Theoretical Study of Singlet Oxygen Molecule Generation via an

Jan 12, 2015 - For Manduca sexta, 10 days is needed for nonirradiated αT at 100 μg/g to kill all larvae. .... MRMP2//CAS(10,8)/cc-pVDZ Energies for ...
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Theoretical Study of Singlet Oxygen Molecule Generation via an Exciplex with Valence-Excited Thiophene Masato Sumita*,†,‡ and Kenji Morihashi*,§ †

Comprehensive Research Organization for Science and Society, 1601 Kamitakatsu, Tsuchiura, Ibaraki 300-0811, Japan National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan § Department of Chemistry, Graduate School of Pure and Applied Sciences, University of Tsukuba, 1-1-1, Tennodai, Tsukuba, Ibaraki 305-8571, Japan ‡

S Supporting Information *

ABSTRACT: Singlet-oxygen [O2(1Δg)] generation by valence-excited thiophene (TPH) has been investigated using multireference Møller−Plesset second-order perturbation (MRMP2) theory of geometries optimized at the complete active space self-consistent field (CASSCF) theory level. Our results indicate that triplet TPH(13B2) is produced via photoinduced singlet TPH(21A1) because 21A1 TPH shows a large spin−orbit coupling constant with the first triplet excited state (13B2). The relaxed TPH in the 13B2 state can form an exciplex with O2(3Σg−) because this exciplex is energetically more stable than the relaxed TPH. The formation of the TPH(13B2) exciplex with O2(3Σg−) whose total spin multiplicity is triplet (T1 state) increases the likelihood of transition from the T1 state to the singlet ground or first excited singlet state. After the transition, O2(1Δg) is emitted easily although the favorable product is that from a 2 + 4 cycloaddition reaction. 1.0.8−12 Electron transfer from triplet αT to methyl viologen dication or tetracyanoethylene with a (5.0−7.0) × 10−9 or 23 × 10−9 M−1 s−1 rate constant was observed.12 This supports the notion that triplet αT is an excellent electron donor. However, triplet αT is a poor electron donor for triplet oxygen molecules [O2(3Σg−)], but its energy transfer generates singlet oxygen molecules [O2(1Δg)]. The more widely believed reason for αT phototoxicity is the ability of αT to generate O2(1Δg)1,8,9,13 similarly to other dyes.13 UV-irradiated αT decreases the activity of glucose-6-phosphate dehydrogenase by 80%. However, quenchers of O2(1Δg) (e.g., histidine, tryptophan, methionine), which coexist with UVirradiated αT, prevent the deactivation of the enzyme (at most 40% of enzymes lost their activation).1 This indicates that O2(1Δg) generated by αT deactivates the enzyme. Excited triplet αT plays an important role in O2(1Δg) generation. O2(1Δg) is detected after triplet αT is produced by irradiation with light of 370 nm (3.35 eV) wavelength under aerobic conditions.8,9 Excited triplet αT is formed rapidly (on a time scale of 1.5−3 ps) with an extremely high quantum yield (0.7−1.0).8−12 The quantum yield of O2(1Δg) is also similarly high. According to the results of laser flash photolysis, the quantum yield of O2(1Δg) is 0.86 in chloroform.8 Similar results have also been obtained by phosphorescence detection, which show little solvent dependence.9 Here, singlet-oxygen [O2(1Δg)] generation is investigated by a multiconfiguration-based calculation using valence excited thiophene (TPH, Scheme 1) as a model and target of αT for

1. INTRODUCTION α-Terthiophene (αT; Scheme 1), a secondary metabolite of Asteraceae plants, shows an allelopathic effect.1−7 Asteraceae Scheme 1. Structural Formulas for α-Terthiophene (αT) and Thiophene (TPH)

plants produce αT, which prevents the germination of competing seedlings and kills some insect larvae. Accordingly, αT is expected to function as a natural insecticide and herbicide. Although αT itself shows an allelopathic effect, near-UV light plays an important role in inducing this effect. The toxicity of αT is enhanced by UV light irradiation.1−7 UVirradiated αT produces higher mortality in various larvae than nonirradiated αT. For Manduca sexta, 10 days is needed for nonirradiated αT at 100 μg/g to kill all larvae. However, nearUV-irradiated αT killed all larvae within 7 days at the same concentration.3 The growth of some seeds is also strongly suppressed by near-UV-irradiated αT, but not by nonirradiated αT.5 It has been suggested that this photoinduced enhancement of αT toxicity mainly depends on whether or not oxygen molecules are involved in the photoinduction of αT. It is presumed that αT phototoxicity is due to the ability of its donor/acceptor to undergo a process not involving oxygen molecules. Long-lifetime triplet αT is rapidly produced by irradiating light of 337 nm wavelength onto singlet αT.8 The high quantum yield of triplet αT has been measured to be 0.7− © XXXX American Chemical Society

