On The Population of Triplet Excited States of 6-Aza-2-Thiothymine

Article pubs.acs.org/JPCA

On The Population of Triplet Excited States of 6‑Aza-2-Thiothymine Joaõ Paulo Gobbo* and Antonio Carlos Borin Instituto de Química, Universidade de São Paulo, and NAP-PhotoTech, the USP Consortium for Photochemical Technology, Av. Prof. Lineu Prestes 748, 05508-900 São Paulo, São Paulo, Brazil S Supporting Information *

ABSTRACT: The mechanisms of population of the lowest excited triplet states of 6-aza2-thiothymine were investigated by means of CASPT2//CASSCF quantum-chemical calculations, with extensive atomic natural orbital basis sets of double-ζ quality (ANO-LVDZP). Several key structures corresponding to equilibrium geometries, surface crossings, minimum energy paths, and linear interpolation in internal coordinates were used to explain the ability to sensitize molecular oxygen. After population of the S2 1(ππ*) state, the system evolves to the state minimum. At this point, and along the minimum energy path of the 1(ππ*) state, two main mechanisms related to the triplet and singlet manifolds can be visualized, leading the system to the lowest triplet state, T1 3(ππ*).

1. INTRODUCTION The photophysical and photochemical properties of biologically relevant naturally occurring nucleobases have received considerable attention due to the need for a better understanding of the mechanisms that prevent the photodamage of DNA caused by the absorption of UV radiation.1−6 Five natural nucleobases exhibit excited state lifetimes on the femtosecond to picosecond time scale, which confers a high photostability to them.7 The low fluorescence quantum yields of the isolated nucleobases, attributed to very efficient nonradiative decay mechanisms, have been explained in terms of internal conversions through conical intersections,8 connecting the brightest 1(ππ*) excited state hypersurface, populated initially by direct absorption of UV radiation, to S1 or S0.3,9−12 The photostability of the natural nucleobases is sensitive to structural modifications, either by tautomerization3,13 or by substitution of atoms (or groups of atoms) of the molecular skeleton,14−19 which can result in species with distinct photochemical and photophysical properties, as for instance high fluorescence quantum yields. From the molecular point of view, these changes impose restrictions on the access to the efficient deactivation pathways present in the biologically relevant analogs.13 As an example, we can compare the photophysical properties of adenine and 2-aminopurine (2AP).3 Unlike adenine, with a very low fluorescence emission quantum yield (ΦF = 2.6 × 10−4),20 2AP is a highly fluorescent species, being employed as a probe in DNA.21,22 Aza- and thio-substituted nucleobases are obtained by substitution of atoms from the natural molecular skeleton by nitrogen or sulfur atoms, respectively. Some of the substituted nucleobases can be used as photosensitizers in photodynamic therapy (PDT)23−25 because they have a high quantum yield population of the triplet state (ΦISC).14−17 Kobayashi et al.15 studied the dynamics of the excited states of four aza analogues © 2013 American Chemical Society

of nucleobases in acetronitrile and proposed the existence of a low-lying and dark 1(nπ*) state below the spectroscopically active 1(ππ*) state as a condition for a high ΦISC, as observed in 8-azaadenine (8-AA) and 6-azauracil (6-AU), while the remaining species (5-azacytosine and 8-azaguanine), according to them, exhibit very low ΦISC due to the absence of this state. In relation to 6-AU,19 our group showed that at least two paths can lead to the lowest T1 3(ππ*) state, the most probable being related to the transfer of the population to the dark 1(nπ*) state described by Kobayashi et al.,15,17 via a conical intersection between this state and the bright 1 (ππ*) state, i.e., (1(ππ*)/1(nπ*))CI, followed by an intersystem crossing to the lowest triplet excited state, (1(nπ*)/3(ππ*))STC. Recently, we described26 the mechanism of triplet excited state population in 8-azaadenine, besides proposing a mechanism for the photoproduction of singlet oxygen (1Δg). Aspects related to thio substitution on the design of photosensitizers and the dynamics of the nucleobase excited states were also studied by Suzuki et al., who investigated the properties of 4-thiothymidine14 (S4-TdR) and 2-thiothymine16 (2TT) by means of experimental and computational methods. The authors proposed similar deactivation paths for both species, starting in the Franck−Condon region with direct population of the S2 1(ππ*) state by absorption of UV radiation, followed by internal conversion to a lower S1 1(nπ*) state, and then an intersystem crossing to a lower 3(ππ*) state with ΦISC = 1.0. It should be mentioned that, although 2TT has a high value of ΦISC, it is only a moderate generator of 1Δg O2 (1O2), as it is suggested by its quantum yield of singlet oxygen Received: April 9, 2013 Revised: June 14, 2013 Published: June 18, 2013 5589

