The Effect of Explicit Solvent on Photodegradation of

Article pubs.acs.org/JPCA

The Effect of Explicit Solvent on Photodegradation of Decabromodiphenyl Ether in Toluene: Insights from Theoretical Study Lu Pan,†,‡ Wensheng Bian,*,† and Jiaxu Zhang*,† †

Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, China University of Chinese Academy of Sciences, Beijing, 100049, China

‡

S Supporting Information *

ABSTRACT: Polybrominated diphenyl ethers (PBDEs) have received special environmental concern because of their potential toxicity to humans and wildlife worldwide. However, their photochemical degradation mechanisms remain largely unknown. Herein, a PCM/TD-DFT scheme (time-dependent density functional theory combined with the polarizable continuum model) augmented with explicit solute− solvent interactions is used to explore the promotive effects of the toluene solvent on the photochemical degradation debromination of deca-BDE (BDE209). The face-toface π−π interactions between penta-bromine-substituted phenyl and toluene are investigated. The calculations indicate that the face-to-face π−π interaction plays an important role in the low-lying π→σ* transitions of BDE209−toluene π-stacking complex at around 300 nm in the sunlight region, which leads to notable changes for the πσ* excited states and which promotes the breaking of the C−Br bonds. The photodegradation reaction via an intermolecular charge-transfer excited state formed by the electronic transition from a π orbital of toluene to a σ* orbital of BDE209 is found to be a dominant mechanism. Our calculation results reveal the mechanism of how the participation of an explicit toluene solvent molecule catalyzes the photodegradation of BDE209 and explain the experimental results successfully. The present study may provide helpful information for the removal of PBDE contamination.

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INTRODUCTION Polybrominated diphenyl ethers (PBDEs) are widely used in electronics and textiles as brominated flame retardants (BFRs).1 In recent years, they have attracted special environmental concern because of their potential toxicity to humans and wildlife worldwide. It was reported that PBDEs can influence liver enzyme activity, induce immunotoxicity, and influence neurological development at a key period of brain growth.2−4 Therefore, it will be of particular interest to figure out a way to eliminate the PBDE contamination. Many experiments have investigated the photochemical degradation of deca-BDE (BDE209). They found that the photolytic debromination reaction could take place in different solvents (methanol, tetrahydrofuran, hexane, some solvent mixtures, etc.) and natural matrixes (sand, silica gel, soil, etc.) induced by solar light and could form less-brominated congeners.5−10 Several experiments mentioned interesting solvent effects on the photodegradation reaction of BDE209.6−10 For example, Davis and Stapleton10 observed a solvent effect on the photochemical decomposition reaction rates of BDE209 and nona-BDEs, and they suggested that the reaction rate in toluene is much more rapid than those in methanol and tetrahydrofuran solutions. Eriksson et al.8 suggested that the hydrogen-bonding (H-bonding) interactions between BDE209 and solvent molecules promote the © XXXX American Chemical Society

debromination reaction, and the difference of the hydrogen donation capabilities of solvents may account for the difference of reaction rates. However, Davis and Stapleton’s studies10 claimed that toluene’s aromatic ring may facilitate the donation of a hydrogen atom from the solvent molecule to replace one bromine atom on the PBDE. Unfortunately, the detailed mechanism for the promotive effects of the solvents on the photodegradation of BDE209 has not been addressed yet in their studies. The work reported here demonstrates that the face-to-face π−π interaction between BDE209 and toluene molecule plays an important role in the photodegradation of BDE209 in the toluene solution. As a typical noncovalent interaction, the face-to-face π−π interaction (also known as π-stacking interaction) has attracted extensive attention in biologies and materials.11−13 It is known that the π-stacking interaction is the result of π−σ attraction overcoming the π−π repulsion and that the total interaction energy is a sum of the electrostatic, exchange, induction, and dispersion components.11 The relevance of π-stacking interaction for determining the structural and dynamical properties of aromatic systems, such as polypeptides, is well recogReceived: April 22, 2013 Revised: May 31, 2013

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dx.doi.org/10.1021/jp403958g | J. Phys. Chem. A XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry A