Received: December 10, 2014 Revised: January 8, 2015

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DOI: 10.1021/jp5123129 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A

To improve the accuracy of the computed energetics by incorporating the effect of a dynamic electronic correlation, we have carried out single-point calculations using the multireference Møller−Plesset second-order perturbation (MRMP2)21 implemented in GAMESS22 on the geometries optimized for energy by CAS(10,8)/cc-pVDZ because CASSCF properly takes the static electronic correlation, but not dynamic electronic correlation, into account. The results of the CASSCF calculation showed that electronic structures have a multiconfiguration property at several of the stationary points we have located. This means that multireference calculation is more suitable for reinforcing the CASSCF energy. A reference CASSCF/cc-pVDZ wave function with the same active space described above was used for all MRMP2 calculations. The weights of states in MRMP2 were equally assigned. To evaluate the possibility of spin-forbidden transitions such as the intersystem crossing (ISC) between the triplet and singlet states, the spin−orbit coupling constant (SOC) was calculated at the same CASSCF level in GAMESS, as noted above using a full Pauli−Breit operator (one- and two-electron SOC).23

future comparison. Thermal reactions between TPH and O2(1Δg) or O2(3Σg−) have been investigated at the singleconfiguration-based calculation level.14 However, there has been no research on the reaction between valence-excited TPH and O2. Although there is a prospective difference in photophysical properties between TPH and αT according to the results of a previous calculation,15 TPH is a good model and a necessary target of αT for comparison. In this research, therefore, we have investigated the photoinduction of TPH and O2(3Σg−) as the first step in clarifying the mechanism of αT phototoxicity. This research will show a possible reaction route to produce O2(1Δg) through the catalytic reaction between photoexcited TPH and O2(3Σg−). Scheme 2. Electronic Configurations for 3Σg−, 1Δg, and 1Σg+ in the π System of Oxygen Molecule (O2)a

a

3. RESULTS AND DISCUSSION The total energies of state-averaged CASSCF and MRMP2 at the stationary points that we found in the triplet and singlet states are shown in Tables 1 and 2, respectively. The representative geometric parameters are summarized in Table 3. We use the atom numbering shown in Figure 1, where the initial structure (T0 Rea) determined by MEP calculation in the T0 state [TPH(11A1) + O2(3Σg−)] at the CASSCF level is depicted, throughout this paper. The structure of TPH at T0 Rea is comparable with the previously reported TPH(11A1) structure18,19 and O2(3Σg−) is located in the place over 3.0 Å apart from TPH. 3.1. Energy Profile of the First Excited Singlet (21A1) State of TPH. To produce singlet O2, it is favorable for TPH(11A1) to be excited to a higher state than its first excited state (21A1). The lowest symmetry allowed excited state of TPH, i.e., the 21A1 state, is calculated as the fourth (T4) state of the TPH(11A1) + O2(3Σg−) system. The vertically excited energy required for TPH(11A1) to reach its 21A1 state at T0 Rea is estimated as 132.9 kcal mol−1, which is comparable to previous computational results.18,19,24 Spin exchange states, i.e., TPH(13B2) + O2(1Σg+) and TPH(13B2) + O2(1Δg), exist as T2/T3 degenerate states that lie at 16.7 kcal mol−1 below the T4 state. The T1 state corresponds to TPH(13B2) + O2(3Σg−), as will be shown later. Because singlet O 2 states [TPH(1 3 B2 ) + O2(1Σg+/1Δg)] lie below the T4 state [TPH(21A1) + O2(3Σg−)] and above the T1 state [TPH(13B2) + O2(3Σg−)], the excitation to the T4 state is reasonable for TPH to produce singlet O2 in the first stage. The energy profile of those states is shown in Figure 2. Although there is no accessible direct channel like conical intersection to the T2 or T3 state from the T4 state of TPH + O2, radiative transition from T4 to T1 is possible. We have calculated SOC between the T4 and T0−T3 states along the energy profile shown in Figure 2. TPH in the 21A1 state is stabilized by pyramidalization around C1 and C4, as shown in Figure 2, and this can be confirmed from the geometrical variation of the ∑C1 and ∑C4 values from T0 Rea to T4 Min in Table 3. As a result of this deformation, TPH + O2 reaches its minimum in the T4 state (T4 Min) at which the value of the summation around C1 and C4 is 348.8° (Table 3). During this relaxation process, the T4 state has a large SOC with the T1 state (over 161 cm−1) in contrast to those with the T0, T2, and T3 states. However, the large gap

Up and down arrows indicate the α and β spin electrons, respectively.