dx.doi.org/10.1021/jp403508v | J. Phys. Chem. A 2013, 117, 5589−5596

The Journal of Physical Chemistry A

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stationary points through minimum energy paths (MEP) and linear interpolation in internal coordinates (LIIC).

generation (ΦΔ) of 0.36. According to the authors a partial contamination of the 3(ππ*) state by n,π* character must be the reason for such a low value of ΦΔ. Suzuki et al. also explored the combined effects of aza and thio substitutions on the dynamics of the excited state of 6-aza2-thiothymine16 (6A2TT; Figure 1) by means of absorption

2. METHODOLOGY All calculations were performed with MOLCAS-7.6 software,32 employing atomic natural orbital basis sets with double-ξ plus polarization functions (ANO-L-VDZP).33 Equilibrium geometries and optimization of most relevant structures, such as conical intersections and singlet−triplet crossings, were obtained at the CASSCF28 level, without imposing any symmetry constraint (C1 point group). Dynamic correlation effects at the optimized geometries were included using the second order multiconfigurational perturbation theory (CASPT2)29 method. Based on tests calculations, intruder states were treated with an imaginary level shift of 0.2 au;34 no IPEA shift35 was used, to be consistent with previous studies. The theoretical approach employed here, the so-called CASPT2//CASSCF protocol, with the single state CASPT2 (SS-CASPT2) approach, has proved to be accurate for this kind of study.19,26,27,30 Spin−orbit coupling (SOC) elements were computed within the AMFI (atomic mean field integrals) framework, as described previously.36 All optimizations and energy point calculations were carried out with a CASSCF reference wave functions constructed by distributing 16 electrons in 11 orbitals (CAS(16,11)), which consisted of all π and π* orbitals and electrons, except the one from the linear combination of the hydrogen orbitals from the methyl group (which was necessary to stabilize the active space for computing the minimum energy paths), plus one lone-pair orbital from the nitrogen, oxygen, and sulfur atoms (three lone pairs in total). The singlet and triplet states were computed by averaging over the lowest 7 states with equal weights, which guarantees all necessary states are included. At all stationary points, CASPT2 calculations on several singlet and triplet states were carried out, as described above. Minimum energy paths on the 1(ππ*) excited state surfaces were obtained at the CAS(16,11) level, with the MüllerBrown37 steepest descendent algorithm in mass-weighted coordinates (in bohr (amu)1/2), which is equivalent to the intrinsic reaction coordinate (IRC). Whenever MEPs calculations were not possible, the evolution of the electronic states along the path connecting the two structures of interest was studied with linear interpolations in internal coordinates. Conical intersections not obtained along the MEPs were computed at the CASSCF level by imposing the restriction of degeneracy between the two states considered, employing the restricted Lagrange multipliers technique38 and nonadiabatic coupling elements were not computed. At the obtained points, CASPT2 calculations on several singlet and triplet states were carried out as described above.