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nized.14,15 Several experiments showed that the π-stacking interaction bonding, such as nucleic acid strands, remarkably modulates the static and dynamical properties of their excited states.16−19 Plenty of theoretical works20−27 have been done to provide an accurate description of the structural and energetic properties of π-stacking systems in their ground electronic states, including hexahalogenbenzene−benzene23 and toluene dimer24,25 systems and so forth. The properties of the low-lying excited states of the π-stacking systems have also been investigated theoretically.26−33 Several works have mentioned the dark excited states produced by intermolecular charge transfer (CT) between two monomers of some π-stacking systems.28−33 For example, Improta has studied the excited states of π-stacked 9-methyladenine oligomers by using the PCM/TD-DFT model (time-dependent density functional theory combined with the polarizable continuum model) showing that the lowest energy excited state of the dimer corresponds to a dark CT excimer.28 Lange and Herbert analyzed with the time-dependent long-range correction density functional theory (TD-LRC-DFT) methods the interstrand charge-transfer states of aqueous B-DNA, for example, adenine→thymine CT states.32 The role of the πstacking was discussed as well. Nowadays, the understanding of the photocatalytic degradation mechanisms of PBDEs still presents a challenge for experimental and theoretical studies partly because of the involvement of heavy bromine atoms although the photodissociation reactions of phenyl rings substituted with fewer bromine atoms, such as bromobenzene and dibromobenzene, have been understood quite well both experimentally and theoretically.34,35 The discussions about the promotive effects of explicit solvent molecule on the photodegradation of PBDEs are rare and inconsistent, and so more detailed studies for the photolytic degradation reactions is urgently needed in order to better understand the environmental debromination of PBDEs. The objective of this work here is to investigate the effects of the participation of an explicit toluene molecule on the photodissociation reaction and the photodegradation mechanisms of BDE209 in the toluene solvent. The detailed theoretical calculations are performed to investigate the photocatalytic debromination of BDE209 in toluene solvent. The face-to-face π−π interactions between BDE209 and toluene (1:1) are analyzed. Then, the low-lying transitions in the sunlight region for both BDE209 and π-stacking complexes (denoted as BDE209-tolu) are simulated, and the effects of the π-stacking interaction on the excited states are discussed. The carbon atoms of BDE209 are numbered as shown in Scheme 1. For BDE209-tolu complexes, the carbon atoms on the phenyl interacting with toluene are numbered as C1−C6.

COMPUTATIONAL METHODS The effects of an explicit toluene solvent molecule on photodegradation reaction of BDE209 are investigated by using the density functional theory (DFT)36,37 and timedependent38−40 density functional theory (TD-DFT). The M06 functional41 combined with the TD method is employed, which has been proven successful in describing the noncovalent interactions and the excitation properties of certain organic compounds.42−48 The hybrid basis set denoted as 6-311+G(d)LANLdp is used for all calculations. For bromine, the LANL2DZ basis set49 is used and is augmented with polarization functions of d symmetry and diffuse functions of p symmetry.50 The 6-311+G(d) basis set is employed for the other atoms. The bulk solvent effects of toluene are evaluated by means of the polarizable continuum (PCM) model.51 The PCM-TD-DFT model has been shown suitable for the studies of photoexcitation properties of some aromatic molecules in solvents.26−31,52−55 The equilibrium geometries of BDE209 and π-stacking complexes BDE209-tolu (1:1) are completely optimized, and the face-to-face π−π interaction energies are obtained by using the M06 functional combined with the PCM model, and the results are listed in Table 1. The optimized Cartesian Table 1. Optimized Structures of BDE209-Tolu π-Stacking Complexes and the π−π Interaction Energies (ΔE)a

a

Energy in kcal mol−1.

coordinates are given in the Supporting Information. The absorption spectra and oscillator strengths (f) of the π→σ* transitions at around 300 nm for BDE209 and BDE209-tolu complexes are simulated at the PCM-TD-M06/6-311+G(d)LANLdp level of theory. The structures of the πσ* excited states are optimized at the same level. For further understanding of the photodegradation mechanism, the potential energy curves (PECs) with respect to the C−Br internuclear distance are calculated for the low-lying excited states of a representative complex.