2. COMPUTATIONAL DETAILS The electron configuration of the ground state of singlet O2 is known as 1Δg (Scheme 2), which almost degenerates with the 1 + Σg state [O2(1Δg) is slightly more stable than O2(1Σg+)].13 Single-determinant-based calculations are not good at describing the electronic structure of singlet diradical species like O2(1Σg+), though these calculations can describe the electronic structures of O2(3Σg−) and O2(1Δg). Therefore, to avoid some predictable artifacts, we have employed the multideterminant based calculation, complete active space self-consistent field (CASSCF),16 whose wave function is not only an antisymmetric function but also an eigenfunction of the spin. Geometry optimization, minimum-energy-path (MEP), and intrinsic reaction coordinate (IRC) calculations were performed using CASSCF method implemented in Gaussian 0917 with the cc-pVDZ basis set. CASSCF features flexibility in selecting an appropriate active space. We used the π system except for a lone electron pair of sulfur in TPH and the all-π system of an oxygen molecule (in Scheme 2) as the active space, which corresponds to 10 electrons in eight orbitals [CAS(10,8)]. Although the radiation-free decay of TPH with ring opening has been well investigated,18−20 the photoinduced ring-opening reaction of TPH is beyond the scope of this research. Hence, no σ orbitals of TPH were placed in the active space. The spin multiplicity in this TPH and O2 system should be described in detail. In this research, we assumed that the spin multiplicity of TPH in the ground state is singlet (11A1). Hence, the initial spin multiplicity of TPH(11A1) with O2(3Σg−) has triplet spin multiplicity and the combination of TPH(11A1) with O2(1Δg) has singlet spin multiplicity. In this study, we used the typical representations S0, S1, ... and T0, T1, ... (the capital letter is the abbreviation of the spin multiplicity and the subscript indicates the index of the state) only for the whole system (TPH + O2) to distinguish between the various states. B

DOI: 10.1021/jp5123129 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A Table 1. MRMP2//CAS(10,8)/cc-pVDZ Energies for Stationary Points and T1/T0 Conical Intersection (T1/T0 DP) species

state

ECASa

EMRMP2b

Three-State-Averaged MRMP2 T0 Rea −701.0137 −702.0030 T0 1 3 − TPH(1 A1) + O2( Σg ) T1 −700.8777 −701.8584 T2 −701.8196 T1 Min −700.9697 −701.9491 T0 TPH(13B2) + O2(3Σg−) T1 −700.9048 −701.8680 T2 −701.8292 T0/T1 DP (TPH•+ + O2•−) T0 −700.8931 −701.8809 T1 −700.8931 −701.8732 T2 −701.7565 T1 Rea (TPH•+ + O2•−) T0 −700.9295 −701.9412 T1 −700.8807 −701.8091 T2 −701.8086 T1 Pro (TPH•+ + O2•−) T0 −700.9390 −701.9030 T1 −700.9179 −701.8710 T2 −701.7820 Five-State-Averaged MRMP2 T0 Rea −701.0137 −701.98863 T0 TPH(11A1) + O2(3Σg−) T1 −700.8777 −701.84228 T2 −701.80347 T3 −701.80347 T4 −701.77685 T4 Min −700.9721 −701.95270 T0 TPH(21A1) + O2(3Σg−) T1 −700.8998 −701.85744 T2 −700.8702 −701.81870 T3 −700.8701 −701.81869 T4 −700.8435 −701.79217

Table 2. MRMP2//CAS(10,8)/cc-pVDZ Energies for S2, S1, and S0 States at Stationary Points

Erelc

species S0 Pro

0.00 90.69 115.02 33.82

S0 TS1

S0 Mid

84.71 109.06 76.58 81.40 154.59 38.78 121.67 121.99 62.75 82.83 138.68

S0 TS2

S0 Rea TPH(11A1) + O2(1Σg+/1Δg)

state

ECASa

EMRMP2b

Erelc

S0 S1 S2 S0 S1 S2 S0 S1 S2 S0 S1 S2 S0

−700.9977 −700.8322

−701.9416 −701.7930 −701.7182 −701.9195 −701.8494 −701.7339 −701.9214 −701.8824 −701.7699 −701.9152 −701.8861 −701.8228 −701.9403