Figure 1. Schematic structure and labeling of 6-aza-2-thiothymine (6A2TT).

and emission spectroscopy, nanosecond transient absorption, time-resolved phosphorescence detection, and DFT calculations. According to them, after 308 nm laser irradiation, 6A2TT is capable of producing singlet oxygen (1O2) with high efficiency (ΦΔ = 0.69), comparable to that observed for 6AU (ΦΔ = 0.63 of 6-AU).17 No emission was observed, indicating a quantum yield of fluorescence (ΦF) smaller than 10−4; however, a structured phosphorescence band was detected and attributed to the lowest T1 3(ππ*) state, with an estimated triplet energy of 21 300 cm−1. Transient absorption spectroscopy showed the existence of a long-lived transient with a lifetime of 25.6 μs and insensitive to O2 and TEMPO (2,2,6,6-tetramethyl-piperidinyloxyl) quenching. Although the authors stated that this transient could be formed by a bimolecular reaction between a molecule in a triplet excited state (36A2TT*) and a ground state molecule, they did not observe any indication of the presence of a dimer and it was assumed that a hydrogen abstracted 6A2TT would be the responsible for the appearance of such transient. In this contribution, the mechanisms of population of the triplet state of 6-aza-2-thiothymine and other aspects related to 1 O2 generation were studied with the CASPT2//CASSCF protocol,27−31 as we have done for other systems.19,26 To this end, deactivation paths were constructed by connecting

Table 1. Bond Distances (Å) for the Ground and Selected Excited States of 6-Aza-2-Thiothymine bond distances S0 ref 16 S1 (1nπ*) S2 (1ππ*) T1 (3ππ*) T2 (3ππ*) T3 (3nπ*)

N1−C2

C2−N3

N3−C4

C4−C5

C5−N6

N6−N1

C5−C7

C4−O8

C2−S9

1.359 1.36 1.412 1.306 1.361 1.400 1.415

1.371 1.37 1.414 1.318 1.367 1.372 1.414

1.375 1.39 1.380 1.447 1.388 1.376 1.375

1.481 1.48 1.492 1.409 1.438 1.453 1.495

1.289 1.30 1.295 1.358 1.455 1.346 1.296

1.341 1.35 1.360 1.368 1.315 1.340 1.362

1.499 1.49 1.500 1.501 1.492 1.498 1.500

1.200 1.23 1.197 1.215 1.209 1.215 1.196

1.642 1.68 1.798 1.765 1.653 1.703 1.799

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3. RESULTS AND DISCUSSION 3.1. Electronic States and Spectra. The most relevant optimized bond lengths are displayed in Table 1, together with the computed B3LYP/6-31+G(d,p)/PCM results of Suzuki et al.16 The 6A2TT optimized ground state (S0) geometry is planar, the largest difference being in comparison to the DFT results16 observed for the C2−S9 and C4−O8 bonding distances (0.04 and 0.03 Å, respectively). The main geometric differences with respect to the natural nucleobase are related to the shorter C5−N6 (1.289 Å) and the longer C2−S9 (1.642 Å) bond lengths in 6A2TT than the corresponding C5−C6 (1.346 Å) and C2−O9 (1.198 Å) bond distances of thymine.12 Computed vertical transition energies, oscillator strengths, and dipole moments at the Franck−Condon geometry (FC) are presented in Table 2, together with the experimental and Figure 2. Most important valence molecular orbitals for 6A2TT in the ground state equilibrium structure. Occupied orbitals are in black and virtuals in red.

Table 2. Singlet and Triplet States of 6-Aza-2-Thiothymine: Computed Vertical Transition Energies in Gas Phase (EVA, eV) and in Acetonitrile (EVAS, eV), Experimental Vertical Transition Energy (EVAE, eV) in Acetonitrile, Oscillator Strengths ( f), and Dipole Moments (μ, D) ref 16a

this work EVA S0 S1 (nπ*) S2 (ππ*) S3 (nπ*) S4 (nπ*) S5 (ππ*) T1 (ππ*) T2 (ππ*) T3 (nπ*) T4 (nπ*) a

