Evidence for nonemissive exciplexes in the singlet quenching of polycyclicaromatic hydrocarbons by polychlorobenzenes in cyclohexane

The quenching of the singlet excited state of several polycyclic aromatic h ydrocarbons (PAH) by polychlorobenzenes was investigated in cyclohexane by static and time-resolved fluorescence measurements. Intensity Stern-Volmer plots were linear, while the corresponding tau(0)/tau plots presented a dow nward curvature. Laser flash photolysis experiments indicated that the only intermediate formed in the quenching process is the triplet state of the P AH. The efficiency of the triplet formation was determined and it was found to be near unity in several cases. These results were interpreted with a m echanism involving the intermediacy of an exciplex formed by the excited si nglet of the PAH as an electron donor and the chlorobenzene as an electron acceptor. Bimolecular quenching rate constants were obtained from the inten sity Stem-Volmer plots or from the initial slope of the lifetime plots. The y could be satisfactorily correlated with the energy of the charge transfer state by using the exciplex model. (C) 1998 Elsevier Science S.A. All righ ts reserved.