∗ minimum is similar to the S1 minimum, as it also shows strong pyramidalization of the selenourea group; thus, we denote this minimum as S2pyr minimum. The other geometry exhibits a slight boat conformation (B3,6 with puckering amplitude Q = 0.08 Å) and bond length alteration in the ring (Figure 2c). This geometry is termed S2boat minimum. The last obtained excited-state minimum is located on the T1 potential energy surface,

13



corresponding to 3 πSe π2∗ character. It is very similar to the S2pyr minimum with regard to the pyramidalization angle, although the ring is more planar in the T1 minimum. Interestingly, no second T1 minimum was identified for 2seUra in the present work, neither for ADC(2) nor for MS-CASPT2. Such a second T1 minimum was found previously for 2tUra, 35,38 with a 3 πS π6∗ wave function character and a B3,6 boat-like geometry (analogous to Figure 2c). For 2seUra, all attempts of optimizing such a minimum converged easily to the pyramidalized T1 minimum. We were also able to optimize five relevant crossing points of 2seUra, including two S1 /S2 MXPs. The first S1 /S2 MXP corresponds to the minimum of the crossing seam between 1 nSe π2∗ and 1 πSe π2∗ and is termed S1 /S2pyr MXP. Its geometry is very similar to the S1 minimum. The second MXP, called S1 /S2boat MXP, connects the 1 nSe π2∗ and 1 πSe π6∗ states. Its geometry is similar to the S2boat , although with additional pyramidalization of the H-C6 CN group, as depicted in Figure 2d. ISC from singlets to triplets might be facilitated by the presence of a crossing between 1

nSe π2∗ and 3 πSe π2∗ at a geometry very similar to the S1 minimum. At this geometry, the

1

nSe π2∗ is S1 and the 3 πSe π2∗ is T2 , hence we term this the S1 /T2 MXP. Subsequent decay to

the T1 surface is enabled by a T1 /T2 crossing between the 3 πSe π2∗ and 3 nSe π2∗ states. Note, however, that this crossing is passed diabatically, since both the T1 minimum and the S1 /T2 MXP involve the 3 πSe π2∗ , whereas the 3 nSe π2∗ state is involved in neither. The final optimized point is a T1 /S0 MXP between the closed-shell configuration and the 3

πSe π2∗ state. The geometry is shown in Figure 2e, displaying the largest pyramidalization

angles of all optimized structures.

3.3

Potential energy surface scans

In order to connect the different optimized geometries and to verify the absence of potential energy barriers, we carried out linear interpolation scans. These scans are presented in Figure 3, with the ADC(2) potential energies (based on the ADC(2) optimized geometries) 14


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S0

S1

S2

T1

T2

T3 Path Minimum Crossing

p7213 (◦ )

Energy (eV)

4.0 1

3.5

πSe π2∗

3.0

1 1

2.5 2.0

nSe π2∗

Path I

S0

650 cm−1

πSe π6∗ 1

nSe π2∗

3

Path II

S0

πSe π2∗ S0

Path III

60 40 20 0

(a) ADC(2)

p7213 (◦ )

Energy (eV)

4.0 1

3.5

πSe π2∗

650 cm−1

3.0

1

2.5 2.0

1

S0

Path I

nSe π2∗ S0

πSe π6∗

1

nSe π2∗

Path II

60 40 20 0

660 cm−1 3

πSe π2∗

Path III

S0

(b) MS-CASPT2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


LIIC coordinate

Figure 3: LIIC scans along optimized minima and crossing points of 2seUra, obtained with (a) ADC(2)/def2-TZVP//ADC(2)/def2-TZVP or (b) MS-CASPT2(12,9)/ANO-RCCVDZP//MS-CASPT2(12,9)/ANO-RCC-VDZP. The relaxation pathway is marked with open black circles, optimized minima with full black circles and involved crossing points with crosses. The black arrows show the vertical excitation from the Frank Condon geometry to the second excited singlet state. The character of the optimized states is given below each minimum. Relevant computed SOCs (in cm−1 ) are given at each singlet/triplet crossing point. shown in panel (a) and the MS-CASPT2//MS-CASPT2 scans in panel (b). We stress that linear interpolation scans do not find the minimum-energy path between two points and hence might overestimate barriers. However, since in Figure 3 all path segments are monotonic, all barriers are given by the energies of the critical points themselves, with no additional barriers in between. In the figure, the critical points are organized in three photophysically reasonable pathways, called Path I, II, and III. Paths I and II both start from the Franck-Condon point 15



(i.e., the S0 minimum) in the bright S2 (1 πSe π ∗ ) state. Path I then leads first to the S2pyr minimum, continues to the S1 /S2pyr MXP, and relaxes to the S1 minimum. On the contrary, Path II leads first to the S2boat minimum and then goes through the S1 /S2boat MXP to also reach the S1 minimum. Hence, both paths describe the relaxation from the Franck-Condon region until the S1 minimum. The main difference is that in Path I, the selenourea group already pyramidalizes while still on the S2 potential energy surface, whereas in Path II pyramidalization only sets in after switching to the S1 state. This distinction can be seen in the lower halfs of panels (a) and (b) of Figure 3, which plot the evolution of the pyramidalization angle along the relaxation pathways. This different evolution of the pyramidalization angle naturally affects the energetics of the two pathways. There are some relevant differences between the ADC(2) and MSCASPT2 results. In Path I, the ADC(2) energies of the involved critical points are 3.97 eV – 3.15 eV – 3.19 eV – 2.86 eV, giving an initial energy release of 0.82 eV and a barrier of 0.04 eV to reach the conical intersection. For MS-CASPT2, the energy release is 0.57 eV and the barrier is 0.05 eV. For Path II, ADC(2) gives an energy release of 0.37 eV and a barrier of 0.14 eV; MS-CASPT2 delivers values of 0.43 eV and a barrier of

Curious Case of 2-Selenouracil: Efficient Population of Triplet States

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