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

A cascade of photochemical reactions between the triplet sensitizer Erythrosin B and the photochrome stilbene-derivative probe exhibiting the phenomenon of cis-trans photoisomerization has been investigated. The cascade triplet cis-trans photoisomerization of the excited stilbene chromophore is sensitized with the excited triplet Erythrosin B and depressed with nitroxide radicals 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 product of quenching rate constant and the sensitizer's triplet lifetime k_q × τ_ph. Calibration of the 'triple' system permits the quantitative detection of the nitroxide radicals in the vicinity of the stilbene-derivative photochrome and sensitizing triplet probes. The experiment is carried out with the constant-illumination fluorescence technique. Sensitivity of the method is close to 10^-12 mole of fluorescence molecules per sample and the minimal 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 μl.