inorganic chemistry

288 nm where most of the light is absorbed by the 16 naphtha-
lene chromophores of 5 , a strong increase of the anthracene emis-
sion at 400 nm is registered together with changes in intensity of
the naphthyl band at 335 nm. A closer look to the titration
profiles shows that (i) changes in absorption and emission bands
of the clip are significant only after addition of ca. 0.2 eq. of
Zn(II); (ii) changes in the naphthyl fluorescence at 335 nm are
effective since the beginning with an increase up to ca. 0.6 eq.
of Zn(II) and then a minimum at 1.1 Zn(II) eq. These results
can be interpreted by formation of a [Zn( 5 ) 2 ]^2 þcomplex at low
Zn(II) ion concentration since naphthyl-based emission
increases, while clip absorption and emission spectra are not
affected. Indeed, as previously discussed, dendrimer 5 forms a
very stable complex with 2:1 ligand to metal stoichiometry. Upon
further addition of metal ion, the [Zn( 5 ) 2 ]^2 þcomplex is disrupted
and Zn(II) is coordinated by both 5 andC^2 forming a [ 5 ZnC]
complex, as demonstrated by the quenching of the naphthyl
emission and the sensitization of the anthracene emission. These
quenching/sensitization processes has an efficiency higher than
95% and it cannot take place by dynamic mechanisms because
of the short lifetime of the fluorescent naphthyl-excited state of

O

O

O

O

O

O

O

OSO 3

O

O

O

O

O

O

O

O

O

{ 5 •Zn2+•C2–}

O

O

O

O

O

O

O

O

N

Zn2+

N

N

N

L

L

OSO 3

C2–

=

FIG. 14. Schematic representation of a metal complex constituted by
dendrimer 5 ,aZn^2 þion, and one molecular clipC^2 , and the corresponding
scheme (Fig. 2f).

128 VINCENZO BALZANIet al.