0.00 3.65 3.80 4.21 4.24 4.39 3.04 3.50 3.59 3.87

f

0.16

0.75

localized on the oxygen atom and a π* molecular orbital localized on the double bond.39 In 6-azauracil, the lowest 1 (nπ*) state comes from a mixture of nonbonding orbitals localized on the oxygen and nitrogen atoms.19 In a narrow energy region, from 4.2 eV up to 4.4 eV, three singlet excited states are found, S3 1(nπ*), S4 1(nπ*), and S5 1(ππ*). The last one is responsible for the most intense absorption band ( f = 0.75), observed experimentally at 4.60 eV (37 040 cm−1).16 The vertical excitation energies of the triplet excited states computed by us (Table 2) are in accordance with the values reported by Suzuki et al..16 It is worth mentioning that besides the T1 3(ππ*) state, vertically 3.04 eV above the ground state, two higher triplet states (T2 3(ππ*) and T3 3(nπ*) at 3.50 and 3.59 eV, respectively) were computed in an energy region close to the S1 1(nπ*) and S2 1(ππ*) states. Therefore, it is likely that singlet−triplet crossings occur and their relevance to the photophysics of 6A2TT will be discussed later. The S1 1(nπ*) and S2 1(ππ*) optimized molecular structure are collected in Table 1. The S1 1(nπ*) state is derived from the ground state by a single excitation (HOMO − 1 → LUMO + 1), located adiabatically 3.08 eV above the ground state equilibrium structure. The computed optimized geometry at the TD-DFT level16 is planar with an elongated C−S bond distance. Our results show a nonplanar S1 1(nπ*) minimum (Figure 3) and exhibit a pyrimidalization of the C2 atom, with a C5−N6−N1−C2 dihedral angle of 25°. As to the bond distances,

μ

EVAS

EVAE

f

2.53 2.81 5.74 6.73 3.19 3.03 1.97 1.67 2.56 3.15

0.00 3.62 3.76

0.00 3.54 4.00

0.06

4.76 2.92 3.50 3.52 3.76

4.60

0.5111

B3LYP/6-31G+G(d,p)/PCM.

theoretical results reported by Suzuki.16 The experimental absorption spectrum exhibits a shoulder (3.54 eV) on the lower energy side of an intense band (4.00 eV), attributed to the S1 1 (nπ*) and S2 1(ππ*) states, in agreement with our results (3.65 and 3.80 eV, respectively). It was suggested that the high intensity of the S1 ← S0 transition is probably due to strong vibronic coupling between the S1 1(nπ*) and S2 1(ππ*) states.16 Based on TD-DFT results, Suzuki et al.16 predicted a gap of 0.16 eV between the S1 1(nπ*) and S2 1(ππ*) states, in agreement with our CASPT2 value of 0.15 eV. In comparison with the excitation energies computed with similar methodology39 for thymine (4.77, 4.89, and 5.94 eV for the 1(nπ*), 1 (ππ*), and 1(ππ*) excited states, respectively), a pronounced red shift it is noted for the 6A2TT excited states. The S1 1(nπ*) and S2 1(ππ*) excited states are derived respectively by a single excitation from HOMO-1 to LUMO+1 and from HOMO to LUMO. HOMO-1 is a lone pair orbital (nS) localized on the sulfur atom; the HOMO orbital is mainly localized on the C2−S9 bond, while the LUMO and LUMO+1 are, respectively, π and π* molecular orbitals localized mainly on the ring. The lower excitation energies of 6-aza-thiothymine, when compared to similar species like thymine, are due to the influence of the sulfur atom. The lowest electronic transitions in 6A2TT (Figure 2) are obtained by transferring electronic density from the sulfur atom to the C5N6 π* electron. For thymine, the S1 1(nOπ*) state is obtained from a lone pair

