Multichromophoric cyclodextrins as fluorescent sensors.: Interaction of heptachromophoric β-cyclodextrins with surfactants

A beta-cyclodextrin derivative CD-NA bearing 7 negatively charged naphthoate chromophores is shown to strongly interact with cationic surfactants. In the absence of surfactant, the CD-NA emission spectrum is composed of two bands; one is assigned to the normal fluorescence and the other one to the fluorescence of intramolecular excimers. Interaction with a cationic surfactant leads to a drop in excimer emission. The ratio of the fluorescence intensities of the monomer and excimer bands is directly related to the concentration of the surfactant. In the case of electroactive surfactants such as cetylpyridinium chloride, the fluorescence quenching arising from photoinduced electron transfer can be additionally used for sensing. CD-NA can thus be used to detect cetyltrimethylammonium chloride (cetyl = hexadecyl) and cetylpyridinium chloride in an aqueous solution at concentrations as low as a few micromoles per litre and up to about 50 micromoles per litre. The interaction between CD-NA and cationic surfactants can be interpreted by a micellization process induced by CD-NA rather than by the formation of 1:1 inclusion complexes. The analogy with the interaction between cationic surfactants and polyelectrolytes bearing negative charges is outlined. It should be noted that addition of the anionic surfactant sodium dodecyl sulfate does not induce any photophysical effect.