Luminescent Dendrimers as Ligands and Sensors of Metal Ions - Advanced Fluorescence Reporters in Chemistry and Biology - page 259

quite suitable for monitoring the formation of complexes in dendrimer/metal
titration experiments (Fig. 4 ). Surprisingly, both dendrimer 1 and 2 give rise to
complexes with 2:1 ligand to metal stoichiometry at low Zn 2+ concentration, as
evidenced by both fluorescence and 1 H NMR titrations. In particular, in the case of
the largest dendrimer 2, at low metal ion concentration, only the species [Zn(2) 2 ] 2+
is present with a high formation constant (log 13). The unexpected [Zn(2) 2 ] 2

+ species shows that the dendrimer branches not only do not hinder, but in fact favor
coordination of cyclam to Zn 2+ with respect to coordination of solvent molecules or
counter ions. Two limiting structures can be proposed for the 2:1 complexes: (1) an
“inward” structure, stabilized by the intermeshing of the branches of the two
coordinated dendrimers; (2) 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 structure. Indeed, an “inward” structure for the 2:1 complex

b 2:1 >

a

1.0

0.8

0.6

0.4

0.2

0.0

b

1.0

0.8

0.6

0.4

0.2

0.0

0.0 0.5 1.0 1.5

[Zn(II)] / [dendrimer]

Fig. 4 Normalized emission intensity changes observed upon addition of Zn(CF 3 SO 3 ) 2 to 1 (a),
and 2 (b)atl em ¼ 337 ( solid circles ), 390 ( empty squares ), and 480 nm ( empty triangles ).
l ex ¼ 275 nm