Effect of protons and metal ions on the fluorescence properties of a polylysin dendrimer containing twenty four dansyl units

Abstract

The interaction of protons, Co²⁺, Ni²⁺ and Zn²⁺ ions (as nitrate salts) with a polylysin dendrimer, D, functionalized in the periphery with 24 5-dimethylamino-1-naphthalenesulfonamido (dansyl) units has been investigated in acetonitrile–dichloromethane solution. The dendrimer consists of a benzene core branched in the 1, 3, and 5 positions. Each branch starts with a (dialkyl)carboxamide-type moiety and carries (i) six aliphatic amide groups and (ii) eight fluorescent dansyl units. For comparison purposes, the behaviour of a monodansyl reference compound (I) has also been investigated. The absorption spectrum and the fluorescence properties of the dendrimer are those expected for a species containing 24 non-interacting dansyl units. Both for the model compound and for the dendrimer, protonation causes a shift of the absorption and fluorescence bands towards higher energies; for the dendrimer, however, the changes in fluorescence intensity during the acid titration reveal the occurrence of intradendrimer quenching processes, with signal amplification. Addition of Co²⁺ or Ni²⁺ ions to a basic solution of the model compound I does not cause any effect in the absorption and emission properties, whereas in the case of dendrimer D a strong fluorescence quenching is observed. At low metal ion concentration each metal ion quenches about 9 dansyl units; the fluorescence quenching takes place by a static mechanism involving co-ordination of metal ions in the interior of the dendrimer. Addition of Zn²⁺ to a basic solution of the dendrimer causes only a very small decrease in the fluorescence intensity. The co-ordinated Co²⁺ and Ni²⁺ ions are fully displaced by addition of Zn²⁺ or H⁺ with revival of the dansyl fluorescence. The results obtained show that a dendrimer can exhibit an unusual co-ordinating ability and sensory signal amplification.