Photoinduced decomposition of the polybromostyryl carbanions

The photoinduced decay in THF of the sodium salt of the polybromostyryl carbanions (PS_B?) was investigated in vacuum at ?78°C. Maximal rates of decay were obtained when the wavelength of the incident light corresponded to the absorption maximum of the PS_B? at 495 nm. The quantum yield of the process was found to be 13 eq./Einstein. Products of the photostimulated decompositions at ?78°C were found to be different from those obtained as result of the “dark” thermal decomposition at –20°C. Decomposition of the PS_B? carbanions was accompanied by evolution of Br^? anions and by formation of polyaryl carbanions absorbing in the 270- to 400-nm range. The mechanism of the photoinduced decomposition of PS_B?, though basically analogous to the S_RN1 reaction mechanism, was found to differ significantly from the latter. The intramolecular dissociative electron transfer was assumed to represent its initiation step. Its propagation was attributed to the addition of PS_B? to the initially formed biradicals, ejection of Br^? from the resulting radical anions, and intramolecular electron transfer yielding polybromostyryl radicals. Products of the photoinduced decomposition of PS_B? remained soluble in THF when the intensity of the incident light was relatively weak, though their molecular weight distribution broadened. Insoluble, highly crosslinked polymers were obtained when the intensity of the incident light was very high.