branches are likely extending outward (Fig. 12). Further, the two
cyclam cores, to account for the coordination number (6) of
Zn^2 þ, are likely forced to adopt a structure in which not all of
the four N atoms are available for Zn^2 þ coordination, thereby
favoring a 2:1 stoichiometry.
In the case of lanthanide ions (Nd^3 þ, Eu^3 þ,Gd^3 þ, Tb^3 þ, Dy^3 þ)
[3636b], the complex stoichiometry is different: at low metal ion
concentration, the formation of 1:3 (metal/dendrimer) complex
(logb1:3¼20.3) is demonstrated by fluorescence and NMR titra-
tion. It is likely that, in the [M( 5 ) 3 ]^3 þcomplex, not all the 12
nitrogens of the three cyclam cores are engaged in metal ion coor-
dination. However, upon metal coordination, the exciplex emis-
sion band completely disappears, as it is was previously
observed upon acid titration. Clearly, as is also shown by NMR
results, the presence of the 3þion is“felt”by all the nitrogens
of the three cyclam moieties, thereby raising the energy of the
exciplex excited state above that of the naphthyl-based one. For
all the lanthanide complexes of 5 , no sensitized emission from
the lanthanide ion was observed. Therefore, energy transfer from
either theS 1 or theT 1 excited state of the naphthyl units of 5 to
the lanthanide ion is inefficient. By contrast, efficient energy
transfer from naphthalene-like chromophores to Eu^3 þhas been
reported in the case in which naphthalene is linked through an
amide or carboxylate bond to the lanthanide( 37 ). Apparently,
the nature of the first coordination sphere plays an important
role concerning energy-transfer efficiency.
FIG. 12. Schematic representation of a metal complex containing a
Zn^2 þ ion coordinated by two cyclam-cored dendrimer 5 , and the
corresponding scheme (Fig. 2e).
PHOTOCHEMISTRY & PHOTOPHYSICS OF METAL COMPLEXES 125