Chemistry Reference
In-Depth Information
nanoparticles is essential, since simple aliphatically capped particles do not
show the effect. TEM images of the composite showed the nanoparticles run-
ning along the sides of tapes formed by the organogelators at a distance con-
sistent with the length of the capping unit being intercalated in the organogels
part. The fluorescence of the OPV gelator is quenched significantly by the
presence of even 1% of the nanoparticles. The decay of luminescence is also
faster in the composite, presumably because the diffusion of the electronic
excitations through the fibres to the particles (that occurs on the nanosecond
timescale) is very ecient. The nature of the particles is essential to providing
this ecient pathway: The noble-metal colloids coated with aliphatic groups
show weak quenching of the OPV luminescence. The self-assembly of the
composite was facilitated by the functionalisation of the nanoparticles with
ligands capable of interaction with the gelator through noncovalent inter-
actions similar to those that hold the fibres of the main component together
(aromatic stacking and hydrogen bonds between the hydroxyl groups). The
metal particles are very close to the stacked chromophores, which makes
electronic communication ecient.
Appropriately substituted nanoparticles can help structure gels and increase
electrical conductivity.
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The combination of a gelator with a tetra-
thiafulvalene (TTF) unit that allows electrical conductivity (when doped) and
gold nanoparticles passivated with tetrathiafulvalene units bearing amide
groups compatible with the same kind of unit in the gelator leads to transparent
gels upon cooling hot solutions in apolar organic solvents when the metal
colloid is present in 1% weight. Even at this low proportion the morphology of
the gel fibres is affected dramatically: The presence of the amide-coated
nanoparticles gives rise to long well-defined fibres, as demonstrated by atomic
force microscope imaging (Figure 7.32). On the contrary, when the nano-
particles were coated with a TTF unit bearing only alkyl chains the corres-
ponding nanocomposites comprise very short fibres and areas where
nanoparticles agglomerate. Importantly, in the transmission electron micro-
scope images of the composites the gold colloid is only observed attached to the
gel fibres in the case of the amide, while with alkyl chains a clear phase sep-
aration takes place. After the materials were partially oxidised using iodine
vapours to give the mixed-valence material (with neutral and cation radical
TTFs on average) it became clear that the nanoparticles are able to induce the
more conducting phase of fibres of the TTF derivative, as proven by electron
paramagnetic resonance spectroscopy. Furthermore, current-sensing atomic
force microscopy showed that the material with amide-containing nano-
particles gave an apparently much more conducting material, probably because
of the increased interfibre connections that must be promoted by the additive.
The nanocomposites containing nanoparticles with no hydrogen bonding units
were very inhomogeneous materials with areas of high conductivity, possibly
caused by agglomerated nanoparticles. The bulk conductivity in these materials
is modified as a result of the changes in fibre structure by the nanoparticles, and
the examples show how a small amount of additive can help structure the gel
and give rise to optimised electrical properties.