38.53 131.78 178.71 52.40 96.39 168.86 51.20 75.68 146.27 55.10 73.36 113.08 39.35

S1 S2

−700.9306

−701.9403 −701.9160

39.34 54.59

−700.9496 −700.8999 −700.9688 −700.9329 −700.9591 −700.9246 −700.9659

a

Two-state-averaged CASSCF energies in atomic units (computed using Gaussian 09). bThree-state-averaged MRMP2 energies for S0, S1, and S2 in atomic units (computed using GAMESS). cMRMP2 relative energies with respect to the T0 equilibrium structure (T0 Rea) in kcal mol−1.

0.0 91.84 116.19 116.19 132.89 22.55

dependence of the chain length will be studied in our future work. 3.2. Energy Profile of the First Excited Triplet (T1) State of TPH(13B2) and O2(3Σg−). The triplet state of αT is important for singlet-oxygen generation, as experimentally suggested.8,9 In the case of TPH, the 13B2 state is accessible from the 21A1 state as already mentioned. This is in agreement with the fact that weak phosphorescence from the 13B2 state has been experimentally detected.10,11 Hereafter, we focus on the reaction between TPH(13B2) and O2(3Σg−). It is an interesting question whether total spin multiplicity in the TPH(13B2) + O2(3Σg−) system is singlet or triplet. However, we ruled out the singlet TPH(13B2) + O2(3Σg−) system because of its nonreactivity (see the Supporting Information). Then, we investigate the triplet TPH(13B2) + O2(3Σg−) state from its vertically excited (VE) point to determine the validity of the T1 state of the TPH + O2 system, which is calculated as the mixed state between ferromagnetic (FM)/antiferromagnetic (AM) TPH and AM/FM O2, that is, TPH(S = 1, Ms = 1, FM) + O2(S = 1, Ms = 0, AM) and TPH(S = 1, Ms = 0, AM) + O2(S = 1, Ms = 1, FM); here S is the spin quantum number and Ms is the eigenvalue of Sz (component of z-axis) of TPH or O2. This mixed state is simply regarded as the state of TPH(13B2) + O2(3Σg−) on the basis of the following discussion. The vertically excited energy required to reach the T1 state at T0 Rea is calculated to be 3.93 eV (90.69 kcal mol−1), which is in agreement with the theoretical and experimental vertical excited energies required to reach the first excited triplet state of isolated TPH (approximately 3.9 eV).15,18,19,24−26 At the VE point of the T1 state, this FM state mixes with the AM state of each molecule; i.e., the leading configurations are 40% TPH(13B2, FM) + O2(3Σg+, AM) and 20% TPH(13B2, AM) + O2(3Σg−, FM). We expected the exchange reaction of spin up−down between excited TPH and O2 to occur easily, and the AM configuration easily relaxes to singlet states. However, we could not find a spin exchange route to TPH(13B2) with O2(1Δg) as the final product. The T1 state is stabilized by only TPH, which reaches the 13B2 relaxed structure previously reported.18,19

82.32 106.63 106.64 123.28

a

Two-state-averaged CASSCF energies in atomic units (computed using Gaussian 09). b Three-state-averaged/five-state-averaged MRMP2 energies for T0, T1, T2, T3, and T4 in atomic units (computed using GAMESS). cMRMP2 relative energies with respect to the T0 equilibrium structure (T0 Rea) in kcal mol−1.

between the T1 and T4 states (more than 40 kcal mol−1) dose not disappear. Therefore, a slow radiative transition from the T4 state (21A1 TPH) to the T1 state (13B2 TPH) is possible; however, a rapid ISC from singlet to triplet seems difficult for 21A1 TPH in contrast to the case of poly-TPHs such as αT.8−12 In this research, we investigate the potential process after the transition of TPH from 21A1 to 13B2. According to the results of theoretical and experimental studies, S1-excited TPH undergoes ring-opening nonradiative decay.18,19 Even if S1-excited TPH(21A1) has a large SOC with the 13B2 state, a rapid transition from TPH(21A1) to TPH(13B2) is hard because of the large energetic gap between them. Hence, the dominant reaction of TPH after its excitation to the 21A1 state is expected to be a ring-opening reaction. S1-excited TPH may undergo ISC with several triplet states during this ring-opening reaction as shown in previous calculations.18 Furthermore, TPH is predicted to reach its relaxed structure in the 13B2 state after the ISC. Indeed, weak phosphorescence from the 13B2 state has also been detected experimentally.10,11 We consider that poly-TPHs such as αT undergo ISC with the five-membered ring maintained because the ISC occurs rapidly after the excitation (1.5−3.0 ps). According to the experimental results, bithiophene shows the strongest possibility of the ISC from singlet to triplet.11 The C