Figure 3. Schematic representations of the nonplanar states minima. 5591



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our results indicate that the N1−C2, C2−N3, and C2−S9 bonds are longer than the corresponding ones in the ground state by 0.05, 0.04, and 0.16 Å, respectively. A planar structure for the S1 1 (nπ*) state, (S1)PL, was located in the higher energy region (3.35 eV), with elongated C2−N3 (1.406 Å) and C2−S9 (1.746 Å) bonds in comparison with the ground state. The S2 1(ππ*) state is derived from the ground state via a HOMO → LUMO excitation. Its optimized geometry ((S2)min) is planar, being located adiabatically 3.08 eV above the ground state, with N3−C4 (1.447 Å), C5−N6 (1.358 Å), and C2−S9 (1.765 Å) bond distances longer than in the ground state, while the N1−C2 (1.306 Å), C2−N3 (1.318 Å), and C4−C5 (1.409 Å) bond distances are shorter. In the Franck−Condon region, the S2 1(ππ*) state exhibits a dipole moment (μ) of 5.74 D. The two lowest 1(ππ*) excited states of thymine are separated by 1.05 eV.39 In 6A2TT, two 1(nπ*) states are found between the first and the second 1(ππ*) excited states. The S3 1(nπ*) state, located vertically 4.21 eV above the ground state, is derived from the ground state by a HOMO − 1 → LUMO single excitation, while the S4 1(nπ*) state, 4.24 eV above the ground state in the Franck−Condon region, originates from the ground state by the n2 → LUMO single excitation (Table 2 and Figure 2). The S5 1(ππ*) state (Table 2) is derived from the HOMO → LUMO + 1 excitation and is located 4.39 eV vertically above the ground state. According to our results, the S5 1(ππ*) state carries most of the energy after irradiation with UV light, as can be concluded from the computed oscillator strengths (0.75), being related to the most intense experimental absorption band (4.60 eV). As to the triplet excited states, T1 3(ππ*) is computed by us, in the FC region to be 3.04 eV (Table 2) higher in energy than the ground state, being best described by a HOMO → LUMO + 1 (Figure 2) single excitation from the ground state. The optimized structure is planar, with elongated C5−N6 (1.455 Å) and shorter C4−C5 (1.438 Å) bond distances in relation to the ground state. The experimental phosphorescence spectrum in 2-methyltetrahydrofuran16 indicates an emission band at 21 300 cm−1 (2.64 eV), which is in accordance with our computed value for the vertical emission (20 485 cm−1). As will be shown below, T1 3(ππ*) is the key state to understand the singlet oxygen generation by 6A2TT. The T2 3(ππ*) (3.50 eV) and T3 3(nπ*) (3.59 eV) states are close energetically in the Franck−Condon region (Table 2). In comparison to the ground state optimized geometry, the T2 3 (ππ*) state optimized geometry is planar, with elongated N1− C2 (1.400 Å), C5−N6 (1.346 Å), and C2−S9 (1.703 Å) bond distances, while the C4−N5 bond (1.453 Å) is shorter. The T3 3 (nπ*) state exhibits a nonplanar equilibrium structure, with a twisted C5−N6−C1−C2 (25.4°) dihedral angle and elongated N1−C2 (1.415 Å), C2−N3 (1.414 Å), and C2−S9 (1.799 Å) bond distances. The wave function for the T2 3(ππ*) state is dominated by the following excitations from the ground state: π5 → LUMO, HOMO → LUMO, and HOMO → LUMO + 1. As for the T3 3(nπ*) state, the wave function is derived from a HOMO − 1 → LUMO + 1 single excitation from the ground state. The band origins calculated by us for the T2 3(ππ*) and T3 3(nπ*) states are, respectively, 3.18 and 3.00 eV. Comparing the vertical excitation energies of the triplet states of 6A2TT with those of thymine (T1 3(ππ*) = 3.59 eV, T2 3 (nOπ*) = 4.75 eV, and T3 3(ππ*) = 5.14 eV),39 the energy differences range from 0.5 eV for T1 up to 1.64 eV for the second 3(ππ*).

