Quenching of the fluorescence of substituted benzenes by halomethanes

Rate constants, kq, for the quenching of the singlet state of benzene, toluene, p-xylene, fluorobenzene, trifluoromethyl benzene, p-bis(trifluoromethyl)benzene, and aniline by CCl4, CH3CCl3, CHCl3, and CFCl3, in the gas phase have been determined. In each instance the quenching is via a charge-transfer complex with the aromatic acting as donor. There is a linear dependence between ln kq and IP (ionization potential) of the aromatics which supports Klein's model for exciplex formation. The most effective quencher is CCl4 while the least effective is CFCl3. The most effective donor was aniline while the least effective one was p-bis(trifluoromethyl)benzene. In general, fluorinated aromatics are less effective donors than the corresponding protonated molecules.