2-Thiouracil deactivation pathways and triplet states population

Computational and Theoretical Chemistry xxx (2014) xxx–xxx

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2-Thiouracil deactivation pathways and triplet states population João Paulo Gobbo, Antonio Carlos Borin ⇑ Universidade de São Paulo, Instituto de Quca, Departamento de Quca Fundamental, NAP-PhotTech, The USP Consortium for Photochemical Technology, Av. Prof. Lineu Prestes 748, 05508-900 São Paulo, São Paulo, Brazil

a r t i c l e

i n f o

Article history: Received 28 January 2014 Received in revised form 11 March 2014 Accepted 13 March 2014 Available online xxxx Keywords: Thiated pyrimidines 2-Thiouracil Modified nucleobases Triplet states

a b s t r a c t The photochemical reaction path approach and the CASPT2//CASSCF protocol have been employed to investigate the nonadiabatic photophysics and photochemistry of 2-thiouracil, with emphasis on the population of the lowest triplet state. Minimum energy structures, conical intersections, intersystem crossings, minimum energy paths, and spin–orbit couplings were discussed along the potential energy profiles, in order to investigate the efficiency of the nonadiabatic deactivation pathways from the initially populated state S2 1 ðnS p Þ, which can be useful to rationalize the absence of fluorescence and the population of the lowest triplet state. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction Thiated pyrimidines, nucleobases derivatives substituted with sulfur atom(s) in the positions of the exocyclic oxygen atom(s), have received considerable attention due to their relevance for numerous metabolic processes [1,2]. 2-Thiouracil (2-TU), a thiated pyrimidine derived by substitution of the oxygen atom from uracil by a sulfur atom (Fig. 1), has been detected in prokaryotic t-RNAs and has been used as cross-linking agent in RNA transcriptional regulation [3–5]. 2-TU has also been of particular interest due to its biological activities [6] and ability to form complexes with some divalent metal ions [7,8], with potential applications as antidote for mercury poisoning [9]. Other therapeutical properties of 2-TU derived from its photophysical properties, as its use as melanomas seeker, is due to its long-wavelength absorption and easy incorporation into biological tissues [10,11]. The photophysical properties of aza and thio modified nucleobases differ from their canonical counterparts, which can be attributed to structural modifications imposed by the substitutions, as we have recently shown for 6-aza-2-thiothymine (6-A2TT) [12], 8-azaadenine (8-AA) [13], and 6-azauracil (6-AU) [14]. The absorption and emission spectra of 2-TU and some derivatives in different solvents were studied by Párkányi et al. [15], who employed the PPP and the CNDO/2 methods to compute excited states dipole moments. More recently, Moustafa et al. [16] reinvestigated the 2-TU absorption spectrum in polar and nonpolar solvents and, with the aid of theoretical results obtained with the ⇑ Corresponding author. E-mail address: [email protected] (A.C. Borin).

INDO/S method, carried out an interpretation of the experimental results in terms of the nature of the molecular orbitals and electronic configurations involved in the ground and excited states. More recently, Khvorostov and co-workers [17] employed the matrix isolation technique to investigate the photoisomerization reactions of a series of thiouracil compounds, including 2-TU. In short, the absorption spectrum of 2-TU can be described generically as being composed by a broad and medium intensity band in the 300–250 nm region, composed by two transitions centred at 267 and 300 nm, and other one in the 200–220 nm region. Both are affected by solvent polarity, but in opposite directions, with the solvatochromic shifts more pronounced in the long wavelength absorption band, which is blue shifted by about 12 nm as the solvent polarity decreases. Increasing the solvent polarity, the intensities of both bands decrease and the relative ratios are reversed. It is also worthing mentioning that no fluorescence has been detected. Cui and Fang [18] used 2-thiouracil as a model to investigate the photophysical mechanisms of 2-thiothymine. Minimum energy structures were optimized at the CASSCF (complete active space self-consistent-field) level, imposing C s symmetry, in addition to several conical intersections and intersystem crossings. Final energies were obtained at the CASPT2 (multiconfigurational second order perturbation theory) level. The computed minimum energy crossing points (MECP) were connected by linear interpolations carried out at the CASSCF level, also imposing C s symmetry. The analysis of the relevant potential energy hypersurfaces, on the basis of conical intersections (CI) [19] and, in the case of formation of triplet excited states, intersystem crossings (ISC) and vibronic spin–orbit coupling (SOC) factors [21] is necessary for a

http://dx.doi.org/10.1016/j.comptc.2014.03.021 2210-271X/Ó 2014 Elsevier B.V. All rights reserved.

