Ultrafast Observation of Isomerization and Complexation in the Photolysis of Bromoform in Solution

Ultrafast photolysis of bromoform (CHBr3) 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 ill 495 urn appears. The wavelength and time evolution of the first absorption is consistent with the formation of iso-bromoform (CHBr2-Br) by recombination of the fragment radicals within the solvent cage. The presence 4 in 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, CHBr3-Br, with other bromoform Molecules. The features in the transient spectra are qualitatively similar ill cyclohexane solutions of bromoform, The wavelength of the transition of iso-bromoform does not change upon dilution, but that of the CHBr3-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.