3.2. Excited State Energy Relaxation Mechanisms. The photophysical properties of the biologically relevant nucleobases depend on the lowest lying 1(ππ*) and 1(nπ*) states, the bright and dark states, respectively.3 On the other hand, if one is interested in a high quantum yield of intersystem crossing, a necessary condition for the efficient generation of 1O2 (1Δg), it is also necessary to take into account the lowest lying 3(ππ*) and 3(nπ*) states. Initially, according to Table 2, two excited states are accessible after laser excitation of 6A2TT: S2 1(ππ*) and S5 1 (ππ*) at 3.80 and 4.39 eV with oscillator strengths of 0.16 and 0.75, respectively. Due to the close energy values (ΔE = 0.59 eV) and the existence of coordinates that lead the S5 1(ππ*) state to a conical intersection (S5 1(ππ*)/S2 1(ππ*))CI region, we can safely assume that the absorbed energy will be transferred to the S2 1(ππ*) hypersurface (vide infra). The minimum energy path (MEP) on the S2 1(ππ*) state hypersurface (Figure 4), as well as the evolution of other singlet

Figure 4. Evolution of the ground and selected singlet and triplet states along the minimum energy path on the S2 1(ππ*) surface from the Franck−Condon region toward the (S2)min.

and triplet states along the same MEP, is barrierless to the planar (S2)min state, located 3.08 eV adiabatically above the S0 state. Despite the barrierless path to the (S2)min, which might suggest the existence of a strong emission in this energy region, no emission was observed experimentally, indicating the existence of other deactivation mechanisms. A closer analysis in the region of the first MEP coordinate point (Figure 4), reveals the presence of a conical intersection connecting the S1 1 (nπ*) and S2 1(ππ*) states, (S2 1(ππ*)/S1 1(nπ*))CI, at 3.5 eV, very close to the Franck−Condon region. In addition, at the (S2)min, the S1 1(nπ*), S2 1(ππ*), and the T3 3(nπ*) states are located in a very narrow energetic region of about 0.1 eV. As expected from the El-Sayed rules,40,41 the spin−orbit coupling (SOC) is very large in this region (130 cm−1), 5592



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favoring an efficient crossing from the S2 1(ππ*) to the T3 (nπ*) state. At this point, it is interesting to make a parallel with the photophysics of thymine. The MEP on the hypersurface of the S2 1(ππ*) excited state of thymine, starting from the Franck− Condon region, has its steepest-descent toward the (S2 1(ππ*)/ S0)CI and, consequently, the population is transferred to the ground state in an efficient ultrafast nonradiative process, which explains its well-know photostability.6 Along the same MEP, two other crossings are also observed, namely, (S2 1(ππ*)/S1 1 (nπ*))CI and (S2 1(ππ*)/T2 3(nπ*))STC, opening the possibility for two other deactivation mechanisms, including one which populates the triplet manifold ((S2 1(ππ*)/T2 3 (nπ*))STC).10,39 Although the mechanisms for population of the lowest singlet and triplet states are very similar in thymine and 6A2TT, the main difference, as revealed by the analysis of the corresponding MEPs, is the accessibility of the (S2 1(ππ*)/ S0)CI. As suggested experimentally by Suzuki,16 who found ΦISC = 1.00 for 6-aza-2-thiothymine, the path directly connecting the S2 1(ππ*) state and the ground state must have minor importance, since almost all energy is transferred to the triplet manifold. For sake of completeness, the geometry of the (1(ππ*)/S0)CI, obtained at the CASSCF level, is shown in Figure 5 and is characterized by a wagging of the C2−S9 bond. 3

Figure 6. Evolution of the ground and selected singlet and triplet states along the minimum energy path on the S1 1(nπ*) surface from the Franck−Condon region toward the (S1)min.

Figure 5. Schematic representations of nonplanar crossing points.