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Fig. 1. Schematic structures and labeling for uracil (U) and 2-thiouracil (2-TU). The structures do not represent detailed bonding characteristics.

comprehensive description of deactivation mechanism. However, as we have shown [12–14], the most relevant CI and ISC are no planar. On the other hand, linear interpolations do not guarantee the presence or absence of energy barriers, constituting an upper bound limit to the energy barriers [20]. From the photochemical point of view, the most effective funnels are the first reached in the seam of crossing points and not its lowest-lying structure (minimum energy crossing points (MECP)). Therefore, it is recommended to explore the deactivation paths followed by the bright state from the Franck-Region employing the minimum energy path approach. In continuation to our previous efforts for describing the photophysical and photochemical relaxation mechanisms of aza and thio modified nucleobases [12–14], in this contribution we have employed the photochemical reaction path approach (PRPA), together with the CASPT2//CASSCF protocol, to investigate the relaxation mechanisms of 2-TU, based on systematic calculations of optimized geometries for the ground and excited states, conical intersections and intersystem crossings structures, spin–orbit couplings, and minimum energy paths. In addition, a comparison to relaxation paths of related species has also been made. 2. Computational methods Double-f plus polarization [22] atomic natural orbitals (ANO-L) basis sets have been employed in all calculations, carried out without imposing symmetry constraints with the MOLCAS-7.8 software [23,24]. Geometry optimizations, minimum energy paths (MEPs), and conical intersections (CI) searches have been performed at the multiconfigurational CASSCF [25] level of theory, averaging over the lowest six singlet and six triplet states. Minimum energy paths (MEPs) have been computed as steepest descent paths, while conical intersections (CI) have been optimized using the restricted Lagrange multipliers technique, as the lowest-energy point obtained under the restriction of degeneracy between the two considered states. Computational details about the MEPs and conical intersections can be found in Ref. [26]. Spin–orbit coupling (SOC) elements have been computed within the AMFI (atomic mean field integrals), as done in previous works [14,13,12]. The multiconfigurational calculations have been carried out employing a valence active space comprising a total of 14 electrons distributed among 2n (one on each oxygen and sulfur atoms) and 8p and p orbitals (CASSCF (14; 10)) (Fig. 2). The multiconfigurational second-order perturbation theory (CASPT2) developed by Roos and co-workers [25,27–31] has been employed to introduce dynamical correlation effects, via single point energy calculations at each computed structure, as defined in the so-called CASPT2//CASSCF protocol employed in other studies [32,33,14,13,12]. Similarly, CASPT2 calculations have also been carried out at selected geometries around the computed CASSCF conical intersection structure, in order to minimize the energy

Fig. 2. Active space of 2-thiouracil in terms of averaged valence natural orbitals computed at the CASSCF level, in the ground-state equilibrium geometry and with the ANO-type doublef plus polarization basis set.

gap between the two intersecting states at the CASPT2 level of theory. Intruder states have been avoided with a level-shift correction of 0.2 a.u. and no modification in the zeroth-order Hamiltonian (IPEA shift [34], ionization potential–electron affinity shift, parameter with a value of 0.0) has been introduced, in consistency with previous results. It is worth mentioning that, whenever MEPs calculations were not possible, linear interpolation in internal coordinates have been computed in order to study the electronic states along the path connecting two structures of interest. 3. Results and discussion 3.1. Excited states properties Our results for vertical absorption and emission energies for the lowest lying singlet and triplet excited states of 2-TU are collected in Table 1, together with others from previous theoretical work Table 1 Singlet and triplet low-lying electronic states of 2-thiouracil: vertical absorption emi energy (Eabs v , eV), Band Origin (T e , eV), vertical emission energy (Ev , eV), oscillator strengths (f), and dipole moments (l, Debye). State

Absorption

Emission

This work Eabs v S0 T1 3 ðpS p Þ T2 3 ðnS p Þ S1 1 ðnS p Þ T3 3 ðpp Þ S2 1 ðpS p Þ S3 1 ðpS p Þ T4 3 ðnO p Þ S4 1 ðnO p Þ S5 1 ðpp Þ a

f

l

[18]

This work

Eabs v

Eemi v

Te

[18] Te

1:84

2:85 3:33 3:36

3:43 3:73 3:81 3:81

2:99

3:55

a

3:14 3:60 3:65 3:67 4:09 4:45 4:57 4:59 4:87

0:000 0:236 0:445 0:000 0:000

4.45 4:07 4:47 4:68 3:75 6:00 4:83 2:90 2:26 7:04

3:83

4:46 4:76

Experimental value in dioxane: 4:21 D [35].

