Chemistry Reference
In-Depth Information
potentials owing to the incorporation of redox active residues, such as ferrocene,
4,4
0
-bipyridinium (viologen) or cobaltocenium. Finally, dendrimers
contain
an ureidopyrimidine residue, introduced to express hydrogen bonding recognition
properties in these macromolecules. The fluorescent dendrimers were prepared for
investigation in aqueous solution, and thus, only the series of dendrimers containing
carboxylic acid groups on the periphery were of interest. In contrast to this, we
prepared both hydrophobic (tert-butyl esters on the surface) and hydrophilic
(carboxylic acids on the surface) series of the redox dendrimers for investigation
in aqueous and nonaqueous solvents. The ureidopyrimidine dendrimers (
25-27
)
were only prepared as tert-butyl esters, since the investigation of hydrogen bonding
interactions is best carried out in low polarity solvents, such as chloroform or
toluene. All these dendrimers were fully characterized by
1
H- and
13
C-NMR
spectroscopies, MALDI-TOF mass spectrometry, and other more specialized
techniques such as FTIR spectroscopy, voltammetric techniques, vapor pressure
osmometry,andsoon.
25-27
7.2.1 Fluorescent Open Core Dendrimers
Dendrimers
were prepared by reaction of dansyl chloride with the focal amine of
first- to third-generation Newkome dendrons [14]. Initially prepared with tert-butyl
ester groups on their surfaces, hydrolysis in formic acid produced the corresponding
dendrimers with terminal carboxylic acids. MALDI-TOF mass spectroscopic data
were extremely important for the characterization of these dendrimers, providing
confirmation of the purity of the amine building blocks and verifying the complete-
ness of the final hydrolysis process. The photophysical behavior of these macro-
molecules was investigated in joint work with the group of Professor Frank Bright, at
the University of Buffalo. Fluorescent measurements on the dendrimers (about 1
1-3
M)
dissolved in aqueous buffer (pH 7.0) demonstrated that the dansyl residue is
progressively shielded from solvent molecules as the dendrimer generation
increases [14]. This is evidenced by the marked changes in spectral parameters
(Table 7.1), such as fluorescence quantumyields, excited-state fluorescence lifetimes,
radiative and nonradiative decay rates, and rotational correlation times. To provide
the 'reader with an approximate measure of the hydrophobic character of the
m
TABLE 7.1 Selected Photophysical Parameters for Dansylamine (DA) and
Dansyl-Containing Dendrimers 1-3 in Aqueous Buffer (pH 7.0)
l
abs
(nm)
a
l
fluor
(nm)
a
F
f
b
t
f
(ns)
c
Compound
DA
324
548
0.028
3.17
1
329
547
0.044
4.90
2
329
542
0.068
6.32
3
324
535
0.086
7.94
a
Wavelengths of maximum absorption and emission.
b
Fluorescence quantum yield.
c
Excited-state fluorescence lifetime.
