Free Radical Reactions in Poly(methyl methacrylate) Films Monitored Using a Prefluorescent Quinoline−TEMPO Sensor

Abstract

The photochemical and photophysical properties of 4-(3-hydroxy-2-methyl-4-quinolinoyloxy)-2,2,6,6-tetramethylpiperidine-1-oxyl free radical (QT) have been studied as a prefluorescent probe to monitor free radical processes in polymer films. This methodology takes advantage of the efficient intramolecular quenching of the fluorescence of quinoline by the paramagnetic nitroxide, which is disabled when TEMPO reacts with carbon-centered radicals. The fluorescence intensity-time profile observed in the thermal decomposition of 2,2′-azobis(isobutyronitrile) (AIBN) in poly(methyl methacrylate) (PMMA) films showed initial increments in the fluorescence with time, according to the trapping of carbon-centered radicals by QT in the polymer films. Comparison of data under nitrogen and oxygen saturation conditions suggests that oxygen trapping of the carbon-centered radicals at 90 °C is about 20 times faster than reaction with nitroxides. The activation energy for AIBN decomposition in PMMA was measured as 34.1 kcal/mol. Analysis of the fluorescence lifetime distribution establishes the involvement of both static and dynamic fluorescence quenching of the diamagnetic reaction product by AIBN.

The First Example of an Azaphenalene Profluorescent Nitroxide

“…A tether between the fluorophore and the radical moiety increases encounters between the fluorophore and the radical, which increases the efficacy of fluorescence quenching. Much work has been done on these tethered units, initially by Blough et al [2][3][4][5][6][7] and more recently by Scaiano et al [8][9][10] Both groups demonstrated that, in the presence of the nitroxide radical, fluorescence is substantially reduced. Loss of the spin by radical trapping, or redox activity, removes this quenching effect, and the fluorescence of the molecule can then be observed.…”

Supramolecular photochemistry in zeolites: From catalysts to sunscreens

“…The quenching mode operative in the paramagnetic probe is lost, and the emission is restored, corresponding to a trapped or "on" state. There are a number of examples in the literature where this methodology has been applied to the study of radicals in other types of heterogeneous systems, e.g., polymers, micelles [114][115][116][117].…”

The Synthesis and Physical Properties of Novel Polyaromatic Profluorescent Isoindoline Nitroxide Probes

“…As many intermolecular quenching mechanisms rely on chance collisions between an excited molecule and the nitroxide radical, the linking together of these moieties increases the rate of interaction which subsequently enhances the efficacy of fluorescence quenching. Work by Blough [5][6][7][8][9][10] and Scaiano [11][12][13] has shown that fluorescence is significantly reduced in the presence of a nitroxide radical. Following radi-cal trapping or redox activity, a diamagnetic species is formed and normal fluorescence emission is restored.…”