DOI: 10.1021/jp5123129 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A Table 3. Representative Geometric Parameters at Stationary Points Located Using CASSCFa T0 Rea

a

T1 Min

C1−C2 C2−C3 C3−C4 S−C1 S−C4 O1−O2 C1−O1 C4−O2

1.362 1.445 1.361 1.746 1.746 1.182 3.902 4.112

1.474 1.354 1.474 1.788 1.788 1.182 3.817 3.94

∑C1 ∑C4

360.0 360.0

352.5 352.4

T1 Rea

T1 Pro

T4 Min

S0 Pro

Bond Lengths (Å) 1.364 1.502 1.49 1.515 1.442 1.383 1.448 1.320 1.363 1.398 1.49 1.515 1.739 1.836 1.768 1.826 1.739 1.765 1.768 1.826 1.437 1.365 1.182 1.545 3.339 1.442 3.815 1.467 4.279 3.619 4.089 1.467 Summation of Bond Angles around C1 and C4 (deg) 360.0 329.1 348.8 339.6 360.0 360.0 348.8 339.6

S0 TS1

S0 Mid

S0 TS2

S0 Rea

1.501 1.329 1.470 1.824 1.747 1.378 1.502 2.294

1.497 1.337 1.430 1.882 1.751 1.344 1.485 3.606

1.443 1.432 1.343 1.763 1.767 1.289 1.761 3.149

1.359 1.448 1.343 1.743 1.739 1.232 3.576 3.694

338.0 356.6

331.2 360.0

348.5 360.0

360.0 360.0

The atom numbering is shown in Figure 1.

pyramidalization around C1 and C4 mainly occurs with this stabilization (see ∑C1 and ∑C4 of T1 Min in Table 3). This indicates that electrons are localized on the C1 and C4 atoms. Hence, C1 and C4 are expected to be potential active sites for an oxygen molecule. Comparing the geometry with the previous calculations,18,19 we conclude that this structure is the relaxed structure of TPH(13B2). On the other hand, the bond length O1−O2 does not change; that is, the property of O2(3Σg−) is maintained. Although the total spin multiplicity is triplet, the T1 state is regarded as the state of TPH(13B2) + O2(3Σg−). At T1 Min in Figure 3, intermolecular electron transfer states, i.e., TPH•+ + O2•− and TPH•− + O2•+, also contribute to the T1 state in addition to TPH(13B2, FM) + O2(3Σg−, AM) and TPH(13B2, AM) + O2(3Σg−, FM). According to the analysis of the electron configuration, TPH(13B2, FM) + O2(3Σg−, AM) is 28% and TPH(13B2, AM) + O2(3Σg−, FM) is 29% at T1 Min. In addition to these valence excited states, ionic structures, i.e., 5.7% TPH•+(2B1) + O2•−(2Πg) and 9.6% TPH•−(2B1) + O2•+(2Πg), are also involved. This result implies that the electron transfer between excited TPH and O2 is possible in the T1 state. 3.3. Energy Profile of Electron Transfer State (TPH•+ + O2•−) in T1 State. The energy profile of the electron transfer state (TPH•+ + O2•−) in the T1 state is shown in Figure 3 with that of TPH(13B2) + O2(3Σg−) in the T1 state. The electron transfer from TPH to O2 is expected to occur easily with shortening the distance between TPH and O2 in the T1 state (exciplex formation). At T1 Min, the distance between TPH and O2 is about 3.8 Å (C1−O1 in Table 3), which seems to be too long for the electron transfer to occur. That is, shortening the distance between TPH and O2 (exciplex formation) is necessary for electron transfer. Hence, we performed a calculation to find a transition state (TS) for the electron transfer. However, we failed to find a TS. We predict that such a TS is extremely low because the electronic structure is very sensitive to the shortening of the C1−O1. By shortening C1−O1 bond by only 0.3 Å from T1 Min (C1−O1 = 3.8 Å), the excited T1 state of TPH(13B2) + O2(3Σg−) converts to the electron transfer state (TPH•+ + O2•−). In the structure where a C1−O1 bond is formed (an exciplex is formed), the electron transfer system (TPH•+ + O2•−) is more stable than the excited system of TPH + O2 (T1 Min) in the T1 state as shown in Figure 3 Although the dissociated state of TPH•+ + O2•− (T1 Rea on the side of 0.0 bohr amu1/2, where C1−O1 = 3.339 Å) is higher than the T1 TPH + O2 system energetically, the product of TPH•+ + O2•− (T1 Pro on the side of 18.0 bohr amu1/2), where the C1−O1 distance is 1.4 Å, is more

