Photophysical properties of novel cationic naphthalimides

Novel positively charged naphthalimide derivatives were synthesized, in which N-methylpyridinium substituent was attached to the dicarboximide moiety. The absorption and fluorescence spectra were studied, fluorescence lifetimes, fluorescence yields and triplet yields were determined in acetonitrile and CH2Cl2. Ion-pairing was found to markedly alter the photophysical properties in the latter solvent. The association with iodide counterion decreased the radiative rate constant. The triplet formation was much more rapid for the 2,3-isomers, whereas the 1,2-derivatives emitted stronger fluorescence. The rate of internal conversion proved to be more than one order of magnitude higher for all N-methylpyridinium derivatives compared with that of the parent compounds, which might indicate that the significantly smaller energy gap between the S1 and S2 excited states led to more efficient excited state relaxation.