Ultrafast observation of isomerization and complexation in the photolysis of bromoform in solution

Ultrafast photolysis of bromoform (CHBr(3)) with a 267 nm pulse of light followed by broadband transient electronic absorption identifies the photoproducts and follows their evolution in both neat bromoform and cyclohexane solutions. In neat bromoform, a species absorbing at 390 nm appears promptly and decays with a time constant of 13 ps as another species absorbing at 495 nm appears. The wavelength and time evolution of the first absorption is consistent with the formation of iso-bromoform (CHBr(2)-Br) by recombination of the fragment radicals within the solvent cage. The presence of an isosbestic point in the transient spectra indicates that this isomer is the precursor of the second absorber. The excess internal energy remaining in iso-bromoform permits release of the weakly bound Br atom to form a complex, CHBr(3)-Br, with other bromoform molecules. The features in the transient spectra are qualitatively similar in cyclohexane solutions of bromoform. The wavelength of the transition of iso-bromoform does not change upon dilution, but that of the CHBr(3)-Br complex systematically decreases with addition of cyclohexane. This trend agrees with the predicted dependence of the energy of a charge-transfer transition on the dielectric constant of the medium. Vibrational relaxation is likely to be the controlling feature of the evolution of the initially formed iso-bromoform.