Chemistry Reference
In-Depth Information
(on-resonance), it is possible to visualise whatever small molecules are bound to
the gel network. Furthermore, any nuclei in direct contact with the gel network
will have larger intensity signals in the STD spectrum. This approach has been
used to characterise a gelation system by irradiating at 100 ppm (''off-reson-
ance'') and -1 ppm (''on-resonance'') in the
1
H NMR spectrum.
90
However,
STD signals were seen for all gelator resonances, and also the solvent molecules
(in this case, acetonitrile and water), suggesting that the whole structure was
involved in gelation, as was the solvent, playing an explicit role in mediating
assembly. This approach may, in the future, prove useful for probing molecular
recognition processes within complex gel mixtures in more detail.
d
n
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r
3
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2.5.2 Infrared Spectroscopy
Infrared (IR) spectroscopy is very useful for probing hydrogen bond inter-
actions between the molecular building blocks. In particular, O-H, N-H and
C
ΒΌ
O stretches all show distinctive responses to hydrogen bonding - with the
bonds changing in strength as hydrogen bonds form resulting in a shift in wave
number. Van der Waals interactions can also sometimes be detected by looking
for changes in C-H stretching interactions. Measuring the IR spectra of gels
can be complicated by the fact that they are solvated materials, with the solvent
potentially obscuring key parts of the IR spectrum. Typically, it is necessary to
compare the IR spectra of the gelator in both the sol and the gel in order to
determine the key noncovalent interactions responsible for assembly. Variable-
temperature IR spectra can provide a very useful way of probing the change in
these interactions as gelation occurs in response to temperature changes.
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Variable-concentration IR spectroscopy can be used to follow the build-up of
the gel network and provide insights into error-checking processes during self-
assembly.
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In some cases, comparing the IR spectrum of the solid gelator with
that of the dried nanostructured xerogel can demonstrate how noncovalent
interactions have changed when gelation has occurred and organised the
molecules into nanoscale morphologies prior to sample drying.
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IR spec-
troscopy has also been used to demonstrate that a gel was broken down by Ag
1
cations because they complexed to alkene groups on the gelator periphery, with
the distinctive change in alkene IR bends reporting on the binding mode.
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3
.
2.5.3 Optical Spectroscopy and Fluorescence
Optical spectroscopy can be used to report on gelation processes - once again
variable concentration and temperature studies can provide valuable infor-
mation about the assembly process. As a simple example, donor-acceptor
systems often form a red colour on complex formation - this visible spectro-
scopic output acts as a direct signature that donor-acceptor interactions are
occurring.
95-97
It has been reported that slowly cooling a two-component
gelation system consisting of donor and acceptor components initially gave rise
to a red colour, indicative of the formation of a donor-acceptor complex, but
no gelation.
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Only on further cooling did gelation take place. Combination of
