Luminescent Dendrimers as Ligands and Sensors of Metal Ions - Advanced Fluorescence Reporters in Chemistry and Biology - page 274

Addition of lanthanide ions to dendrimer solutions showed that: (a) the
absorption spectrum of the dendrimer is almost unaffected, (b) the fluorescence
of the dansyl units is quenched; (c) the quenching effect is very large for Nd 3+
(Fig. 7 )andEu 3+ , moderate for Er 3+ and Yb 3+ ,smallforTb 3+ , and very small for
Gd 3+ ; (d) in the case of Nd 3+ (Fig. 7 ), Er 3+ ,andYb 3+ quenching of the dansyl
fluorescence is accompanied by the sensitized near-infrared emission of the
lanthanide ion [ 42 ].
Interpretation of the results obtained on the basis of energy levels (Fig. 8 ) and
redox potentials of the dendrimer and of the metal ions have led to the following
conclusions: (1) at low metal ion concentrations, each dendrimer hosts only one
metal ion; (2) when the hosted metal ion is Nd 3+ or Eu 3+ , all the 24 dansyl unit of
the dendrimer are quenched with unitary efficiency; (3) quenching by Nd 3+ and Er 3+
takes place by direct energy transfer from the fluorescent ( S 1 ) excited state of dansyl
to a manifold of Nd 3+ energy levels, followed by sensitized near-infrared emission
from the metal ion ( 1,525 for Er 3+ ); (4)

quenching by Eu 3+ does not lead to any sensitized emission since the lowest excited
state of the system is an electron-transfer excited state; upon protonation of the dansyl
units, however, the electron-transfer excited state moves to very high energy and at
77 K a sensitized Eu 3+ emission is observed, which originates from the quenching of
the T 1 excited state of the protonated dansyl units; (v) in the case of Yb 3+ ,the
sensitization of the near-infrared metal-centered emission occurs via the intermediate
formation of an electron-transfer excited state; at 77 K the electron-transfer excited
state moves to higher energy, thus preventing the population of the Yb 3+ emitting
excited state; (vi) the small quenching effect observed for Tb 3+ is partly caused by a
direct energy transfer from the fluorescent ( S 1 ) excited state of dansyl; on protonation
of the dansyl units, a strong Tb 3+ sensitized emission is observed at 77 K, originating
from the T 1 excited state of the protonated dansyl units; (vii) the very small quenching
effect observed for Gd 3+ is assigned to either induced intersystem crossing or, more
likely, to charge perturbation of the S 1 dansyl excited state.
Another interesting example of dendrimer containing multiple amide coordinat-
ing units for lanthanide ions is a third generation poly(amido amine) dendrimer (12)
containing 60 amide units in its interior and 32 luminescent 2,3-naphthalimide
groups at the periphery [ 43 ]. Upon titration of dendrimer 12 in dimethylsulfoxide
(DMSO) solution with Eu 3+ , slow changes in the emission spectra were obtained.
No fragmentation of the naphthalimide groups or the aliphatic structure takes place,
as demonstrated by 1 H NMR and 13 C NMR. Dendrimer 12 is able to coordinate up
to 8 Eu 3+ ions as an average, over a period of 7 days. The slow coordination process
has been ascribed by the authors to the internal location of the amide binding units.
A relatively low sensitized emission quantum yield of Eu 3+ (

1,064 nm for Nd 3+ and l max ¼ l max ¼

0.06%) was
obtained, but this was compensated for by the high molar absorption coefficient of
dendrimer 12, so that Eu 3+ red emission could be observed by naked eye under a
conventional UV lamp for a 8.7

F em ¼

10 5 M dendrimer solution in DMSO contain-
ing 8 equivalents of lanthanide ions.