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3:86

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[18]. As it can be observed, we have computed two triplet states below the singlet excited states, being the lowest singlet excited state of 1 ðnS p Þ character. This is an important characteristic for describing the population mechanisms of the lowest triplet states. The T13 ðpS p Þ state (l ¼ 4:07 D) is the lowest excited state (Table 1), computed vertically at 3.14 eV from the ground-state, with a wavefunction dominated by an electronic configuration derived from the ground-state by a one-electron promotion from the pS orbital, localized to a large extent on the sulfur atom, to a p orbital delocalized over the N1, C2, and N3 atoms. The T2 3 ðnS p Þ (l ¼ 4:47 D) state is 3.60 eV vertically above the ground state, being best represented by a singly excited electronic configuration obtained from the ground-state by an excitation from the sulfur nS orbital to the p delocalized orbital described previously. A similar pattern is found in uracil [36], for which the 3 ðpp Þ (3.80 eV) and 3 ðnO p Þ (4.71 eV) are also the lowest-lying excited states at the Franck– Condon region. Nevertheless, the thio substitution strongly stabilized the triplet states in 2-TU. The S1 1 ðnS p Þ state is the lowest-lying 2-TU singlet excited state, placed 3:65 eV (l ¼ 4:68 D) above the ground state. It is derived from the ground-state by a single excitation from the nS orbital, localized on the sulfur atom, to a p delocalized orbital in the neighborhood of the sulfur atom (around the C3  C2  N1 atoms). The T3 3 ðpp Þ state (Table 1), at 3:67 eV (l ¼ 3:75 D) vertically above the ground state, has a stronger multiconfigurational nature with the dominant electronic configuration derived from the ground state by a single p ! p excitation, involving molecular orbitals localized on the ring; two other configurations, singly excited from the pS orbital localized on sulfur, contribute to the description of the T3 3 ðpp Þ state. The next two excited states at the Franck–Condon (Table 1) region exhibit 1 ðpp Þ character. In relation to the ground-state, the S2 state is best described by a singly excited configuration derived by the pS ! p excitation (4:09 eV, f ¼ 0:236), with the p molecular orbital delocalized over the C4  C5  C6 atoms. Thus, the S0 ! S2 transition involves a charge-transfer transition to the heterocyclic ring, resulting in a larger dipole moment on the S2 state (l ¼ 6:00 D) in comparison to the ground state (4:45 D). The S3 state wavefunction is also dominated by the pS ! p single excitation, but the p is delocalized over the N1, C2, and N3 atoms, that is, in sulfur atom neighborhood. Transition to the S3 1 ðpS p Þ is predicted to carry most of the intensity (f ¼ 0:445). The two bands observed experimentally can be attributed to the S2 and S3 states computed by us. In agreement with the experimental result, solvatochromic shifts would be larger in the long than in the short wavelength absorption band, since the S2 state has a higher dipole moment (6:00 D) than the ground state (4:45 D), which corresponds to the transition to our computed S3 state (4:83 D). The energetic ordering of the states at the Franck–Condon region reported in the previous study [18] differs from that computed by us (Table 1), probably due to symmetry restrictions imposed in the earlier calculations. Cui and Fang [18] computed a single 1 ðpp Þ delocalized around the C@S double bound, between the S1 1 ðnS p Þ and S4 1 ðnO p Þ 2-TU excited states. Our results, however, place the S2 1 ðpS p Þ and S3 1 ðpS p Þ between them. The electronic structure of the 1 ðpp Þ reported by Cui and Fang [18] compares to the S3 state computed by us, which means that they were not able to reproduce the broad and medium intensity band observed experimentally in the 300–250 nm region. The T4 3 ðnO p Þ and S4 1 ðnO p Þ (Table 1) states have been computed to be, respectively, 4.57 eV and 4.59 eV above the ground state at the Franck–Condon region. As they are located in a higher energetic region, they were not taken into account for describing the deactivation paths of 2-TU. To compute the band origins (Table 1), the minima of the singlet and triplet states have been optimized. In relation to the previous

3

computed values [18], a pronounced blue-shift is observed for all states amounting to 0.58 eV for the 3 ðpp Þ state. According to our results, the band origins for the 3 ðnS p Þ and 1 ðnS p Þ are predicted to be 0:17 eV apart and the 3 ðpp Þmin is stabilized by 0:70 eV with respect to 1 ðpS p Þmin . 3.2. Deactivation pathways and triplet states population To guarantee an efficient population of triplet states, small singlet–triplet energy gaps and large spin–orbit coupling (SOC) between the corresponding singlet and triplet states are required. According to El-Sayed rules [37,38], spin–orbit coupling is expected to be large between pp and np states (two states with different spatial symmetry). On the other hand, spin–orbit coupling is expected to be small between pp and pp states or between np and np states (two states with the same spatial symmetry). Since the spectroscopic bright state is of 1 ðpp Þ nature, large SOC is expected between a low-lying 1 ðnp Þ state, on which n represents the nonbonding orbital on the heterogen atom, and a 3 ðpp Þ state, indicating that the first relevant event is an ultrafast radiationless decay from the spectroscopic 1 ðpp Þ state to the low-lying 1 ðnp Þ. We have therefore followed the main energy path events starting from the Franck–Condon absorption region and evolving along the MEP related to the bright spectroscopic state, on which the most relevant crossings have been located. In the case of 2-TU, the most relevant photoinduced events begin with the absorption of the energy to the bright S2 1 ðpS p Þ singlet state, computed vertically at 4:09 eV, with the corresponding transition associated with an oscillator strength of about 0:2. It is worth recalling that the spectroscopic state considered previously by Cui and Fang [18] compares to the S3 state here reported. Nonetheless, according to Kasha’s rule [39], the lowest singlet state is generally responsible for the observed fluorescence. Exploratory MEP calculation from the Franck–Condon region along the S3 state leads to a conical

Fig. 3. Evolution of the low-lying singlet and triplet excited states of 2-thiouracil along the 1 ðpS p Þ MEP, computed at the CASPT2//CASSCF level, from the Franck– Condon ground state geometry. Full lines with filled symbols indicate singlet states (S0 : j, 1 ðnS p Þ: , 1 ðpS p Þ ). Dashed lines with open symbols indicate triplet states (3 ðpS p Þ: , 3 ðnS p Þ: , 3 ðpS p Þ: ).

