complexes that cannot exhibit MC or CT excited states at low
energy. Nevertheless, Zn^2 þcoordination by the dendrimer causes
strong changes in the emission spectrum of the dendrimer since
engagement of the nitrogen lone pairs in the coordination of
the metal ion prevents exciplex formation, with a resulting
increase of the naphthyl fluorescence. Such a fluorescent signal
is quite suitable for monitoring the formation of the complexes
in dendrimer/metal titration experiments.
Complexation of dendrimer 5 with lanthanide ions (Nd^3 þ, Eu^3 þ,
Gd^3 þ, Tb^3 þ, Dy^3 þ) [3636b] leads to qualitatively similar results.
V. Coordination of Dendrimers Around Metal IonsWhen a dendrimer contains a precise coordination site, as pre-
viously reported for case 2c, more than one dendrimer (Fig. 2e) or
a dendrimer and other ligands (Fig. 2f) can be assembled around
a metal ion.
A. COORDINATION OFDENDRIMERSALONE
We have seen in SectionIV.B that dendrimer 5 is able to form
1:1 complex with Zn^2 þions in acetonitrile/dichloromethane solu-
tion. The [Zn( 5 )]^2 þcomplex is stable as long as the Zn^2 þconcen-
tration is equal or higher than that of dendrimer 5 , while at low
Zn^2 þ concentration, dendrimer 5 gives rise only to complexes
with 2:1 dendrimer/metal stoichiometry with a high formation
constant (> 1013 M^2 ), as evidenced by both fluorescence and
(^1) H NMR titrations. The [Zn( 5 )
2 ]
2 þspecies is an example of case
2e: the dendrimer branches not only do not hinder but also in
fact favor coordination of cyclam to Zn^2 þwith respect to coordi-
nation of solvent molecules or counter ions. Two limiting
structures can be proposed for the 2:1 complexes: (i) an“inward”
structure, stabilized by the intermeshing of the branches of the
two coordinated dendrimers; (ii) an“outward”structure in which
the branches of the two coordinated dendrimers do not interact
but impose to the cyclam core a very specific coordination struc-
ture. Indeed, an“inward”structure for the 2:1 complex stabilized
by branch intermeshing should increase the probability of
excimer formation compared with the 1:1 species. In such a case,
the intensity of the excimer band (lmaxca. 390 nm) should grow
more rapidly at the beginning of the titration, when formation
of a 2:1 species is favored. This seems not to be the case for den-
drimer 5. Therefore, in the [Zn( 5 ) 2 ]^2 þ species, the dendrimer
124 VINCENZO BALZANIet al.