Dendrimers based on a bis-cyclam core as fluorescence sensors for metal ions

Abstract

We have synthesized a novel dendrimer (1) based on two covalently linked cyclam units as a core appended to six branches, each one consisting of one dimethoxybenzene and two naphthyl units (cyclam = 1,4,8,11-tetraazacyclotetradecane). Such a dendrimer shows three fluorescence bands which can be assigned to naphthyl localized excited states (λ_max = 336 nm), naphthyl excimers (λ_maxca. 390 nm), and naphthyl–amine exciplexes (λ_max = 510 nm). Protonation or complexation of the bis-cyclam core with Zn²⁺ does not affect the absorption spectrum of the dendrimer, but causes noticeable changes in the fluorescence intensity of the three component bands. Complexation with Cu²⁺ not only causes changes in the relative intensities of the fluorescence bands, but also the appearance of a new absorption band in the near UV spectral region. Analysis of the titration curves has allowed us to obtain clear evidence for the formation of 1 : 1 (1(H⁺), [Zn(1)]²⁺, [Cu(1)]²⁺) and 2 : 1 (1(2H⁺), [Zn₂(1)]⁴⁺, [Cu₂(1)]⁴⁺) species. Comparison with the behaviour of a previously investigated parent monocyclam dendrimer (2) suggests that in the 1 : 1 species of 1 both the cyclam units are involved in the complexation with Zn²⁺ and Cu²⁺.