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Fig. 4. Structural illustration of the (a) ground state minimum, (b) 1 ðpS p Þmin , and (c) ð1 pS p =GSÞCI .

intersection involving the S2 and S3 states at the Franck–Condon region, indicating that even if the energy is absorbed by system bringing the population to the S3 state, the population would be transferred very efficiently to the S2 state immediately after excitation. Evolution of the low-lying singlet and triplet states along the MEP on the 1 ðpS p Þ state hypersurface is displayed in Fig. 3. A careful analysis of the initial region of the MEP reveals an ISC between the 1 ðpS p Þ and the 3 ðpp Þ (corresponding to the T3 at the Franck– Condon region) states. Despite the small energy gap, this is not an ISC favorable region because it involves states of same spatial symmetry (pp states). In addition, we have found a conical intersection ðð1 pS p =1 nS p ÞCI Þ with the 1 ðnS p Þ state, opening the possibility for a partial population transfer to the 1 ðnS p Þ state. The next feature observed in the MEP on the 1 ðpS p Þ state is its barrierless evolution to the 1 ðpS p Þ minimum energy region (rep-

resented by the 1 ðpS p Þmin structure), on which the 1 ðnS p Þ and 3 ðnS p Þ states are predicted to be degenerated, indicating that the 1 ðpS p Þ minimum energy region is prone to ISC to the 3 ðnS p Þ (via the ð1 pS p =3 nS p ÞISC intersystem crossing) and transfer to 1 ðnS p Þ (via the ð1 pS p =1 nS p ÞCI conical intersection) states. Based on these findings, the study of two possible ISC mechanisms from the 1 ðpS p Þmin structure was also undertaken: (1) the MEP on the 1 ðnS p Þ and (2) the MEP on the 3 ðnS p Þ potential energy surfaces. It is worth mentioning that a ð1 pS p =GSÞCI conical intersection, similar to the ethylene-like conical intersection found in uracil (Fig. 4), whose accessibility was evaluated by linearly interpolated paths connecting the 1 ðpS p Þ to the ð1 pS p =GSÞCI structure from (i) the Franck–Condon region and (ii) from the 1 ðpS p Þmin structure. As can be seen in Fig. 5, in both cases it is necessary to overcome an energetic barrier, hampering the access to the ð1 pS p =GSÞCI . A closer analysis of the structures can elucidate the origin of the

Fig. 5. Evolution of the ground, 1 ðpS p Þ, and 1 ðnS p Þ states along the linearly interpolated paths connecting the 1 ðpS p Þ to the ð1 pS p =GSÞCI structure from (a) the Franck– Condon region and (b) from the 1 ðpS p Þmin minimum structure region

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Fig. 7. 1 ðnS p Þ minimum energy structure optimized at the (a) CASSCF level and (b) last point along the MEP on the 1 ðnS p Þ potential energy surface exhibits a slightly distorted structure.

Fig. 6. Evolution of the low-lying singlet and triplet excited states of 2-thiouracil along the 1 ðnS p Þ MEP, computed at the CASPT2//CASSCF level, from the 1 ðpS p Þmin region.

energetic barriers. The 1 ðpS p Þmin structure, obtained as the last point in the MEP on its potential energy surface exhibits a slightly distorted structure (Fig. 4), in comparison to the ground state; the dihedral angle C4  C5  C6  H, involving the C5 ¼ C6 double bond, amounts to 155:6 , but the six-membered ring is quasi planar with the N3 and C6 atoms exhibiting a small out-of-plane distortion (N3  C4  C5  C6 ¼ 4:3 ). Moreover, at the 1 ðpS p Þmin structure, the 1 ðpS p Þ state is described mainly by the pS ! p singly excitated configuration, with the p orbital delocalized over the C4  C5  C6 atoms, which elongates the C2  S ¼ 1:753 Å and C5  C6 ¼ 1:430 Å bond lengths in comparison to the ground-state equilibrium geometry (1.646 Å and 1.343 Å, respectively). The C4  C5 is shortened by 0.075 Å and the C4  O is elongated by 0.016 Å respectively. On the other hand, the ð1 pS p =GSÞCI (Fig. 4) exhibits a much more distorted geometry. The dihedral angle C4  C5  C6  H is 126.0° and the N3  C4  C5  C6 ¼ 37:4 , indicating a much more distorted ring. It is also interesting to note that in uracil the 1 ðpp Þ state also relaxes into a distorted configuration (boat-like deformation) [40–43]. The photophyscial behavior of the 1 ðpS p Þ spectroscopic state of 2-TU contrasts to that observed in uracil, on which the deactivation to the 1 ðpp Þ can be explained by two mechanisms: (i) a barrierless path along the 1 ðpp Þ toward the ethene-like conical intersection [41,42,44], which is responsible for the short time constant observed experimentally [45] and (ii) relaxation to the 1 ðpp Þmin stationary point, from which it evolves to the lower lying 1 ðpp Þ state via conical intersection [46], explaining the experimentally observed larger time constant [47,48,45]. The step to be described now is related to the nonradiative decay to the 1 ðnS p Þ state, from the ð1 pS p =1 nS p ÞCI structure. The minimum energy path computed on the 1 ðnS p Þ state from the 1 ðpS p Þmin region is displayed in Fig. 6. The 1 ðnS p Þ state relaxes barrierless to the 1 ðnS p Þ minimum energy region, represented by

