A step further in cyclam-based dendritic ligands for metal ions
is constituted by dendrimer 6 (Fig. 13), containing two covalently
linked cyclam units as a core, appended to six branches, each one
of them consisting of a dimethoxybenzene and two naphthyl
units ( 38 ). Its photophysical properties are qualitatively similar
to that observed for 5. For example, the emission spectrum
evidences the presence of naphthyl-localized excited states
(lmax¼337 nm), naphthyl excimers (lmax ca. 390 nm), and
naphthyl-amine exciplexes (lmax¼480 nm).
Dendrimer 6 ability to coordinate Zn(II) and Cu(II) in acetoni-
trile/dichloromethane 1:1 (v/v) solution has been carefully
investigated ( 38 ).
Upon titration with Zn(CF 3 SO 3 ) 2 , no change was observed
in the absorption spectrum, whereas strong changes were
observed in the emission spectrum. Such changes, qualitatively
similar to those caused by protonation, indicate that a 1:1 com-
plex, [Zn( 6 )]^2 þ, is first formed and then replaced by a 2:1 species
[Zn 2 ( 6 )]^4 þ(logb1:1¼9.7 and logb2:1¼16.1 for these two species,
respectively). In the 1:1 complex [Zn( 6 )]^2 þ, the metal ion is likely
sandwiched between the two cyclam units. As previously
observed in the case of dendrimer 5 , apparently, the dendrimer
FIG. 13. Formula of a dendrimer constituted by a bis-cyclam core.126 VINCENZO BALZANIet al.