DISTANCE-DEPENDENT FLUORESCENCE QUENCHING OF P-BIS[2-(5-PHENYLOXAZOLYL)]BENZENE BY VARIOUS QUENCHERS

We report results of frequency-domain and steady-state measurements of the fluorescence quenching of p-bis[2-(5-phenyloxazolyl)]benzene (POP OP) when quenched by bromoform (CHBr3), methyl iodide (CH3I), potassiu m iodide (KI), 1,2,4-trimethoxybenzene (TMB), or N,N-diethylaniline (D EA). The quenching efficiency of these compounds decreased in the orde r DEA, TMB, KI, CH3I, CHBr3. In the case of DEA and TMB the measuremen ts clearly confirm the applicability of the exponential distance-depen dent quenching (DDQ) model, in which the bimolecular quenching rate k( r) depends exponentially on the fluorophore-quencher separation r, k(r ) = k(a) exp[-(r - alpha)lr(e)], where alpha is the distance of closes t approach. Simultaneous analysis of the frequency-domain and steady-s tate data significantly improved resolution of the recovered molecular parameters k(a) and r(e). The data for DEA and TMB cannot be satisfac torily fit using either the Smoluchowski or Collins-Kimball radiation boundary condition (RBC) model. The quenching behavior of the less eff icient quenchers KI, CH3I, and CHBr3 can be adequately described with both the DDQ and RBC models, but this may be a simple consequence of l ess efficient quenching. The efficiency of quenching is discussed on t he basis of the mechanisms of interaction between the fluorophore and quencher molecules, which involves electron transfer and/or heavy atom effects.