MULTIPHASE FLUORESCENCE QUENCHING BY A SURFACTANT NITROXYL RADICAL

The self-association of 4-[N,N-dimethyl-N-(n-hexadecyl)ammonium]-2,2,6,6-tetramethylpiperidinyl-N’-oxyl bromide (1), a surfactant nitroxyl radical, into micelles was studied by ESR spectroscopy giving an estimate of its critical micelle concentration (cmc) as (4.6±0.5) × 10-4 M, in the absence of added salt. The fluorescence quenching of a series of cationic fluorophors of the structure [(1-pyrenyl)(CH2)nN(CH3)3]+ X- (2-n) by 1 was investigated over a concentration range below and above the cmc. A comparison of fluorescence lifetime and yield measurements indicates that 2-1 and 2-5 are quenched by a simple diffusional mechanism while 2-11 is quenched by a static mechanism below the cmc. Discontinuities occur in the Stern-Volmer plots at the cmc owing to the onset of static quenching of fluorophors solubilized in the micelle phase. Analysis of a kinetic scheme incorporating the several quenching mechanisms operating below and above the cmc leads to estimates of Kb/N for the 2-n fluorophors: (2-1), 27; (2-5), 336; (2-11), 38 000 M-1, where Kb is the association constant with the micelle and N is the aggregation number of the host micelle. © 1979, American Chemical Society. All rights reserved.