the 1 ðnS p Þmin structure, where an intersystem crossing with the lowest lying 3 ðpS p Þ state takes place. The 1 ðnS p Þmin minimum structure obtained as the last point in the MEP on the 1 ðnS p Þ potential energy surface is distorted (Fig. 7), with a dihedral angle C4  C3  C2  S ¼ 165:8 . At the 1 ðnS p Þmin region, the 1 ðnS p Þ state is best described by a singly excited electronic configuration involving an excitation from the nS orbital to the p antibonding orbital delocalized over the C3  C2  N1 atoms, the C2  N3 ; N1  C2 , and C2  S bond lengths are elongated (0:047 Å, 0.047 Å, and 0.154 Å, respectively) in relation the ground-state minimum structure. Other feature of the MEP on the 1 ðnS p Þ potential energy surface (Fig. 6) is the barierless evolution of the 3 ðpS p Þ and 3 ðnS p Þ states to the 1 ðnS p Þ minimum energy region, represented by the 1 ðnS p Þmin structure, where they are isoenergetic. Two intersystem crossing minimum energy structure were indeed computed in this region, that is the ð3 pS p =1 nS p ÞISC and ð3 nS p =1 nS p ÞISC structures,

Fig. 8. Evolution of the low-lying singlet and triplet excited states of 2-thiouracil along the 3 ðnS p Þ MEP, computed at the CASPT2//CASSCF level, from the 1 ðpS p Þmin region.

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with energy gaps between the 1 ðnS p Þ and the 3 ðpp Þ and 3 ðnS p Þ states amounting to, respectively, 3.9 kcal/mol and 1.2 kcal/mol, with the corresponding SOCs amounting to 275 cm1 and 37 cm1, respectively. Based on these findings, we conclude that the transfer of the population from the 1 ðnS p Þ to the 3 ðpS p Þ is favorable around the 1 ðnS p Þmin region. The other deactivation possibility has been explored by computing the MEP on the 3 ðnS p Þ potential energy surface from the 1 ðpS p Þmin region, depicted in Fig. 8. As it can be noticed, in this region the energy gap (DEST ) between the 3 ðnS p Þ and the 1 ðpS p Þmin is computed to be 1:1 kcal/mol at the CASPT2 level, with a very high spin–orbit coupling (SOC ¼ 188 cm1), favouring the population transfer to the 3 ðnS p Þ state. As can be noticed in Fig. 8, the 3 ðnS p Þ state evolves to the 3 ðnS p Þmin minimum energy structure region, where a conical intersection connecting the 3 ðnS p Þ and 3 ðpp Þ (ð3 nS p =3 pp ÞCI ), located 3:33 eV adiabatically, allows population transfer to the 3 ðpS p Þ state, which evolves to the 3 ðpS p Þmin minimum energy structure. It is worth noting that our results compare to the previous [18] description, except the path originating at the intersystem crossing between the bright ð1 pS p Þ state and a higher 3 ðnp Þ triplet state. A summary of the proposed deactivation mechanism of 2-TU is displayed in Fig. 9. 3.3. Comparison with related compounds The 2-TU deactivation pathways and triplet states population described above can be compared with those proposed for uracil and 6-azauracil. According to Climent et al. [36], the T1 3 ðpp Þ lowest triplet state of uracil can be populated by two paths. One beginning with population transfer from the bright 1 ðpp Þ state to the 3 ðnO p Þ state, with a high spin–orbit interaction (SOC ¼ 25 cm1), from which it is transferred to the T1 3 ðpp Þ state via the ð3 nO p =3 pp ÞCI conical intersection. A secondary mechanism, based on a direct population transfer from the spectroscopic bright 1 ðpp Þ state to the T1 state was also described, but due to the small

