Influence of the alkyl chain length on the fluorescence properties of N-alkyl-2,3-naphthalimides included in beta-cyclodextrin

Absorption spectra of N-alkyl-2,3-naphthalimides (1, R = CH3; 2,: R = n-C6H 13; 3, R = n-C12H25; 4, R = n-C18H37) show absorption maximum independent o f the alkyl chain length but dependent on the nature of the solvent. In the polar aprotic solvent acetonitrile N-alkyl-2,3-naphthalimides 1-4 exhibit a vibrationally resolved fluorescence emission, while in polar protic solve nts (methanol or water) their fluorescence is broad and shows a bathochromi c shift when compared to the one in acetonitrile. This behaviour is in agre ement with previously reported data on N-methyl-2,3-naphthalimide. By the a ddition of aqueous solutions of beta-cyclodextrin (beta-CD) (concentration ranging from 1 x 10(-3) to 1 x 10(-2) M) to an aqueous solution of the prob e, a hypsochromic shift and a strong increase of the fluorescence intensity were observed for short, i.e. 1, and long alkyl chains, i.e. 2-4. From the se data we can conclude that an inclusion complex between 1-4 and beta-CD i s formed. The inclusion geometry for these complexes is dramatically affect ed by the alkyl chain length. Thus, for the short chain N-alkyl-2,3-naphtha limide, 1, a broad emission is revealed, whereas for long chains, 2-4, a vi brationally structured emission was observed. This indicates a tighter geom etry for the latter case, which is a consequence of the inclusion of both t he naphthalimide moiety and the alkyl group into the beta-CD cavity. (C) 19 99 Elsevier Science B.V. All rights reserved.