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Excited States and photodebromination of selected polybrominated diphenyl ethers: computational and quantitative structure--property relationship studies.

Luo J, Hu J, Wei X, Li L, Huang X - Int J Mol Sci (2015)

Bottom Line: This paper presents a density functional theory (DFT)/time-dependent DFT (TD-DFT) study on the lowest lying singlet and triplet excited states of 20 selected polybrominateddiphenyl ether (PBDE) congeners, with the solvation effect included in the calculations using the polarized continuum model (PCM).The results obtained showed that for most of the brominated diphenyl ether (BDE) congeners, the lowest singlet excited state was initiated by the electron transfer from HOMO to LUMO, involving a π-σ* excitation.Based on the previously reported kinetic data for the debromination by ultraviolet (UV) and sunlight, obtained QSPR models exhibited a reasonable evaluation of the photodebromination reactivity even when the BDE congeners had same degree of bromination, albeit different patterns of bromination.

View Article: PubMed Central - PubMed

Affiliation: Guizhou Provincial Key Laboratory for Information System of Mountainous Areas and Protection of Ecological Environment, Guizhou Normal University, Guiyang 550001, China. luojin@gznu.edu.cn.

ABSTRACT
This paper presents a density functional theory (DFT)/time-dependent DFT (TD-DFT) study on the lowest lying singlet and triplet excited states of 20 selected polybrominateddiphenyl ether (PBDE) congeners, with the solvation effect included in the calculations using the polarized continuum model (PCM). The results obtained showed that for most of the brominated diphenyl ether (BDE) congeners, the lowest singlet excited state was initiated by the electron transfer from HOMO to LUMO, involving a π-σ* excitation. In triplet excited states, structure of the BDE congeners differed notably from that of the BDE ground states with one of the specific C-Br bonds bending off the aromatic plane. In addition, the partial least squares regression (PLSR), principal component analysis-multiple linear regression analysis (PCA-MLR), and back propagation artificial neural network (BP-ANN) approaches were employed for a quantitative structure-property relationship (QSPR) study. Based on the previously reported kinetic data for the debromination by ultraviolet (UV) and sunlight, obtained QSPR models exhibited a reasonable evaluation of the photodebromination reactivity even when the BDE congeners had same degree of bromination, albeit different patterns of bromination.

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Frontier orbitals of BDE-181 (A: HOMO; B: LUMO) (iso-surface value = 0.02, arbitrary unit).
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ijms-16-01160-f002: Frontier orbitals of BDE-181 (A: HOMO; B: LUMO) (iso-surface value = 0.02, arbitrary unit).

Mentions: For ten of the selected twenty BDE congeners (Table 1), the electron excitation from highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO) made the biggest contribution to S0→S1 excitation. The HOMOs for most of the BDE congeners were found to be π-based, and preferred to spread mainly over the aromatic ring of fewer bromine substituents (especially for the lower brominated BDE congeners). Conversely, the LUMOs for the BDE congeners under study generally have the σ* character, and were mainly located on the phenyl group which has relatively more bromine substituents (e.g., BDE-181, Figure 2). This should be caused by the electron-withdrawing effects of the bromine atoms on the two phenyl rings. According to the assumption of Kasha’s rule, higher electronic states (SN) should decay exclusively via internal conversion to the lowest excited state in a solvent where collisions are common and energy dissipation is fast [33]. Therefore, after excitation, the molecules could convert to S1whichproceed from π orbital to σ* orbital showing the characteristics of the coherent HOMO-LUMO electronic transition. A similar phenomenon was found in the photoreductive debromination of a halogen-binding-based complex between decabromodiphenyl ether (BDE-209) and carboxylate under visible light irradiation [24]. Unlike other halogenated aromatic compounds, such as polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs), the conjugated system of the two aromatic rings for PBDEs is relatively weak. For the BDE congeners not symmetrically brominated on the two aromatic rings, the phenyl group with more Br substituents becomes more electron-deficient than the other one. Therefore, the electronic excitation from HOMO to LUMO of PBDEs would make the electron of electronic-rich ring transferred to the electron-deficient ring, resulting in a situation similar to the BDE anionic species which captured an additional electron in the LUMO [34,35,36]. Consequently, this transition of electrons into these anti-bond orbitals will reduce the C–Br bond order, and the C–Br bonds become weakened and easy to break in the excited states of BDE congeners. This contribution of S1 to the photolysis of PBDEs could be supported by previous theoretical calculations, the structure of PBDEs in singlet excited state and the structure of PBDE anions have the same geometrical characteristic, i.e., the significant lengthening and out-of-plane bending of C–Br bonds [28,37,38]. Zeng et al. [39] also found the major debromination products of PBDEs have a greater similarity between photodebromination and Fe0 reduction and the linear relationships are significantly high between the LUMO energies and the debromination rates of PBDEs by treatment using UV light or zero-valent iron. In addition, it can be inferred that with one aromatic ring having relatively more bromine substituents than the other one, the electronic excitation of this kind of BDE congeners might be relatively easy, and the photochemical debromination, according to the characteristics of S1, would prefer to occur on the benzene ring having a higher bromination. These assumptions could be in agreement with the results observed in the previous photodebromination experiments [40].


