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Photophysical Properties of the Prefluorescent Nitroxide Probes QT and C 343 T
Author(s) -
Bueno Claudia,
Mikelsons Larisa,
Maretti Luca,
Scaiano J. C.,
Aspée Alexis
Publication year - 2008
Publication title -
photochemistry and photobiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.818
H-Index - 131
eISSN - 1751-1097
pISSN - 0031-8655
DOI - 10.1111/j.1751-1097.2008.00381.x
Subject(s) - nitroxide mediated radical polymerization , chemistry , photochemistry , physics , nuclear magnetic resonance , radical polymerization , polymer , copolymer
Abstract 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 (C 343 T) were evaluated as a function of pH and solvent properties. The absorbance of QT showed high pH sensitivity. The p K 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 C 343 T 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 C 343 T as sensors for pH and micropolarity, respectively, in addition to their role as monitors for free radicals or hydrogen transfer from phenols.