Scheme 1. The Serial Number of the Carbon Atoms in BDE209a

a

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For clarity, Br atoms are omitted. B



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To test the reliability of the DFT calculations, two levels of numerical integration, fine and ultrafine, are used to calculate the excitation energies of BDE209 and its complex as shown in Table 1s of the Supporting Information. It is found that the absorption wavelengths and oscillator strengths for both BDE209 and its complex are almost the same when the integration grid is set to fine and ultrafine, respectively, and are in very good agreement with the experimental values. This suggests that the characters of the system are insensitive to the DFT grid setting, and thus, the fine integration grid is used for all calculations. All the calculations are performed using the Gaussian 09 suite of programs.56

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RESULTS AND DISCUSSION Face-to-Face π−π Bonding. Three BDE209-tolu πstacking complexes (denoted as complex I, complex II, and complex III) are located by varying the orientation of the methyl group in toluene as shown in Table 1. The optimized structures exhibit that the monomer BDE209 in π-stacking BDE209-tolu complexes does not suffer appreciable variation compared with BDE209 with the C−Br and C−C bond lengths almost unchanged. Meanwhile, two phenyl rings of monomer BDE209 remain in the planar form no matter whether it is stacked with toluene or not. This indicates that the face-to-face π−π interactions have a slight effect on the geometries of the ground states of monomer BDE209. From Table 1, we notice that the orientation of the methyl group almost has no effects on the geometries and energies of all three complexes. The toluene plane is almost parallel to the plane of the interacting phenyl, and the π−π interaction energies of three π-stacking complexes are −12.5, −12.2, and −12.5 kcal/mol−1, respectively. The hydrogen-bonding contribution has been estimated between Br in BDE209 and H in toluene at the PCM-M06/6-311+G(d)-LANLdp level. The interaction energy is about −1.4 kcal/mol−1 (the optimized geometry is shown in Figure 1s in the Supporting Information), which is much weaker compared to the face-to-face π−π interactions. Previous theoretical works have investigated the face-to-face π−π interactions of some analogous systems, for example, hexabromobenzene−benzene system23 and toluene dimers,24 showing that the dispersion interaction between two stackers is the major source of attraction. Similarly, the strong π−π interactions for the BDE209-tolu complexes may be attributed to the large dispersion interaction between pentabrominesubstituted phenyl and toluene ring. Features of Low-Lying π→σ* Transitions. Typical frontier molecular orbitals, in particular the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), are shown in Figure 1 for BDE209 and BDE209-tolu complex I. Frontier molecular orbitals of complex II and complex III are similar to those of complex I and are shown in Figure 2s of the Supporting Information. The left column of Figure 1 shows that all five frontier orbitals of BDE209 have distributions over the whole molecular configuration. The HOMOs are the π-bonding (denoted as πBDE209) orbitals, while the LUMOs correspond to the σantibonding (σ*) orbitals. In contrast, with an explicit toluene molecule for BDE209-tolu complex, the electronic cloud distributions of the three occupied orbitals are totally different and are strongly involved with the toluene ring, whereas the unoccupied MOs remain the BDE209 σ* character. The

Figure 1. Frontier molecular orbitals of BDE209 (left) and BDE209tolu complex I (right).

HOMO of BDE209-tolu complex shows small distributions over the interacting phenyl besides the large distributions over the toluene ring (denoted as πtolu), and similarly, HOMO-1 and HOMO-2 of complex I are essentially π orbitals located on both BDE209 and the toluene moieties (denoted as πcpx). This implies the strong π−π interaction bonding between BDE209 and toluene. The molecular orbitals involved in low-lying transitions, vertical excitation wavelengths, and oscillator strengths (f) of the π→σ* transitions for BDE209 and three π-stacking BDE209-tolu complexes computed by us are given in Table 2. The lowest excitation of BDE209 is at ∼333 nm with f of 0.0136 and is assigned as πBDE209→σ* transition from the π (HOMO) to σ* (LUMO+1) orbital. For the three BDE209tolu complexes, the lowest energetic excitations show a slight Table 2. Properties of Low-Lying Transitions for BDE209 and Three BDE209-Tolu Complexes in the Toluene Solvent BDE209 complex I

complex II

complex III

a

C

transition

absorptiona

fb

HOMO→LUMO+1 (πBDE209→σ*) HOMO−1→LUMO, LUMO+1 (πcpx→σ*) HOMO−2, HOMO−1→LUMO (πcpx→σ*) HOMO→LUMO+1 (πtolu→σ*) HOMO−1→LUMO, LUMO+1 (πcpx→σ*) HOMO-2, HOMO-1→LUMO (πcpx→σ*) HOMO→LUMO+1 (πtolu→σ*) HOMO−1, HOMO→LUMO (πcpx→σ*) HOMO−2→LUMO, HOMO−1→ LUMO + 1 (πcpx→σ*) HOMO→LUMO+1 (πtolu→σ*)