computed spin–orbit interaction (SOC ¼ 1 cm1) it is considered to be of minor importance. In the present study, we have proposed two competitive mechanisms for populating the lowest 2-TU 3 ðpp Þ state. In the first one, the conical intersection ð1 pS p =1 nS p ÞCI is responsible for population transfer from the bright 1 ðpS p Þ state to the dark 1 ðnS p Þ state, from which the lowest 3 ðpp Þ state is populated via the ð1 np =3 pp ÞISC with SOC ¼ 275 cm1. In the other one, the first event involves the population transfer from the spectroscopic ð1 pS p Þ active state to the 3 ðnS p Þ state, via the ð1 pS p =3 nS p ÞISC intersystem crossing (SOC ¼ 188 cm1), from which the population is transferred to the lowest 3 ðpp Þ state via the ð3 nS p =3 pp ÞCI conical intersection. As can be noticed, these pathways compare to those proposed by us for 6-azauracil, but with larger spin–orbit couplings. Based on our description, the mechanism proposed for uracil is enhanced in 2-TU, since the ð1 pp =3 np ÞISC present in both nucleobases has a much larger spin–orbit interaction in 2-TU. Interestingly, in the case of 2-TU, the related pp and np states are derived by exciting electrons from orbitals localized exclusively on the sulfur atom. The photophysics and photochemistry of 2-TU represents a nice example of the heavy-atom effect. Other comparison can be made considering the 6-azauracil, an aza-substituted nucleobase exhibiting a high quantum yield of intersystem crossing, for which two possible mechanisms have been earlier proposed [14]. The first involves a conical intersection between the bright 1 ðpp Þ and the lowest 1 ðnp Þ states (ð1 pp =1 np ÞCI ), followed by population transfer from the 1 ðnp Þ state to the 3 ðpp Þ triplet state via a singlet–triplet crossing (1 ðnp =3 pp ÞISC ) (SOC ¼ 64 cm1). This nonadiabatic pathway is similar to that proposed for uracil, but it is predicted much more efficiently due to the relatively larger computed SOC. Furthermore, the population transfer from the bright 1 ðpp Þ state to the 3 ðnp Þ in 6-azauracil can occur via the ð1 pp =3 np ÞSTC (SOC ¼ 16 cm1) singlet–triplet crossing and, from the 3 ðnp Þ state, the lowest-lying 3 ðpp Þ state is populated via the ð3 np =3 pp ÞCI . The mechanisms are the same as those proposed here for 2-TU and the heavy-atom effect is noticed again, playing a fundamental role. 6-Aza-2-thiothymine, other thiated pyrimidine, whose nonadiabatic mechanisms of population of the lowest excited triplet states were recently proposed by us [12], can also be considered. After excitation to the spectroscopic 1 ðpp Þ state, two possible triplet state population mechanisms were described: (i) an indirect path relaxing along the 1 ðpp Þ state to the ð1 pp =1 np ÞCI conical intersection, with population transfer to the 1 ðnp Þ state, and subsequent population of the 3 ðpp Þ triplet state via the ð1 np =3 pp ÞSTC (SOC ¼ 169 cm1) intersystem crossing and (ii) a path on the 1 ðpp Þ state relaxing to the ð1 pp =3 pp ÞSTC (SOC ¼ 130 cm1), involving a higher triplet state, followed by population transfer to the lowest triplet state via the ð3 pp =3 pp ÞCI conical intersection. Thus, we can also find a similarity between the proposed mechanisms for triplet population of 6A2TT and 2-TU with, with enhancement spin–orbit coupling due to heavy atom effect. It is also worth mentioning that similar deactivation pathways and triplet states population have been proposed for 6-thioguanine by Martínez–Fernández et al. [49,50].

4. Conclusions

Fig. 9. 2-Thiouracil mechanisms.

deactivation

pathways

and

triplet

states

population

The photochemical reaction path approach and the CASPT2// CASSCF protocol were employed to probe the 2-TU deactivation pathways and triplet states population mechanisms. According to our results, two mechanisms lead to the population of the T1 3 ðpS p Þ state: (i) the first event is the population transfer from

