THE PHOTOCHEMISTRY OF POLYHALOARENES .10. THE PHOTOCHEMISTRY OF 4-BROMOBIPHENYL

The photodebromination of 4-bromobiphenyl (BpBr) in a variety of solvents has been characterized by measurement of the dependence of quantum yield upon substrate concentration, concentration of quenchers cis-1,3-pentadiene and fumaronitrile, and determination of the quantum yield for intersystem crossing, phi-isc. The dependence of quantum yield upon light intensity reveals no evidence for biphotonic processes. The rate constant for triplet excimer formation (k2) is dependent upon the polarity of the solvent as measured by E(T). By use of the linear solvation energy relationship of Abraham, Kamlet, and Taft, the rate constant for excimer formation is dependent upon the Hildebrand solubility parameter (delta-H-2) and solvent hydrogen bond acceptor basicity (beta-1), log k2 = 1.94 + 0.00242-delta-H-2 + 0.248-beta-1. The rate constant for triplet excimer formation is much less sensitive to change in the E(T) value of the solvent than is the rate constant (k(ex) for exciplex formation form BpBr3 and triethylamine.