333 342

0.0136 0.0051

333

0.0093

305 342

0.0102 0.0119

333

0.0063

304 339

0.0164 0.0052

333

0.0033

304

0.0019

Vertical excitation wavelength in nanometers. bOscillator strengths. dx.doi.org/10.1021/jp403958g | J. Phys. Chem. A XXXX, XXX, XXX−XXX


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energies of optimized ground states are set zero. The Cartesian coordinates of these excited states are given in the Supporting Information.

red-shift compared with BDE209, and all occur at around 340 nm with the f values of 0.0051−0.0119 corresponding to the electronic transition from a π orbital of the whole complex to a σ* orbital of monomer BDE209 (denoted as πcpx→σ* transition). Moreover, the other πcpx→σ* transition is found at around 333 nm for each complex. It can also be seen that the low-lying transitions of complex III exhibit some difference from those of complexes I and II as illustrated in Table 2, that is, the components and the f values of the two πcpx→σ* transitions. Notably, for each π-stacking BDE209-tolu complex, an additional excitation at a shorter absorption wavelength around 305 nm corresponding to the πtolu (HOMO)→σ* (LUMO+1) transition can be found. This πtolu→σ* transition is produced by intermonomer charge transfer. Similar situations were also found in previous theoretical studies of some π-stacking systems.26−33 Among all the low-lying π→σ* transitions, the πtolu→σ* transition of complex II features the highest value f of 0.0164 (at ∼304 nm) and thus becomes a local maximum absorption in the BDE209−toluene solution. Above all, we find that the low-lying transitions of BDE209tolu in the sunlight region all correspond to π→σ* absorptions, and the local maximum absorption is around 304 nm, which arises from the πtolu→σ* transition of complex II. Many experiments5−10 have demonstrated that BDE209 can undergo reductive debromination to lower bromo congeners in solvents by UV light in the sunlight region. Eriksson et al.8 have measured the absorption spectra of BDE209 in different solvents such as tetrahydrofuran, methanol, and so forth and have found that the local maximum absorptions are all around 300 nm. Our calculation results are in good agreement with the experiments. In the following discussion, we concentrate on the photodegradation reaction of typical BDE209-tolu complex I compared with that of BDE209 with implicit solvent. Different πσ* excited states are optimized by using TD-M06 theory combined with the PCM model. The πBDE209σ* state of BDE209 and the πcpxσ* and πtoluσ* (including two isomeric structures) states of complex I are located as depicted in Figure 2, and the representatively geometrical parameters and energies of the optimized excited states are given in Table 3. The

Table 3. Structures and Energies of Excited States of BDE209 and π-Stacking Complex Ia BDE209 C2−Br C3−Br C4−Br C5−Br C6−Br C2′-Br C3′-Br C4′-Br C5′-Br C6′-Br energy

BDE209-tolu

πBDE209σ*

πcpxσ*

πtoluσ*-Pb

πtoluσ*-Mc

1.89 1.89 1.89 1.89 1.89 1.95 1.94 1.92 1.94 1.91 75.1

1.89 1.89 1.89 1.89 1.89 1.95 1.95 1.92 1.94 1.91 74.9

1.92 1.94 2.36 1.91 1.93 1.89 1.89 1.90 1.89 1.89 71.2

1.90 1.91 1.93 2.38 1.95 1.89 1.89 1.89 1.89 1.89 71.4

Bond lengths are in Angstroms, and energies are in kcal mol−1. bThe largely elongated C−Br bond is at the para position. cThe largely elongated C−Br bond is at the meta position. a