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the bright to the 1 ðnS p Þ state, from which the T1 3 ðpS p Þ can be reached through the ð1 nS p =3 pS p ÞISC , due to a very strong spin–orbit coupling (SOC ¼ 275 cm1) and (ii) due to a strong spin–orbit coupling between the bright and the 3 ðnS p Þ states ð1 pS p =3 nS p ÞISC (SOC ¼ 188 cm1), the 3 ðnS p Þ state is populated and the T1 3 ðpS p Þ state is reached by the ð3 nS p =3 pS p ÞCI conical intersection. As the pp and np states have a well-defined localized character on the sulfur atom, the heavy-atom effect is pronounced, resulting in stronger spin–orbit couplings. Acknowledgements J.P.G. thanks Fundação de Amparo á Pesquísa do Estado de São Paulo (FAPESP, Project 2010/16043-2) for financial support. A.C.B. thanks continuous academic support from Conselho Nacional de Desenvolvimento Cientifíco Tecnológico (CNPq) and the Laboratory of Advanced Scientific Computation (LCCA) of the University of São Paulo. The authors also thank the support by Spanish MINECO. References [1] A. Favre, C. Saintome, J.L. Fourrey, P. Clivio, P.J. Laugaa, Thionucleobases as intrinsic photoaffinity probes of nucleic acid structure and nucleic acid-protein interactions, Photochem. Photobiol. B 42 (1998) 109–124. [2] C. Ritter, G. Jedlitschky, H. Schwabedissen, M. Grube, K. Kock, H. Kroemer, Cellular export of drugs and signaling molecules by the ATP-binding cassette transporters MRP4 (ABCC4) and MRP5 (ABCC5), Drug Metab. Rev. 37 (2005) 253–278. [3] R. Martin, J. Schneller, A. Stahl, G. Dirheimer, Studies of odd bases in yeast mitochondrial tRNA: Ii. Characterization of rare nucleosides, Biochem. Biophys. Res. Commun. 70 (1976) 997–1002. [4] M. Altwegg, E. Kubli, The nucleotide sequence of glutamate TRNA4 of drosophila melanogaster, Nucl. Acids Res. 8 (1980) 215–223. [5] Z. Wang, T.M. Rana, RNA conformation in the TAT-TAR complex determined by site-specific photo-cross-linking, Biochemistry 35 (1996) 6491–6499. [6] M.-Y.W. Yu, J. Sedlak, R.H. Lindsay, Metabolism of the nucleoside of 2 thiouracil (2-thiouridine) by rat liver slices, Arch. Biochem. Biophys. 155 (1973) 111– 119. [7] B.N. Singh, U.P. Singh, R. Ghose, A.K. Ghose, Complexes of 2-thioracil with some divalent metal ions, Asian J. Chem. 5 (1993) 262–265. [8] M.S. Masoud, O.H.A. El-Hamid, Z.M. Zaki, 2-Thiouracil-based cobalt(II), nickel(II) and copper(II) complexes, Transit. Metal Chem. 19 (1994) 21–24. [9] M.A. Basinger, J. Casas, M.M. Jones, A.D. Weaver, N.H. Weinstein, Structural requirements for Hg(II) antidotes, J. Inorg. Nucl. Chem. 43 (1981) 1419–1425. [10] B.S. Larsson, B. Larsson, A. Roberto, Boron neutron capture therapy for malignant melanoma: an experimental approach, Pigm. Cell Res. 2 (1989) 356–360. [11] F. Waetjen, O. Buchardt, E. Langvad, Affinity therapeutics. 1. Selective incorporation of 2-thiouracil derivatives in murine melanomas. Cytostatic activity of 2-thiouracil arotinoids, 2-thiouracil retinoids, arotinoids, and retinoids, J. Med. Chem. 25 (1982) 956–960. [12] J.P. Gobbo, A.C. Borin, On the population of triplet excited states of 6-aza-2thiothymine, J. Phys. Chem. A 117 (2013) 5589–5596. [13] J.P. Gobbo, A.C. Borin, On the mechanisms of triplet excited state population in 8-azaadenine, J. Phys. Chem. B 116 (2012) 14000–14007. [14] J.P. Gobbo, A.C. Borin, L. Serrano-Andrés, On the relaxation mechanisms of 6azauracil, J. Phys. Chem. B 115 (2011) 6243–6251. [15] C. Párkányi, C. Boniface, J.-J. Aaron, M. Gaye, R. Ghosh, L. Szentpaly, K.S. RaghuVeer, Electronic absorption and fluorescence spectra and excited singlet-state dipole moments of biologically important pyrimidines, Struct. Chem. 3 (1992) 277–289. [16] H. Moustafa, M.F. Shibl, R. Hilal, Electronic absorption spectra of some 2thiouracil derivatives, Phosphorus, Sulfur, Silicon Relat. Elem. 180 (2005) 459– 478. [17] A. Khvorostov, L. Lapinski, H. Rostkowska, M.J. Nowak, UV-induced generation of rare tautomers of 2-thiouracils: a matrix isolation study, J. Phys. Chem. A 109 (2005) 7700–7707. [18] G. Cui, W.-h. Fang, State-specific heavy-atom effect on intersystem crossing processes in 2-thiothymine: a potential photodynamic therapy photosensitizer, J. Chem. Phys. 138 (2013) 044315. [19] M. Klessinger, Conical intersections and the mechanism of singlet photoreactions, Ang. Chem. Int. Ed. Eng. 34 (1995) 549–551. [20] A. Giussani, M. Merchán, D. Roca-Sanjuán, R. Lindh, Essential on the photophysics and photochemistry of the indole chromophore by using a totally unconstrained theoretical approach, J. Chem. Theory Comput. 7 (2011) 4088–4096. [21] M. Klessinger, J. Michl, Excited States and Photochemistry of Organic Molecules, VCH Publishers, Inc., New York, 1995.

