Angewandte
Chemie
of great research interest and will be dis-
cussed in future publications.
In summary, we have demonstrated the
preparation of chromophore amphiphile/
silica self-assembled nanocomposite films
of PPP-C11 and FL-stilbene-(C11)2 with
enhanced emission, and have thus tried to
solve the problem of solid-state quenching of
organic chromophores. These nanocompo-
sites show a nanosheet or nanorod morphol-
ogy. The ability to tailor the orientation of
the chromophore surfactant amphiphiles
within the mesoscopic structures by sol-
vent-evaporation induced self-assembly in
thin films, and the subsequent formation of
well-ordered chromophore nanopackets,
dramatically affects the photophysical prop-
erties of these materials. Moreover, this
chromophore amphiphile/silica self-assem-
bly approach to overcome the problem of
solid-state quenching in the development of
organic light emitting devices with high
efficiencies should be broadly applicable
and will enable new physical studies and
new devices with these nanocomposite mate-
rials. Furthermore, this method obviates the
chromophore leaching problem that is
Figure 4. UV/Vis spectra for a THF solution (dotted lines) and chromophore (dashed
lines) and nanocomposite films (solid lines) of PPP-C11 (a) and FL-Stilbene-(C11)2 (c).
Photoluminescence spectra for a THF solution (dotted lines) and chromophore (dashed
lines) and nanocomposite films (solid lines) of PPP-C11 (b) and FL-Stilbene-(C11)2 (d).
(The PL intensities of solutions were normalized for comparison.)
However, when the nanocomposite film of FL-stilbene-(C11)2
is formed by EISA, its emission intensity is enhanced more
than twofold compared to the chromophore amphiphile film
alone, with lmax at 445 nm and shoulders at around 428 and
465 nm. The external quantum efficiency (F) for the FL-
stilbene-(C11)2 chromophore film is 24%, and this increases
to 31% in the FL-stilbene-(C11)2 nanocomposite film. This
increase in absorbance in the nanocomposite indicates an
increase in the effective conjugation length of the chromo-
phores and, consequently, enhancement in the fluorescence is
observed. The “controlled” aggregation of the chromophore
groups within the hydrophobic core of the nanocomposite
minimizes the formation of larger crystals and thus enhances
the fluorescence. The nanopackets in the hybrid composites
could provide a controlled concentration of active dots or
chromophores, which are better defined systems, and could
prevent coalescence into larger, ill-defined aggregates. A
similar enhancement has been observed for C540A residing in
the organic, hydrophobic regions of a nanocomposite.[20]
Although Forster quenching[31,32] is observed in most cases
for fluorescent molecules in molecular sieves, the fluores-
cence enhancement induced in this case is attributed to the
well-ordered nanopackets of “controlled” chromophore
aggregates formed in the self-assembled nanocomposites, as
is evident from the above results. The intensity increase may
be due to an increased quantum efficiency of the chromo-
phore nanoaggregates within silica. Such a fluorescence
enhancement has also been observed for the well-packed
chains and aggregates in conjugated polymers[33] and for
fullerene aggregates in organic–inorganic nanocomposites.[5]
The mode of interaction amongst the chromophore amphi-
phile molecules in functional hybrid nanocomposites is a topic
common in inclusion chemistry. The design of further
surfactant amphiphiles with other chromophores at the tail-
ends will allow the variation and improvement of the
architecture on a molecular and nanometer scale. The
demonstration of “controlled” chromophore aggregation
and emission enhancement in self-assembled functional
hybrid nanocomposites may stimulate new molecular engi-
neering endeavors in the design of luminescent organic
compounds with highly emissive aggregation states.
Received: August 29, 2005
Revised: November 18, 2005
Published online: January 30, 2006
Keywords: aggregation · fluorescence · nanostructures ·
.
organic–inorganic hybrid composites · self-assembly
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ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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