520 Chu and Pang
Macromolecules, Vol. 38, No. 2, 2005
absorption band indicated that the chromophore-chro-
mophore interactions in both 1 and 4 were very similar.
Spin-cast film 4 exhibited two absorption bands at
about 433 and 456 nm (Figure 3), corresponding to the
single chromophore and its aggregate in the solid state.
Clearly, the ordered structure formed in the aggregate
remained intact in the film state. The aggregate absorp-
tion band of the spin-cast polymer film 1, however,
occurred at 450 nm (Figure 5), which was about 12 nm
blue-shifted from that of its solution. It appeared that
the chromophores in the film state had slightly looser
contact than that in the solution, since the chromo-
phores along the polymer chain did not have sufficient
time to orient themselves for optimum interaction
during the spin-casting process.
Aggregate formation from 4 under the similar con-
centration as 1 indicated that intermolecular interaction
remained to be a major factor for aggregation under
high dilution. Difference in the solvent compositions for
4 and 1 could be largely offset by their solubility, as the
polymer required a significantly higher content of good
solvent to reach the same solubility level as the model
compound. A similar level of chromophore-chromophore
interaction observed from both 1 and 4 suggested that
intermolecular interaction remained to be predominant
in the aggregate formation. The assumption was sup-
ported by the optical spectra of 4 in film state, which
closely resembled that of its solution aggregate.
information about the aggregate structure. Since the
molecules of 4 were freely mobile, the rodlike molecules
are preferably to assemble in a parallel fashion13,14 to
achieve a maximum degree of interaction. A similar
degree of chromophore-chromophore interaction was
observed from 1 and 4, as characterized by nearly the
same bathochromic shift from their respective 0-0
absorption bands (Figures 3 and 5). The observed
similarity in the chromophore-chromophore interaction
suggested that the chromophores in the aggregate of
polymer 1 were also assembled in a parallel (or nearly
parallel) fashion. Although intramolecular interaction
was still a permitted pathway, the chromophores along
the single chain are required to be relatively apart from
each other in order for them to comfortably fold toward
each other and to reach the preferable parallel align-
ment.
Acknowledgment. Support of this work has been
provided by AFOSR (Grant F49620-00-1-0090), NIH/
NIGMS/MBRS/SCORE (Grant S06GM08247), and NASA
(Grant NCC3-911).
Supporting Information Available: Synthesis and char-
acterization of 4. This material is available free of charge via
References and Notes
(1) Handbook of Luminescence, Display Materials, and Devices:
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Conclusion
Molecular aggregation of an oligo(2,5-dialkoxy-1,4-
phenyleneethynylene) 4 was induced by using a com-
bination of solvent/nonsolvent with low temperature.
The current technique was shown to be superior than
by changing the solvent composition (or polarity) alone,
thereby providing an effective method to study molec-
ular aggregation. The improved detection of aggregate
could be partially attributed to the more planar confor-
mation that chromophore achieved at the low temper-
ature, which facilitates the chromophore-chromophore
interaction. Spectroscopic comparison of 4 in both solu-
tion and film states (UV-vis and fluorescence) showed
that the aggregate structure developed in the solution
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the solid state.
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Comparison of aggregation behavior between the
model compound 4 and polymer 1 revealed useful
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