Figure 1. Structure of T0 Rea [T0 TPH(11A1) + O2(3Σg−)], located through MEP calculation, and atom numbering used throughout this study.

Figure 2. MRMP2//CASSCF/cc-pVDZ energy profile of minimum energy path (MEP) from the vertically excited (VE) point in the T4 state [T0 TPH(21A1) + O2(3Σg−)] of T0 TPH(11A1) + O2(3Σg−). Note that T2 and T3 states are degenerate. Atom indices are also given.

The T1 state of the TPH + O2 system is stabilized by the deformation of TPH rather than O2. The energy profile of the T1 state of TPH + O2 from the VE point of T0 Rea at the MRMP2// CASSCR/cc-pVDZ level is shown in Figure 3. The T 1 TPH(13B2) + O2(3Σg−) system reaches a stationary point (T1 Min) that lies at approximately 20 kcal mol−1 below the T1 VE point. TPH excited to the 21A1 state reaches T1 Min via ring opening18 or radiative transition, as already mentioned. The D

DOI: 10.1021/jp5123129 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A

Figure 3. MRMP2//CASSCF/cc-pVDZ energy profile of minimum energy path (MEP) from the vertically excited (VE) point in the T1 state of TPH(1A1) + O2(3Σg−) (T0 Rea) and additional reaction of TPH•+ + O2•− in the T1 state. In both energy profiles, dissociated species are set to 0.0 bohr amu1/2. TPH(1A1) + excited O2(3Σu−) is calculated as the ground state of TPH•+ + O2•−. Atom indices are also given.

stable than T1 Min. The stability of T1 Pro relative to T1 Min suggests that the electron transfer occurs easily in the T1 state. However, there is no channel to produce O2•− species in the ground state. TPH•+ + O2•− is the T1 state whose ground state is TPH(11A1) and excited O2(3Σu−). Although we can easily succeed in finding the conical intersection (T1/T0 DP) with O2(3Σg−) and TPH(11A1), T1/T0 DP is the channel leading back to T0 Rea. This is in agreement with the experimental suggestion regarding αT8,9 that radical oxygen molecule is not produced, and that TPH(13B2) has the temporary ability to act as an electron donor for O2(3Σg−). To produce singlet oxygen (1Δg), T1 Pro should undergo transition from the triplet state to the singlet state. The electronic structure at T1 Pro is represented by a combination of allyl (the moiety of C2−C3−C4) and the oxygen molecule, as shown in Figure 4. Because the leading configuration at T1 Pro is (1π)2(σ)2(1π*)2(2π*)2(1b2)1(1a2)1, positive and negative charges are located at the moieties of C2− C3−C4 and O1−O2, respectively. Furthermore, significant SOC with some singlet states at T1 Pro can be expected owing to the excited configuration [(1b2)1(1a2)1] of the allyl cation moiety (C2−C3−C4). 3.4. Profiles of Vertical Singlet States and Spin−Orbit Coupling (SOC) Constant. In the ionic TPH•+ + O2•− system of the T1 state, the transition from the T1 state to the S1 or S0 state becomes possible with creation of the bond between TPH•+ and O2•−. Figure 5 shows vertical S1 and S0 energy profiles along the energy profile of the additional reaction of TPH•+ + O2•− in the T1 state. Here, the S0 state is calculated as TPH(11A1) + O2(1Σg+), and the S1 state is calculated as TPH(11A1) + O2(1Δg). The S0 and S1 states are degenerate in the dissociated state (the side of 0.0 bohr amu1/2). The energy difference between the T1 state of TPH•+ + O2•− and its vertical S1 state is less than 0.5 kcal mol−1 on the side of T1 Pro because the leading configuration of the vertical S1 state [TPH(11A1) + O2(1Δg)] is the same as that of T1 Pro [(1π)2(σ)2(1π*)2(2π*)2(1b2)1(1a2)1, see Figure 4 about orbitals]. On the other hand, the energy difference between the T1 state and its vertical S0 state is more than 20 kcal mol−1 on the