7

[22] P.-O. Widmark, P.-Å. Malmqvist, B.O. Roos, Density matrix averaged atomic natural orbital (ANO) basis sets for correlated molecular wave functions. I. First row atoms, Theor. Chem. Acc. 77 (1990) 291–306. [23] F. Aquilante, L. De Vico, N. Ferré, G. Ghigo, P.-Å. Malmqvist, P. Neogrády, T.B. Pedersen, M. Pitonak, M. Reiher, B.O. Roos, L. Serrano-Andrés, M. Urbán, V. Veryazov, R. Lindh, Software news and update molcas 7: the next generation, J. Comput. Chem. 31 (2010) 224–247. [24] F. Aquilante, T.B. Pedersen, V. Veryazov, R. Lindh, Molcas: a software for multiconfigurational quantum chemistry calculations, WIREs Comput. Mol. Sci. 3 (2013) 143–149. [25] B.O. Roos, The complete active space self-consistent field method and its applications in electronic structure calculations, in: K.P. Lawley (Ed.), Adv. Chem. Phys.: Ab Initio Methods in Quantum Chemistry – II, 69, John Wiley & Sons Ltd., Chichester, England, 1987, pp. 399–446. [26] L. De Vico, M. Olivucci, R. Lindh, J. Chem. Theory Comp. 1 (2005) 1029. [27] K. Andersson, P.-Å. Malmqvist, B.O. Roos, A.J. Sadlej, K. Wolinski, J. Phys. Chem. 94 (1990) 5483. [28] K. Andersson, P.-Å. Malmqvist, B.O. Roos, 2nd-Order perturbation-theory with a complete active space self-consistent field reference function, J. Chem. Phys. 96 (1992) 1218–1226. [29] B.O. Roos, K. Andersson, M.P. Fülscher, P.-Å. Malmqvist, L. Serrano-Andrés, K. Pierloot, M. Merchán, Multiconfigurational perturbation theory: applications in electronic spectroscopy, in: I. Prigogine, S.A. Rice (Eds.), Adv. Chem. Phys.: New Methods in Computational Quantum Mechanics, vol. 93, Wiley, New York, 1996, pp. 219–332. [30] M. Merchán, L. Serrano-Andrés, M. Füscher, B.O. Roos, Multiconfigurational perturbation theory applied to excited states of organic compounds, in: K. Hirao (Ed.), Recent Advances in Multireference Methods, vol. 4, World Scientific Publishing Company, Amsterdam, 1999, pp. 161–195. [31] M. Merchán, L. Serrano-Andrés, Ab initio methods for excited states, in: M. Olivucci (Ed.), Computational Photochemistry, Elsevier, Amsterdam, 2005, pp. 35–91. [32] L. Serrano-Andrés, M. Merchán, Photostability and photoreactivity in biomolecules: quantum chemistry of nucleic acid base monomers and dimers, in: M. Shukla, J. Leszczynski (Eds.), Radiation Induced Molecular Phenomena in Nucleic Acid: A Comprehensive Theoretical and Experimental Analysis, vol. 5, Springer, Berlin, 2008, pp. 435–472. [33] L. Serrano-Andrés, M. Merchán, Are the five natural DNA/RNA base monomers a good choice from natural selection? A photochemical perspective, J. Photochem. Photobiol. C 10 (2009) 21. [34] G. Ghigo, B.O. Roos, P.-Å. Malmqvist, A modified definition of the zeroth-order hamiltonian in multiconfigurational perturbation theory (CASPT2), Chem. Phys. Lett. 396 (2004) 142. [35] W.C. Schneider, I.F. Halverstadt, The dipole moments of thiouracil and some derivatives, J. Am. Chem. Soc. 70 (8) (1948) 2626–2631. [36] T. Climent, R. González-Luque, M. Merchán, L. Serrano-Andrés, On the intrinsic population of the lowest triplet state of uracil, Chem. Phys. Lett. 441 (2007) 327–331. [37] M.A. El-Sayed, Spin–orbit coupling and the radiationless processes in nitrogen heterocyclics, J. Chem. Phys. 38 (12) (1963) 2834–2838. [38] M.A. EL-Sayed, Triplet state – its radiative and nonradiative properties, Acc. Chem. Res. 1 (1968) 8–16. [39] M. Kasha, Characterization of electronic transitions in complex molecules, Discuss. Faraday Soc. 9 (1950) 14–19. [40] M. Shukla, P. Mishra, A gas phase ab initio excited state geometry optimization study of thymine, cytosine and uracil, Chem. Phys. 240 (1999) 319–329. [41] S. Matsika, Radiationless decay of excited states of uracil through conical intersections, J. Phys. Chem. A 108 (37) (2004) 7584–7590. [42] M.Z. Zgierski, S. Patchkovskii, T. Fujiwara, E.C. Lim, On the origin of the ultrafast internal conversion of electronically excited pyrimidine bases, J. Phys. Chem. A 109 (2005) 9384–9387. [43] S. Yamazaki, T. Taketsugu, Nonradiative deactivation mechanisms of uracil, thymine, and 5-fluorouracil: a comparative ab initio study, J. Phys. Chem. A 116 (2012) 491–503. [44] M. Merchán, R. Gonzalez-Luque, T. Climent, L. Serrano-Andrés, E. Rodriuguez, M. Reguero, D. Pelaez, Unified model for the ultrafast decay of pyrimidine nucleobases, J. Phys. Chem. B 110 (2006) 26471–26476. [45] C. Canuel, M. Mons, F. Pluzzi, B. Tardivel, I. Dimicoli, M. Elhanine, J. Chem. Phys. 122 (2005) 074316. [46] D. Nachtigallová, A.J.A. Aquino, J.J. Szymczak, M. Barbatti, P. Hobza, H. Lischka, Nonadiabatic dynamics of uracil: population split among different decay mechanisms, J. Phys. Chem. A 115 (2011) 5247–5255. [47] H. Kang, K.T. Lee, B. Jung, Y.J. Ko, S.K. Kim, J. Am. Chem. Soc. 124 (2002) 12958. [48] S. Ulrich, T. Schultz, M.Z.Z.A. Stolow, Electronic relaxation dynamics in DNA and RNA bases studied by time-resolved photoelectron spectroscopy, Phys. Chem. Chem. Phys. 6 (2004) 2796–2801. [49] L. Martínez-Fernández, I. Corral, G. Granucci, M. Persico, Competing ultrafast intersystem crossing and internal conversion: a time resolved picture for the deactivation of 6-thioguanine, Chem. Sci. 5 (2014) 1336–1347. [50] L. Martínez-Fernández, L. González, I. Corral, An ab initio mechanism for efficient population of triplet states in cytotoxic sulfur substituted DNA bases: the case of 6-thioguanine, Chem. Commun. 48 (2012) 2134–2136.

Please cite this article in press as: J.P. Gobbo, A.C. Borin, Comput. Theoret. Chem. (2014), http://dx.doi.org/10.1016/j.comptc.2014.03.021