Photophysical Properties of the Prefluorescent Nitroxide Probes QT and C343T

The photophysical properties of the nitroxide prefluorescent probes 4-(3-hydroxy-2-methyl-4-quinolinoyloxy)-2,2,6,6-tetramethyl-piperidin-4-yl) ester free radical (QT) and 2,3,4,6,7,8-hexahydro-quinolizino [1,10,9-gh] coumarin-3-carboxylic acid (1-oxyl-2,2,6,6-tetramethyl-piperidin-4-yl) ester free radical (C343T) were evaluated as a function of pH and solvent properties. The absorbance of QT showed high pH sensitivity. The pK(a) values for the different ionization forms involved in the acid-base equilibrium of the quinoline chromophore were determined in the ground and excited states. The fluorescence lifetimes of QT, and N-hydroxylamine (QTH) and quinoline methyl ester (QMe) derivatives, showed that the intramolecular quenching efficiency by the nitroxide moiety is independent of the quinoline ionization form. The fluorescence and absorbance of C343T were highly sensitive to solvent polarity in agreement with a charged transfer excited state of the chromophore. However, we noted a decrease in the intramolecular fluorescence quenching efficiency by the nitroxide moiety when changing the polarity of the solvent from hexane to water. This behavior has been attributed to a suppression of an energy transfer mechanism in the nitroxide quenching process in very polar solvents. The results obtained in micelles allow us to propose QT and C343T as sensors for pH and micropolarity, respectively, in addition to their role as monitors for free radicals or hydrogen transfer from phenols.