Figure 4. Orbitals within the active space of CASSCF at T1 Pro. The left column shows the orbitals from the oxygen molecule. The right column shows the orbitals from the allyl moiety of thiophene (C2−C3−C4). C2V symmetry is assumed in the allyl moiety.

same side. The energetically favorable nonradiative ISC is from the T1 state to the S1 state. The following singlet oxygen emission is expected to occur easily with heterolytic C1−O1 bond breaking (Scheme 3) because there is no barrier to the TPH(1A1) + O2(1Δg) dissociated state. However, the principal SOC calculated at the CASSCF level indicates that the transition from the T1 state to the S0 state is favorable. Figure 6 shows computed SOC between the T1 state and the S0 or S1 state along the additional reaction coordinate of TPH•+ + O2•− in Figures 3 and 5. In the dissociated region of TPH•+ + O2•− (on the 0.0 bohr amu1/2 side; T1 Rea), the SOC is almost zero. At approximately 9.0 bohr amu1/2, the ⟨T1|HSO|S0⟩ SOC shows rapid marked growth, whereas the ⟨T1|HSO|S1⟩ SOC shows only a small peak (the maximum is about 10 cm−1). From this, we can surmise that the C1−O1 bond is formed at approximately 9.0 bohr amu1/2. After passing this region, the ⟨T1| E

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transition in the TPH + O2 system from the T1 state to the S0 state. This transition is radiative because there is a sufficient energy gap (20 kcal mol−1) between the T1 and S0 states. Our calculation shows that the SOC is large between the states that have different electronic structures from each other. As already shown, the leading configuration of T1 Pro and its vertical S1 state is (1π)2(σ)2(1π*)2(2π*)2(1b2)1(1a2)1. On the other hand, the vertical S0 state at T1 Pro is (1π)2(σ)2(1π*)2(2π*)1(1b2)2(1a2)1, which shows that both moieties of C2−C3−C4 and O1−O2 have radical electrons (diradical). 3.5. Singlet Oxygen Molecule (1Δg) Emission in the S0 State. After the radiative transition from the T1 state to the S0 state, T1 Pro is expected to reach the intermediate structure upon the 2 + 4 cycloaddition reaction between O2(1Δg) and TPH. Figure 7 shows the energy diagram of the coordinate of the

Figure 5. Vertical S1 and S0 states along the energy profile of the additive reaction of TPH•+ + O2•− in the T1 state shown together. The S1 state is calculated as TPH(1A1) + O2(1Δg), and the S0 state is calculated as TPH(1A1) + O2(1Σg+). The S1 and S0 states are degenerate on the dissociated side (0.0 bohr amu1/2). On the exciplex side (18.0 bohr amu1/2), the S1 state is close to the T1 state energetically. Atom indices are also given.

Scheme 3. Heterolytic Dissociation Process after the Transition from T1 to S1

Figure 7. Schematic energy diagram of TPH(11A1) and O2(1Δg) in the S0 state. Values are energies (in kcal mol−1) relative to T0 Rea at the MRMP2//CASSCF level. Atom indices are also given.

reaction between TPH and O2 in the S0 state. According to the previous calculation at the CCSD level,14 the 2 + 4 cycloaddition between O2(1Δg) and TPH occurs via only one TS. In contrast to the previous calculation, our MCSCF calculation shows that the reaction proceeds via two TSs. Consequently, an intermediate (S0 Mid) appears between two TSs. The 2 + 4 cycloaddition reaction between butadiene and O2(1Δg) similarly shows an intermediate between two TSs.27 S0 Mid has the diradical character because the leading configuration at S0 Mid is (1π)2(σ)2(1π*)1(1b2)2(1a2)2(2b2)1, i.e., one radical located to the O2 atom and the other radical is delocalized on the C2−C3− C4 allyl moiety. The structure of S0 Mid is almost identical to that of T1 Pro (see Table 3). The root-mean-square of delta for bond lengths between S0 Mid and T1 Pro is only 0.031 Å. Therefore, the exciplex of TPH•+ + O2•− (T1 Pro) is predicted to reach around S0 Mid after the radiative transition. O2(1Δg) is easily emitted by S0 Mid. From S0 Mid, the reaction route bifurcates. One route leads to a product of 2 + 4 cycloaddition (S0 Pro), in which the barrier to S0 Pro (S0 TS1) lies 1.2 kcal mol−1 above S0 Mid. The other is a dissociative route (homolytic C1−O1 bond breaking as shown in Scheme 4) to TPH(11A1) and O2(1Σg+) (S0 Rea); that is, O2(1Σg+) is emitted. The emitted O2(1Σg+) immediately changes to O2(1Δg). The