Photodegradation with Implicit Solvent. A low-lying πBDE209σ* excited state is formed after the πBDE209→σ* electronic transition induced by UV light with the relative energy of 75.1 kcal mol−1. The structure changes of the πBDE209σ* excited state with respect to the ground state mainly happen on one of the phenyl rings, whereas the other one keeps almost the same geometry as the ground state. As shown in Figure 2A, the C2−C3 and C5−C6 bond lengths of the πBDE209σ* state are about 1.37 Å, and the other four bonds are about 1.43 Å in length, which means that two C−C bonds show double-bonding character whereas the other four have the feature of single-bonding. Obviously, this phenyl ring becomes nonplanar and in the form of quinoid with some charge-transfer characteristics indicating that the electron of the π-bonding orbital is primarily excited to this phenyl ring. In addition, a general elongation by about 0.02−0.06 Å of all five C−Br bonds of this phenyl is observed compared with the ground state of 1.89 Å as shown in Table 3. Interestingly, a C−Br bond at one of the ortho positions is the longest, whereas the one at the other ortho position is the shortest in length. In general, all five C−Br bonds of the electron-accepted phenyl are increased and are close to one another; the position of Br-substitution may not have much influence on the debromination process of BDE209. One can predict that all the C−Br bonds of the electron-accepted phenyl might be broken via the πBDE209σ* excited state no matter whether it is at the ortho, meta, or para position. Photodegradation with Explicit Solvent Molecule. (a). πcpxσ* State. The πcpxσ* state of complex I exhibits a similar geometrical character as the πBDE209σ* state of BDE209 as shown in Table 3. The electron-accepted phenyl (unstacked phenyl) has the quinoid form and the prolonged C−Br bonds. The relative energy is ∼74.9 kcal mol−1, which is also similar to the value of the πBDE209σ* state. A minimum of the πcpxσ* state cannot be located with the structure changes mainly happening on the stacked phenyl of monomer BDE209. This type of the πcpxσ* state may be correlated to a higher electronic state, which is difficult for the TD-DFT optimization or out of the sunlight region. Herein, we suppose that for a low-lying πcpxσ*

Figure 2. Structures of (A) the πBDE209σ* state of BDE209 and the (B) πcpxσ*, (C) πtoluσ*-P, and (D) πtoluσ*-M states of BDE209-tolu complex I (bond lengths are in Angstroms). D



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complex I. The electron clouds of the σ* orbitals largely gather around one of the C−Br bonds, which is opposite to the C−O bond of BDE209 for πtoluσ*-P, or at the meta position of BDE209 for πtoluσ*-M. This is quite different from the orbitals at the ground-state equilibrium geometries. The large elongation of the C−Br bond at the para (or meta) position might be caused by the intramolecular electron transfer from the oxygen side to the opposite side C−Br bond (or from one of the ortho positional C−Br bonds to the opposite meta positional side) after the πtolu→σ* transition. Apparently, the intramolecular electron transfer stimulates the breaking of the C−Br bonds at the para and meta positions of the electronaccepted phenyl and then promotes the photochemical degradation of BDE209 in toluene. Sun et al. have reported the TiO2-catalytic debromination experiment of BDE209.9 They pointed out the essential step of the electron transfer from the bromine to TiO2, and they indicated that the Br atom behaves as the electron donor. In this work, besides the intermolecular electron transfer, the intramolecular electron transfer also plays a prominent role in the debromination mechanism via the πtoluσ* state. In contrast to Sun’s experiment, the Br atom behaves as the electron acceptor for the system studied here. As discussed above, all the πσ* excited states share similar features, that is, only one phenyl ring of BDE209 exhibits large changes in geometry, whereas the other phenyl stays almost the same structure as that of the ground state. For the πBDE209σ* state of BDE209, the structure changes can happen on either phenyl ring. For the πcpxσ* state of the BDE209-tolu complex, the structure changes happen on the phenyl unstacked with the toluene ring, whereas for the πtoluσ* state, the electron can only be excited to the phenyl interacting with the toluene ring. (c). Photodegradation Mechanism. It will be useful to examine the potential energy curves (PECs) with regard to the C−Br internuclear distance because PECs provide a reasonable potential energy profile of the C−Br bond dissociation and present a qualitative indication of the photodegradation debromination mechanism. The PECs of the πcpxσ* state with regard to the C−Br distance at the ortho, meta, and para positions of the electron-accepted phenyl and a representative C−Br distance of the stacked phenyl are presented in Figure 4A. For πtoluσ*-M, the PECs along with the C−Br distance at three representative positions on the two phenyls are depicted in Figure 4B. The points are scanned rigidly without optimization, and for convenience, here we refer to the binding energy of a C−Br bond as the energy difference between the bottom of well and asymptotic region or the top of the barrier of the curve for a πσ* state. As shown in Figure 4A, the breakage of the C2′−Br, C3′−Br, and C4′−Br bonds on the electron-accepted phenyl of the πcpxσ* state requires similar energies, which suggests that all the C−Br bonds of the electron-accepted phenyl might be broken in the mechanism via the πcpxσ* state regardless of the substitution position. This is in the same situation as in the πBDE209σ* state as mentioned above and is also consistent with the experimental results that the debromination of ortho, meta, and para bromines can happen on pentabrominated phenyl in methanol, tetrahydrofuran, or toluene solvent, and three kinds of nona-BDEs can be formed.5−8 Moreover, Figure 4A shows that the breaking of the C2−Br bond requires much higher energy than the C−Br bond breakage on the unstacked phenyl. Therefore, the photochemical debromination reaction would

