Photoexcitation dynamics of bromodiphenyl ethers in acetonitrile-d studied by femtosecond time-resolved infrared spectroscopy.

The efficient decomposition of polybrominated diphenyl ethers (PBDEs), onetime prevalent flame retardants, is central to the reduction of their harmful effects on human health. PBDE photodecomposition is a promising method, but its mechanism and products are not well understood. The photoexcitation dynamics of 3- and 4-bromodiphenyl ethers (BDE-2 and BDE-3) in CDCN were studied from 0.3 ps to 10 μs using time-resolved infrared spectroscopy. An excitation at 267 nm dissociated the Br atom from BDE-2 and BDE-3 within 0.3 ps and 14 ± 3 ps, respectively, producing a radical compound (R) and a Br atom. About 85% of R formed an intermediate (IM) that weakly interacted with the Br atom and the surrounding CDCN solvent in 7-12 ps. The remaining R separated from the dissociated Br and underwent slow geminate rebinding (GR) with Br within 35 to 54 ns. The IM competitively engaged in GR with the interacting Br in 40-60 ps or formed CDCN-bound radical compounds (RS) in 100-130 ps. The RS further degraded either the dissociation of CD-producing a cyano-bound diphenyl ether (DE) in 150 or 550 ns-or the deuterium abstraction of CDCN in 180 or 430 ns-producing a deuterated DE. Overall, 33 ± 3 (22 ± 3)% of the photoexcited BDE-2 (BDE-3) decomposed in CDCN under 267 nm excitation. Efficient binding of the CDCN solvent to R deterred the yield-diminishing GR and slowed the rate of product formation. The observed photoexcitation dynamics of BDE suggest methods for the efficient decomposition of PBDE.