Quenching of the cascade reaction between triplet and photochrome probes by nitroxide radicals

A cascade of photochemical reactions between the triplet sensitizer Erythro sin B and the photochrome stilbene-derivative probe exhibiting the phenomen on of cis-trans photoisomerization has been investigated. The cascade tripl et cis-trans photoisomerization of the excited stilbene chromophore is sens itized with the excited triplet Erythrosin B and depressed with nitroxide r adicals quenching the excited triplet state of sensitizer. The rate of the cascade photoisomerization is sensitive to the molecular dynamics of media and radicals' concentration. The proposed method allows to measure the prod uct of quenching rate constant and the sensitizer's triplet lifetime k(q) x tau(ph). Calibration of the 'triple' system permits the quantitative detec tion of the nitroxide radicals in the vicinity of the stilbene-derivative p hotochrome and sensitizing triplet probes. The experiment is carried out wi th the constant-illumination fluorescence technique. Sensitivity of the met hod is close to 10(-12) mole of fluorescence molecules per sample and the m inimal local concentration of nitroxide radicals being detected is about 10 (-5) M. This method allows to investigate any chemical, biological objects and surface processes of microscopic scale when the minimal volume is about 10(-3) mu l. (C) 1999 Elsevier Science S.A. All rights reserved.