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K. Zhang et al. / Polymer 51 (2010) 6107e6114
polymers have been prepared upon anodic electropolymerization.
The study shows that the polymer growth takes place randomly
through the 2-, 3-, 5-, and 6-position of the DPP core. Compared with
the monomers, P1, P2 and P3 showed broad longer wavelength
absorption bands, which indicates an enlargement of the
p
ꢁconjugation in the polymers. However, compared with linear
polymers only grown through the 3,6-positions, the tetrafunction-
alized monomers lead to polymers with less econjugation and
only little pronounced electrochromic properties.
In polymers P1, P2 and P3, conjugated blocks through coupling
of thiophene units in the 3,6-positions were obtained, which are
separated by non-conjugated blocks. As a result, the polymers are
not as fully conjugated as the linear polymers prepared previously
through coupling in the 3,6-positions.
p
Fig. 9. Sulfur oxygen interaction between adjacent EDOT and EDTT units.
The broad absorption shoulder of P1 starting from 650 nm
increased. In a comparison, P2 showed no change in the absorption
characteristics with increasing p-doping. It could be possible that
the polymer produced was highly cross-linked and, due to SeO
interactions adjacent monomer units were densely packed so that
counter ions could not diffuse into the film, balance the charge, and
allow any change to occur (Fig. 9) [33].
The spectroelectrochemistry experiment for P3 showed signif-
icant changes in the electronic absorption spectra with increasing
potential. At þ1.0 V, one can see the formation of polarons and
bipolarons in the polymer chain which results in a new peak
between 500 and 700 nm, followed by a drop in absorbance of the
peak at 510 nm. Between þ1.0 and þ1.4 V there was a weak broad
peak appearing from 700 nm. The SeO interaction between the
adjacent EDOT units in P2 does not exist in P3, so the EDTT-con-
taining polymer is less likely to be densely packed, allowing easier
access for the counter ions to diffuse into the polymer to balance
the charge.
Acknowledgements
Financial support by Ciba Specialty Chemicals, Basle,
Switzerland, is gratefully acknowledged. Drs. M. Düggeli and
R. Lenz from Ciba are kindly thanked for helpful discussions.
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The reasoning for the large discrepancy between the optically
and electrochemically determined band gaps is supported by the
spectroelectrochemical results. For all three polymers, there is
relatively little change in the absorption spectra and this indicates
that oxidation takes place at localized, short-conjugation sites. The
LUMO of the materials is therefore dominated by the DPP core and
the HOMO is derived from the thiophene segments. The absorption
spectra provide information about the
pep* transitions and the
corresponding orbitals are clearly not representative of the HOMO
and LUMO orbitals (since the gaps determined by the two methods
differ by 0.2e0.9 eV). In conjugated polymers, this situation often
arises when there is a strongly redox-active unit in the structure
which is poorly conjugated to the main chain [34].
As an interesting comparison, the linear polymers containing
2,5-bis(40-t-butylphenyl)-3,6-diphenylpyrrolo[3,4-c]pyrrole-1,4
(2H,5H)-dione [25] with EDOT only attached to the 3,6-positions
showed fully reversible oxidative and reductive waves. The poly-
mer was fully conjugated and electrochromic, giving a HOMOe
LUMO gap of 1.32 eV. The other polymer containing DPP units with
EDOT only attached to the 2,5-positions showed irreversible
oxidative and reductive waves, giving a HOMOeLUMO gap of 3 eV.
This polymer was not conjugated and electrochromic. The observed
behaviour suggests that the three polymers P1, P2 and P3 are not
fully conjugated through the four cross-linked 2,3,5,6-directions.
4. Conclusion
In summary, a series of electropolymerizable tetrafunctionalized
DPP monomers have been synthesized, and the corresponding
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J Mater Chem 2004;14:1964.