The out-of-plane deformation generates a (1(ππ*)/S0)CI structure with a dihedral angle (N6−N1−C2−S9) of 139°. In comparison to the ground state geometry, this out-of-plane deformation is the most striking feature. Population of the S1 1(ππ*) state is likely to occur via the (S2 1 (ππ*)/S1 1(nπ*))CI conical intersection, detected in the MEP of the S2 1(ππ*) state, or from the CI connecting the S2 1(ππ*) and S1 1(nπ*) states in the vicinity of the S2 1(ππ*) state minimum. In order to obtain a better interpretation of the photophysical processes, two MEPs on the S1 1(nπ*) state surface were computed, i.e., from the (S2 1(ππ*)/S1 1(nπ*))CI (Figure 6) and from (S2 1(ππ*))min (Figure 7) structures. As to the (S2 1(ππ*)/S1 1(nπ*))CI initial structure, since the energy difference between the S2 1(ππ*) and S1 1(nπ*) states in the Franck−Condon region is small (0.15 eV), and the CI arises very close to this region, we decided to start the MEP from the Franck−Condon structure (Figure 6). Starting in the Franck−Condon region (Figure 6), the MEP on the S1 1(nπ*) state surface, after crossing the T2 3(ππ*) and T3 3(nπ*) hypersurfaces, evolves barrierlessly to (S1)min. Once

Figure 7. Evolution of the ground and selected singlet and triplet states along the minimum energy path on the S1 1(nπ*) surface from (S2)min toward the (S1)min.

(S1)min is reached, two triplet states are close by within 0.16 and 0.07 eV, these being the T1 3(ππ*) and T3 3(nπ*) states, 5593



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respectively. Again, in accordance with El-Sayedś rules,40,41 the SOC between the S1 1(nπ*) and T1 3(ππ*) states is the largest (169 cm−1). Therefore, the (S1 1(nπ*)/ T1 3(ππ*))STC is very efficient in this region, opening the possibility for the population of the T1 3(ππ*) state. It is interesting to note that, starting in the FC region, the coordinates responsible for reaching (S1)min can also lead the S5 1(ππ*) state to a (S5 1 (ππ*)/S2 1(ππ*))CI conical intersection in a barrierless way, corroborating our hypothesis that the energy passes to the S2 1 (ππ*) hypersurface. The MEP on the S1 1(nπ*) hypersurface starting at the S2 1 (ππ*) state minimum (Figure 7) exhibits a similar pattern; the S1 1(nπ*) state evolves to its minimum and the T1 3(nπ*) state hypersurface has a parallel and similar profile. An important conclusion is that at all geometries along the S2 1(ππ*) MEP, transfer of the energy to S1 1(nπ*) is possible, with subsequent evolution to (S1)min opening the possibility of accessing the (S1 1 (nπ*)/ T1 3(ππ*))STC. The T3 3(nπ*) state can also be populated directly in the S2 1 (ππ*)min region, because in this region we located an intersystem crossing between the T3 3(nπ*) and S2 1(ππ*) states ((S2 1(ππ*)/T3 3(nπ*))STC). The MEP of the T3 3(nπ*) state starting at the (S2)min geometry, is depicted in Figure 8. A

the (T3)min region, a population transfer to the lowest T1 (ππ*) state is also possible. For sake of completeness, an optimized CASSCF (T3 3(nπ*)/ T1 3(ππ*))CI has also been found in the same energy region (Figure 5), with a more pronounced out-of-plane bending character. As mentioned before, the photophysical mechanisms described here for 6A2TT are analogous to those proposed for the population of the thymine triplet manifold,39 which are based on two key crossings: (S2 1(ππ*)/S1 1(nπ*))CI and (S2 1 (ππ*)/T2 3(nπ*))STC. In both cases, thymine can reach the minimum of the respective states, 1(nOπ*)min and 3(nOπ*)min, where nearby crossings, ( 1 (n O π*)/ 3 (ππ*)) S T C or (1(nOπ*)/3(ππ*))CI, are readily accessible. From the lowest T1 3(ππ*) state, the possibility of deactivation via the (T1 3(ππ*)/S0)STC was also investigated. At the CASSCF level, a twisted geometry (Figure 5) was obtained, with out-of-plane deformation of the methyl group, forming a N1−N6-C5−C7 dihedral angle of 86.8°. Besides, a pyramidalization of the N6 atom with a bond angle of 109° occurred with the C5−N6-N1 atoms. The singlet−triplet crossing is placed at approximately 2.6 eV above (S0)min. Although our CASSCF results produced a small energy difference between the states (0.03 eV), our CASPT2 results gave a reasonably large splitting of 0.40 eV; nonetheless, these values can provide valuable qualitative insights. A linear interpolation in internal coordinates connecting the minimum of the T1 3(ππ*) state with the (T1 3(ππ*)/S0)STC is shown in Figure 9. As can be seen, no energetic barrier need be surmounted, other than the energy difference between the two geometries. Although the (T1 3(ππ*)/S0)STC is accessible with little excess vibrational energy, the spin−orbit coupling between them is rather weak when compared to the other values for 3