state the πcpx electron is primarily excited to the σ* orbital of phenyl unstacked with the toluene ring. (b). πtoluσ* State. The effects of the π-stacking interaction on the low-lying transitions of BDE209 in toluene solvent lead to an additional excitation from the π orbital of monomer toluene to the σ* orbital of monomer BDE209 at ∼305 nm under the UV-light irradiation, and the πtoluσ* excited state can be formed after the πtolu→σ* transition. Thus, the πtoluσ* state is an intermolecular charge-transfer excited state. Two isomeric structures are obtained for the πtoluσ* state of complex I and are denoted as πtoluσ*-P and πtoluσ*-M, respectively. The relative energies of the two conformations of the πtoluσ* state, 71.2 and 71.4 kcal mol−1 as presented in Table 3, are obviously lower than those of the πBDE209σ* and πcpxσ* states. From Figure 2C and D, we find that for both πtoluσ*-P and πtoluσ*-M the positions of methyl groups in toluene are above the C3−C4 bond of BDE209, which avoids the steric hindrance between the unstacked phenyl and the methyl group. Compared with the ground state, the structural differences of πtoluσ*-P and πtoluσ*-M states primarily occur on the phenyl interacting with the toluene ring, which indicates that the electron of a πtolu state is mainly excited to the σ* orbital located on the stacked phenyl. The particular striking difference between the excited and ground states is that the C4−Br bond (2.36 Å) in πtoluσ*-P at the para position and the C5−Br bond (2.38 Å) in πtoluσ*-M at the meta position are largely elongated by about 0.46 and 0.48 Å in length, respectively. A slightly elongation can also be seen for the other four C−Br bonds of the electron-accepted phenyl. This geometrical character suggests that the debromination process of BDE209 involving the toluene solvent is largely dependent on the position of Brsubstitution. A πtoluσ* state with the structure changes mainly happening on the unstacked phenyl ring cannot be located. This is reasonable that the electron of the πtolu orbital can be excited to the stacked phenyl much easier than to the one far away from the toluene ring. This result is different from those of the πcpxσ* and the πBDE209σ* states. For the former, the electron of the πbonding orbital mainly goes to the unstacked phenyl after the π→σ* transition, whereas for the latter, the electron can be excited to a σ* orbital of either phenyl ring. The π and σ* molecular orbitals at the two isomeric structures of the πtoluσ* state are depicted in Figure 3 for

Figure 3. π (lower) and σ* (upper) molecular orbitals at the two isomeric structures of the πtoluσ* state for complex I. E



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states. The debromination reaction can proceed on the phenyl interacting with the toluene via the πtoluσ* state, and the intramolecular charge transfer after the photoexcitation significantly promotes the elongation of the C−Br bond at the para or meta position. The PECs show that breaking this prolonged C−Br bond is much easier than breaking the other C−Br bonds, and this becomes the most feasible degradation pathway. In contrast, for the mechanism via the πcpxσ* state, the electron of the π orbital is mainly excited to the σ* orbital of the unstacked phenyl. The debromination reaction can happen at the ortho, meta, or para position of this phenyl, which is consistent with the analysis that the C−Br bond lengths at these positions are close to each other, and thus three kinds of nona-BDE products might be formed from BDE209. Interestingly, the photodegradation mechanism of BDE209 in toluene is similar to the results of nona-BDEs in methanol.50 In that work, the photodegradation reaction of nona-BDEs can take place either on the penta- or the tetrabromine-substituted phenyl via different πσ* excited states. Similar to the debromination via the πcpxσ* state, on the pentabromine phenyl, all five C−Br bonds can be broken, while on the tetrabromine phenyl, the way undergoing breakage of the elongated C−Br bond is the dominant channel as the case via the πtoluσ* state. The two photodegradation reaction mechanisms via the πtoluσ* and πcpxσ* states are competitive. It is useful to make comparison for the absorption character to determine the most feasible mechanism. Again in Table 2, the πtolu→σ* transitions with f of 0.0164 have stronger local absorption than the πcpx→ σ* transitions having f of 0.0119 for BDE209-tolu. Moreover, the vertical excitation wavelengths of the former transitions (∼304 nm) are shorter than those of the latter ones for all three complexes. It could be suggested that the way via the πtoluσ* state is expected to be the more favorable pathway.

