Luminescent Dendrimers as Ligands and Sensors of Metal Ions - Advanced Fluorescence Reporters in Chemistry and Biology - page 269

dansyl model compound under the same experimental conditions: l max ¼ 522 nm,

F ¼ 13 ns), and no dimethoxybenzene emission is present due to a

quenching process by the nearby dansyl units. Double protonation of the cyclam
ring takes place before protonation of the dansyl units, and the oligoethylene glycol
chains do not interfere with protonation of the cyclam core and the dansyl units in
the ground state, but affect the luminescence of the protonated dansyl units by
reversible proton transfer.
As to metal ion coordination, dendrimer 8 possesses three distinct types of
multivalent, potentially coordinating sites, as previously observed: the cyclam
core, the 16 oligoethylene glycol chains appended in the periphery, and the
8 amine moieties of the dansyl units. Upon complexation of Co 2+ ,Ni 2+ , and Cu 2+ ,
three quenching mechanisms are active, namely (1) an effect of the positive charge
of the metal ion on the dansyl emission transition moment, (2) an energy transfer
quenching since the investigated metal ions have low-lying excited states, and (3) an
electron transfer quenching for metal ions that can be easily reduced, as in the case of
Cu 2+ . Changes in absorption and emission spectra are quite complex, but some
general conclusions can be drawn: (1) the first equivalent of metal ion is coordinated
by the cyclam core; (2) coordination of metal ions by the dansyl units follows at high
metal ion concentrations. The interaction of the resulting cyclam complex with the
appended dansyl units depends on the nature of the metal ion and the counter ion. For
example, in the case of Cu 2+ coordination, a strong quenching of the dansyl excited
state takes place and it is related to the occurrence of an electron transfer process.
This example demonstrates that dendrimers may exhibit complex functionality
resulting from the integration of the specific properties of their component units.
Therefore, such design is interesting for understanding relations between complex
structure and multiple functionality.

0.30, and t ¼

3 Luminescent Dendrimers Hosting Luminescent Metal Ions

Lanthanide ions are known to show very long lived luminescence, which is quite a
useful property for several applications (e.g., sensors [ 33 ], imaging [ 34 ], and
photonics) [ 35 ]. Because of the forbidden nature of their electronic transitions,
however, lanthanide ions exhibit very weak absorptions bands, which is a severe
drawback for applications based on luminescence. In order to overcome this
difficulty, lanthanide ions are usually coordinated to ligands containing organic
chromophores whose excitation, followed by energy transfer, causes the sensitized
luminescence of the metal ion (antenna effect) [ 36 ]. Such a process can involve
either direct energy transfer from the singlet excited state of the chromophoric
group with quenching of the chromophore fluorescence [ 37 ], or, most frequently,
via S 1 ! T 1 intersystem crossing followed by energy transfer from the T 1 excited

state of the chromophoric unit to the lanthanide ion [ 33 ].