Quenching of the fluorescence of substituted benzenes by halomethanes

Rate constants, k q , for the quenching of the singlet state of benzene, toluene, p-xylene, fluorobenzene, trifluoromethyl benzene, p-bis(trifluoromethyl)benzene, and aniline by CCl 4 , CH 3 CCl 3 , CHCl 3 , and CFCl 3 , 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 k q and IP (ionization potential) of the aromatics which supports Klein's model for exciplex formation. The most effective quencher is CCl 4 while the least effective is CFCl 3 . 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.