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

Jin Luo,Jiwei Hu,Xionghui Wei,Lingyun Li,Xianfei Huang

doi:10.3390/ijms16011160

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.

Highlights

Polybrominateddiphenyl ethers (PBDEs) are organobromine compounds that are widely used as an additive flame retardant in polymers and are widespread in the environment [1,2,3,4,5]
Since PBDEs share the basic structure of diphenyl ether, the properties of brominated diphenyl ether (BDE) congeners will all depend on the bromination pattern
For most of the BDE congeners, the singlet excited state was initiated by the electron transfer from highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO), involving a π–σ*

Summary

Introduction

Polybrominateddiphenyl ethers (PBDEs) are organobromine compounds that are widely used as an additive flame retardant in polymers and are widespread in the environment [1,2,3,4,5]. Several quantitative structure-property relationship (QSPR) studies were previously carried out to investigate the photodebromination rates and quantum yields of PBDEs [26,27]. In these QSPR models, the excitation energy was not included as the molecular descriptors, and the studies on the relationship between the excitation states and the photodebromination of PBDEs are still insufficient and incomplete [28]. To better investigate the relationship between the excited states and the photodebromination of PBDEs, 20BDE congeners were selected from their reported experiments for a computational study in the present work. Linear and nonlinear models were employed for a quantitative structure-property relationship (QSPR) study based on the reported photodebromination rate constants of PBDEs by UV [19]. We considered and studied the over fitting problems encountered when using the BP-ANN methods since this might be useful for significantly improving the performance of the BP-ANN model

Results and Discussion

The Lowest-Lying Triplet Excitations for Selected BDE Congeners

Performance of the QSPR Models

Experimental Section

Conclusions