Synthesis and spectral characteristics of di-substituted 1,8-naphthalimides: Bi-radical formation

Abstract

Novel probes representing connection of 1,8-naphthalimides as chromophore and two structural units of sterically hindered amine stabilizer (HAS) were synthesized. One HAS unit was situated on naphthalene ring in position 4 and second HAS created imide composition. HASes were in the form of parent amine – DINH, stable nitroxyl radical – DINO and substituted hydroxylamine DINOR. Absorption and fluorescence spectra of probes were taken in solution as well as in polymer films. In absorption spectrum the longest wavelength band was shifted from 410 nm in non-polar cyclohexane to 440 nm in polar methanol. In fluorescence spectrum the maximum of the broad band is red shifted from 460 nm in cyclohexane to 530 nm in methanol. All derivatives exhibit similar solvatochromism in absorption as well as in fluorescence spectrum. The fluorescence is observed for all probes but there is distinctly much higher fluorescence intensity for parent amine DINH and alkoxyamine DINOR than for adequate bi-radical DINO. The extent of fluorescence enhancement comparing amine and alkoxyamine derivatives with bi-radical is in the range 10–70 depending on the medium. Using laser flash photolysis the probe DINH exhibited transient absorption after 266 nm excitation in the nanosecond time scale region which is ascribed to triplet state. This transient absorption is red shifted in going from non-polar cyclohexane (460 nm) to polar methanol (520 nm). No (or extremely weak transient) absorption spectra was observed for stable nitroxyl bi-radical DINO.