Wavelength dependence of luminescence and quantum yield in dechlorination of PCBs

Direct photolysis of many PCBs has a non quenchable component which survives oxygen quenching. In this study, the nature of the excited states of PCB 138 and PCB 77 was analyzed to understand the origins of reaction from singlet as well as triplet excited states. Data on luminescence quenching, luminescence profiles, reaction quantum yields, and product distributions as functions of excitation wavelength were collected. The most striking and unambiguous observation was the excitation wavelength dependence of the vibronic structure of a fluorescence. Dependence of the apparent quantum yields on the reaction conditions (concentration, intensity) was also recognized during this study. Upon correlating all collected data, sufficient evidence for slow vibrational relaxation in a 2,2′-dichloro congener emerged. This could be seen to open opportunities for reaction from the singlet in competition with intersystem crossing. The slow vibrational relaxation can be understood through analogy to quantitatively modeled biphenyl spectra, to arise from steric hindrance and solvent resistance to rotation about the central C–C bond.

► There is some strong evidence that a singlet excited state might be involved in the dechlorination of PCBs.
► Following the trends in reaction yields, product distributions and analyzing emission line shapes were the approaches taken for this study.
► The results show that large geometric changes create the opportunities for reaction from a singlet excited state.
► Dependence of the apparent quantum yields on the reaction conditions, concentration and intensity, was also recognized during this study.