Excited States and photodebromination of selected polybrominated diphenyl ethers: computational and quantitative structure--property relationship studies.

Luo J, Hu J, Wei X, Li L, Huang X - Int J Mol Sci (2015)

Frontier orbitals of BDE-181 (A: HOMO; B: LUMO) (iso-surface value = 0.02, arbitrary unit).
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4307296&req=5

ijms-16-01160-f002: Frontier orbitals of BDE-181 (A: HOMO; B: LUMO) (iso-surface value = 0.02, arbitrary unit).
Mentions: For ten of the selected twenty BDE congeners (Table 1), the electron excitation from highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO) made the biggest contribution to S0→S1 excitation. The HOMOs for most of the BDE congeners were found to be π-based, and preferred to spread mainly over the aromatic ring of fewer bromine substituents (especially for the lower brominated BDE congeners). Conversely, the LUMOs for the BDE congeners under study generally have the σ* character, and were mainly located on the phenyl group which has relatively more bromine substituents (e.g., BDE-181, Figure 2). This should be caused by the electron-withdrawing effects of the bromine atoms on the two phenyl rings. According to the assumption of Kasha’s rule, higher electronic states (SN) should decay exclusively via internal conversion to the lowest excited state in a solvent where collisions are common and energy dissipation is fast [33]. Therefore, after excitation, the molecules could convert to S1whichproceed from π orbital to σ* orbital showing the characteristics of the coherent HOMO-LUMO electronic transition. A similar phenomenon was found in the photoreductive debromination of a halogen-binding-based complex between decabromodiphenyl ether (BDE-209) and carboxylate under visible light irradiation [24]. Unlike other halogenated aromatic compounds, such as polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs), the conjugated system of the two aromatic rings for PBDEs is relatively weak. For the BDE congeners not symmetrically brominated on the two aromatic rings, the phenyl group with more Br substituents becomes more electron-deficient than the other one. Therefore, the electronic excitation from HOMO to LUMO of PBDEs would make the electron of electronic-rich ring transferred to the electron-deficient ring, resulting in a situation similar to the BDE anionic species which captured an additional electron in the LUMO [34,35,36]. Consequently, this transition of electrons into these anti-bond orbitals will reduce the C–Br bond order, and the C–Br bonds become weakened and easy to break in the excited states of BDE congeners. This contribution of S1 to the photolysis of PBDEs could be supported by previous theoretical calculations, the structure of PBDEs in singlet excited state and the structure of PBDE anions have the same geometrical characteristic, i.e., the significant lengthening and out-of-plane bending of C–Br bonds [28,37,38]. Zeng et al. [39] also found the major debromination products of PBDEs have a greater similarity between photodebromination and Fe0 reduction and the linear relationships are significantly high between the LUMO energies and the debromination rates of PBDEs by treatment using UV light or zero-valent iron. In addition, it can be inferred that with one aromatic ring having relatively more bromine substituents than the other one, the electronic excitation of this kind of BDE congeners might be relatively easy, and the photochemical debromination, according to the characteristics of S1, would prefer to occur on the benzene ring having a higher bromination. These assumptions could be in agreement with the results observed in the previous photodebromination experiments [40].

Bottom Line: This paper presents a density functional theory (DFT)/time-dependent DFT (TD-DFT) study on the lowest lying singlet and triplet excited states of 20 selected polybrominateddiphenyl ether (PBDE) congeners, with the solvation effect included in the calculations using the polarized continuum model (PCM).The results obtained showed that for most of the brominated diphenyl ether (BDE) congeners, the lowest singlet excited state was initiated by the electron transfer from HOMO to LUMO, involving a π-σ* excitation.Based on the previously reported kinetic data for the debromination by ultraviolet (UV) and sunlight, obtained QSPR models exhibited a reasonable evaluation of the photodebromination reactivity even when the BDE congeners had same degree of bromination, albeit different patterns of bromination.

View Article: PubMed Central - PubMed

Affiliation: Guizhou Provincial Key Laboratory for Information System of Mountainous Areas and Protection of Ecological Environment, Guizhou Normal University, Guiyang 550001, China. luojin@gznu.edu.cn.

ABSTRACT
This paper presents a density functional theory (DFT)/time-dependent DFT (TD-DFT) study on the lowest lying singlet and triplet excited states of 20 selected polybrominateddiphenyl ether (PBDE) congeners, with the solvation effect included in the calculations using the polarized continuum model (PCM). The results obtained showed that for most of the brominated diphenyl ether (BDE) congeners, the lowest singlet excited state was initiated by the electron transfer from HOMO to LUMO, involving a π-σ* excitation. In triplet excited states, structure of the BDE congeners differed notably from that of the BDE ground states with one of the specific C-Br bonds bending off the aromatic plane. In addition, the partial least squares regression (PLSR), principal component analysis-multiple linear regression analysis (PCA-MLR), and back propagation artificial neural network (BP-ANN) approaches were employed for a quantitative structure-property relationship (QSPR) study. Based on the previously reported kinetic data for the debromination by ultraviolet (UV) and sunlight, obtained QSPR models exhibited a reasonable evaluation of the photodebromination reactivity even when the BDE congeners had same degree of bromination, albeit different patterns of bromination.

Show MeSH
Related in: MedlinePlus