Synthesis and spectral characteristics of substituted 1,8-naphthalimides: Intramolecular quenching by mono-nitroxides

Novel series of chromophore-nitroxide adducts were synthesized for the monitoring of photophysical and photochemical processes in solution as well as in a polymer matrix. These adducts were: (1) N-butyl-4-(1′-oxy-2′,2′,6′,6′-tetramethyl-4′-piperidylamino)-1,8-naphthalimide, BUNO – paramagnetic probe has been synthesized having the nitroxyl radical substituent acting as a quencher attached to the naphthalene in position 4 of 1,8-naphthalimide while imino nitrogen was substituted by n-butyl group. (2) N-(1′-oxy-2′,2′,6′,6′-tetramethyl-4′-piperidyl)-4-dimethylamino-1,8-naphthalimide – DMANO, where nitroxyl unit is on imino nitrogen of 1,8-naphthalimide while in position 4 of 1,8-naphthalimide there is dimethylamino substituent. Diamagnetic parent amine (BUNH, DMANH) were used as standard samples for comparison of the efficiency of intramolecular quenching. Absorption and fluorescence spectra of probes were taken in solution as well as in polymer films. In their absorption spectrum the longest wavelength band lies in the range from 400 up to 440 nm depending on the medium. In fluorescence spectrum the maximum lies in the range from 460 to 530 nm. When compared with less polar media the maxima of absorption and fluorescence spectra in polar media are red-shifted. The fluorescence is observed for all probes but there is distinctly much higher for parent amine and alkoxyamine than for respective mono-nitroxyl radical. The extent of intramolecular quenching, represented by the ratio of quantum yields of diamagnetic and paramagnetic forms, for both pairs BUNH/BUNO and DMANH/DMANO is in the range 2–15 depending on the medium. Intramolecular quenching in solvents for both pairs of mono-nitroxyl radical (BUNH/BUNO and DMANH/DMANO) exhibits an additive effect when compared with bi-nitroxyl radical (DINH/DINO) investigated previously.