Figure 8. Evolution of the ground and selected singlet and triplet states along the minimum energy path on the T3 3(nπ*) surface from (S2)min toward the (T3)min.

slopped (T3 3(nπ*)/ T2 3(ππ*))CI is found at the beginning, while the T2 3(ππ*) state energy increases steeply and the T3 3 (nπ*) state reaches its twisted minimum. Since the energy of the T2 3(ππ*) state increases fast in the (T3 3(nπ*)/T2 3 (ππ*))CI region, we claim that it is not a very efficient funnel. Then, the most plausible event is the evolution of the T3 3 (nπ*) state to is minimum energy region, (T3)min, which is, essentially, a (T3 3(nπ*)/ T1 3(ππ*))CI. Thus, after reaching

Figure 9. Evolution of the ground and selected singlet and triplet states along the linear interpolation in internal coordinates from T1 3 (ππ*) minimum toward the (T1 3(ππ*)/S0)STC. 5594



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6A2TT (∼4 cm−1), but high enough to be able to deactivate the system back to the ground state. Just for comparison, the SOC of thymine for the analogous (T1 3(ππ*)/S0)STC is close to 2 cm−1.33

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4. CONCLUSIONS In this contribution we have used the CASPT2//CASSCF protocol with extensive basis set of double-ζ quality to understand the photophysics of 6-aza-2-thiothymine, especially the mechanisms of population of the lowest triplet state, 3 (ππ*), and its potential for using in photodynamic therapy due to its capacity of sensitize molecular oxygen. For this purpose, stationary equilibrium structures of the ground and excited states, minimum energy paths, and linear interpolations had to be characterized. Our computations suggest that, after laser excitation, several events can take place. Along the S2 1(ππ*) MEP, two main paths may occur. The first one is through population of the lowest singlet state via a (S2 1(ππ*)/S1 1(nπ*))CI that exists along the path and at the (S2)min geometry. Once in the S1 1 (nπ*) state, the system evolves to the state minimum, (S1)min, where an efficient and easily accessible (S1 1(nπ*)/ T1 3 (ππ*))STC populates the triplet state. The second mechanism transfers the population to the triplet manifold directly from S2 due to a (S2 1(ππ*)/T3 3(nπ*))STC. Once in the T3 3(nπ*) state, another crossing, (T3 3(nπ*)/ T1 3(ππ*))CI, may lead the system to the T1 3(ππ*) surface. In short, no matter which path the system utilizes, it will always reach the lowest triplet state, explaining the experimental ΦISC = 1.00.13

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ASSOCIATED CONTENT

S Supporting Information *

Cartesian coordinates of the optimized structures. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

Notes. The authors declare no competing financial interest.

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ACKNOWLEDGMENTS J.P.G. acknowledges FAPESP (Fundaçaõ de Amparo à Pesquisa do Estado de São Paulo) for financial support. A.C.B. acknowledges continuous academic support from the CNPq ́ (Conselho Nacional de Desenvolvimento Cienti fico e Tecnológico) and LCCA (Laboratory of Advanced Scientific Computation) of the University of São Paulo. J.P.G. and A.C.B also acknowledge the support by project CTQ2010-14892 of the Consolider-Ingenio in Molecular Nanoscience of the Spanish MEC/FEDER.

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REFERENCES

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On The Population of Triplet Excited States of 6-Aza-2-Thiothymine

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