Figure 6. ⟨T1|HSO|S0⟩ and ⟨T1|HSO|S1⟩ spin−orbit coupling (SOC) constants (in cm−1) computed using GAMESS at the CASSCF level along the energy profile of the additive reaction of TPH•+ + O2•− in the T1 state shown in Figures 3 and 5. The value of SOC from the T1 state to the S0 state increases along this reaction coordinate, whereas that from the T1 state to the S1 state does not show significant growth. That is, the transition from the T1 state to the S0 state in the TPH•+ + O2•− system probably occurs by the formation of a bond between TPH•+ and O2•−.

Scheme 4. Homolytic Dissociation Process after the T1 to S0 Transition

HSO|S0⟩ SOC reaches its upper limit, 72.09 cm−1 which is sufficient for the transition between the S0 state and the T1 state in the TPH + O2 system to occur. Therefore, bond formation between TPH•+ and O2•− (exciplex formation) induces the F

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The Journal of Physical Chemistry A barrier for S0 Rea (S0 TS2) lies 4.1 kcal mol−1 above S0 Mid. Although the dominant reaction proceeds to S0 Pro from S0 Mid, the reaction to S0 Rea is also possible and O2(1Δg) is easily emitted.

+ O2(3Σg−) system is discussed. This material is available free of charge via the Internet at http://pubs.acs.org.

4. CONCLUSION We have investigated O2(1Δg) generation through multiconfiguration based calculation using the valence excited TPH as the model and target of αT for future comparison. According to the results of our calculation, it is favorable for TPH to be excited to the 21A1 state or one of the higher states because singlet O2 states [TPH(13B2) + O2(1Σg+/1Δg)] lie below the 21A1 state of TPH [TPH(21A2) + O2(3Σg−)]. However, there is no channel from TPH(21A2) + O2(3Σg−) to TPH(13B2) + O2(1Σg+/1Δg). Alternatively, the TPH excited to the 21A1 state can transit to 13B1 radiatively or via the ring opening reaction.18−20 The relaxed TPH in the 13B2 state forms an exciplex with O2(3Σg−) occurring in the electron transfer from TPH to O2. The formation of the exciplex in the T1 state increases the likelihood of transition between T1 and S0/S1. An energetically favorable route is a nonradiative ISC from the T1 state to S1 state because the energy difference between the T1 and vertical S1 states disappears with the exciplex formation. The following O2(1Δg) emission occurs easily without a barrier. From the viewpoint of SOC, a possible route is the radiative T1-to-S0 transition because the SOC between the T1 and S0 states increases with exciplex formation. After the radiative transition, the resulting compound of TPH and O2 assumes an intermediate structure upon the 2 + 4 cycloaddition reaction in the S0 state. From this intermediate structure, O2(1Δg) is emitted though O2 emission might not be the main reaction. In Figure 8, the O2(1Δg) generation process clarified in this study is shown.

Corresponding Authors

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*(M.S.) E-mail: [email protected]. Tel.: +81 29 859 2490. *(K.M.) E-mail: [email protected]. Tel.: +81 29 853 5771. Notes

The authors declare no competing financial interest.

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REFERENCES

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Figure 8. Summary of processes of singlet O2(1Δg) generation by excited TPH. TPH in the T1 state forms an exciplex with triplet O2(3Σg) while electron transfer from TPH to O2 occurs. Once this exciplex forms in the T1 state, the likelihood of both nonradiative intersystem crossing (ISC) and radiative transition from the T1 state to the S0/S1 state increases. Nonradiative ISC from the T1 state to the S1 state directly leads to O2(1Δg) emission as expected. After radiative transition from the T1 state to the S0 state, reaction route in the S0 state bifurcates into routes involving 2 + 4 cycloaddition and singlet O2(1Δg) emission.

According to our results, TPH can produce O2(1Δg) in spite of the small quantum yield. To elucidate the phototoxicity of αT, we should compare this process of O2(1Δg) generation by TPH with that by αT. This is our future work.

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AUTHOR INFORMATION

ASSOCIATED CONTENT

S Supporting Information *

Cartesian coordinates at stationary points and conical intersection are tabulated. Nonreactivity of singlet TPH(13B2) G

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