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

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 (lambda max = 336 nm), naphthyl excimers (lambda(max) ca. 390 nm), and naphthyl - amine exciplexes (lambda(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(+)), [Zn-2(1)](4+), [Cu-2(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+.