Luminescent Dendrimers as Ligands and Sensors of Metal Ions - Advanced Fluorescence Reporters in Chemistry and Biology - page 256

species for this kind of application [ 20 ]. The signal amplification that got the name
'superquenching' can be realized by different mechanisms. In dye-doped nanopar-
ticles it can be homo-FRET with excitation energy transfer to nonfluorescent
acceptor [47]. In conjugated polymers this effect can be due to a trap in exciton
propagation [48]. Dendritic structures allow realizing both mechanisms.
The aim of this chapter is limited to reviewing some recent developments
concerning luminescent dendrimers that can play the role of ligands and sensors
for luminescent and nonluminescent metal ions, mainly investigated in our labora-
tories, with particular references to transition metal or lanthanide ions. We will not
discuss dendrimers constituted by polypyridine metal complexes [ 21 ] and porphyr-
ins [ 22 ] since it is outside the scope of the present paper.
2 Luminescent Dendrimers Hosting Nonluminescent
Metal Ions
Among nonluminescent metal ions, we can distinguish two classes: metal ions that
can quench dendrimer luminescence once they are coordinated inside the dendrimer
and metal ions that cannot. The latter class is constituted by metal ions that do not
possess low-lying excited states and do not show oxidation or reduction processes
at easily accessible potentials so that they cannot be involved in photoinduced
energy/electron transfer processes with the dendritic luminescent units. Also in the
latter case, the presence of the metal ion inside the dendrimer can be revealed by
changes in the luminescence spectra due to charge perturbation effect or to the
shutting down of quenching processes, so that metal coordination cause the revival
of dendrimer luminescence.
We will discuss in the following selected examples of the two cases.
2.1 Luminescent Dendrimers Hosting Innocent Metal Ions
2.1.1 Dendrimers with a Cyclam Coordinating Core
Cyclam is one of the most extensively investigated ligands in coordination chem-
istry [ 23 ]. Both cyclam and its 1,4,8,11-tetramethyl derivative in aqueous solution
can be mono and diprotonated and can coordinate metal ions such as Co 2+ ,Ni 2+ ,
Cu 2+ ,Zn 2+ ,Cd 2+ , and Hg 2+ with very large stability constants [ 24 ]. Furthermore,
cyclam and its derivatives have been studied in medical applications [ 25 ], as
carrier of metal ions in antitumor [ 26 ], imaging contrast agents [ 27 ], and as
antiHIV agents [ 28 ].