Figure 4. Potential energy curves with regard to the C−Br internuclear distance for the (A) πcpxσ* and (B) πtoluσ*-M states of complex I computed using the TD-M06 method combined with the PCM model (in toluene).

hardly happen on the electron-unaccepted phenyl via the πcpxσ* state. It can be seen from Figure 4B that the energy required to break the largely elongated C5−Br bond (2.38 Å) on the electron-accepted phenyl is the least among all the C−Br bonds for πtoluσ*-M. This is the dominant debromination channel in the photodegradation reaction of BDE209-tolu complexes. Similarly, the C4−Br of πtoluσ*-P with a bond length of 2.36 Å could be expected to be broken at low-energy cost. Moreover, the binding energy of C4−Br bond is ∼20 kcal mol−1 higher than that of the C5−Br bond, although the C4−Br bond is slightly increased to 1.93 Å after excitation. Also, the C−Br bonds on the unstacked phenyl show not surprisingly much higher binding energies than those of the longer C−Br bonds on the stacked phenyl. Thus, it can be concluded that the breakage of the C−Br bond prefers to happen on the elongated one rather than on the ones with shorter C−Br bond length via the πtoluσ* state, and the debromination via the πtoluσ* state shows a very strong selectivity. Several PECs with high-binding energies seem to lose smoothness at around 2.5 Å of the C−Br distance in Figure 4, for example, the PECs with regard to the C3′−Br distance for πtoluσ*-M and with regard to the C2−Br distance for the πcpxσ* state. This may be caused by a curve crossing with higher electronic states in that region. Here, we do not discuss them in detail because this kind of surface crossing, which is believed to be widely existent in the study of excited electronic states of polyatomic molecules,57−59 happens in potential energy regions about which we are not concerned. Above all, the photocatalysis degradation reaction of BDE209-tolu follows two mechanisms via different πσ* excited

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CONCLUSIONS The detailed photodegradation mechanisms of BDE209 in the toluene solvent have been investigated using the DFT and TDDFT methods. The participation of the toluene molecule plays an important role in the photocatalytic debromination of BDE209 through the promotive effects caused by the face-toface π−π interactions between BDE209 and toluene. Two main photodegradation mechanisms are identified, which proceed via the πtoluσ* and πcpxσ* excited states of the BDE209-tolu π-stacking complexes, respectively, formed by photoexcitation at ∼300 nm in the sunlight region. For the mechanism via the πtoluσ* state, the intramolecular charge transfer to the π-stacking phenyl results in a significant elongation of the C−Br bond at the para or meta position, and then the debromination reaction proceeds through the breakage of these bonds. For the one via the πcpxσ* state with the electronic excitation from the π orbital distributed over the whole complex skeleton to the σ* orbital of the phenyl far away from toluene, the debromination would not be influenced by the Br-substituted position and may happen at the ortho, meta, or para position. The mechanism via the πtoluσ* state is expected to be the more feasible pathway owing to the stronger local absorption.

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ASSOCIATED CONTENT

S Supporting Information *

The structure of H-bonding BDE209-tolu complex, the frontier molecular orbitals of BDE209-tolu complexes II and III, the F



Article

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optimized geometries in terms of Cartesian coordinates (in Angstrom) for the S0 and excited states, and the test of the effect of the integration grid. 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] (W.B.), [email protected] (J.Z.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the Chinese Ministry of Science and Technology (no. 2013CB834601), the National Natural Science Foundation of China (nos. 21173232, 91221105), and the Chinese Academy of Sciences. Many computations are carried out at Virtual Laboratory of Computational Chemistry, Computer Network Information Center of Chinese Academy of Sciences.

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