results in a blue shift of the PL emission as in Fig. 3B, in
contrast with the red-shift reported for more ordered polymers
such as PPV.27
different solubilities and so equilibrium fractionation might
differentiate between fractions on the basis of cis : trans
vinylene distribution as well as on the basis of molecular mass.
There were small but significant differences in infrared and
NMR spectra of the as-made polymers as a function of
cis : trans content in the feed, these differences provided
an analytical probe for the effects of fractionation and
enabled us to determine that PDPV made via the McMurry
route, Fig. 1, pathway (ia), had a roughly 50 : 50 cis : trans
ratio. This method of analysis was sensitive enough to
establish that some of the low molecular weight fractions
of PDPV prepared by the McMurry route had chain end
Coupling of rotation about the vinylene linkage to the
electronic excited state, which becomes important when the
polymer is in solution or in a non-crystalline state, is of general
importance. Firstly, it is necessary to obtain high PL efficiency
in solid films of disordered polymers of this type. Excitons in
the disordered regions would be expected to be higher in
energy than in ordered regions, but are able to lower their
energy through ring torsional coupling. We therefore associate
high PL efficiency in glassy polymers with the combined effects
of (i) disorder-hindering of aggregation and (ii) ring-torsion
coupling. Secondly, it provides a design rule for the selection of
polymers for use in LEDs. PAVs provide red to green
emission, but not blue, because the Stokes’ shift is found to
be large for the larger-gap PAVs reported. (Blue emitters are
better obtained using polymers which avoid a vinylene linkage
in the polymer chain, and which have extra chain rigidity
imposed by formation of a ‘ladder’ structure, at least in part,
along the chain. These include ‘ladder’ poly(phenylene)s28 and
the poly(fluorene)s.29) Thirdly, it may provide insight into the
investigations of the charged excited states in polyaniline, for
which a model was developed by Epstein and co-workers of
charged polarons localised by phenylene ring rotation about
the nitrogen coupling sites.30 We note that the importance of a
non-crystalline environment for such rotations to be strongly
coupled to the electronic excitations should not be limited to
charge neutral states, such as those investigated in our work.
Therefore we suggest that it is likely that these excitations on
the polyaniline chain are present in the non-crystalline regions
of the material (which is found to show both amorphous and
crystalline components).
carbonyls detectable at 1660 cm21
.
The solid state photoluminescence efficiencies were mea-
sured by means of an integrating sphere technique as described
elsewhere,19 using two UV lines (351 and 363.8 nm) from an
Ar ion laser for excitation.
Acknowledgements
We thank the Royal Society for the award of a University
Research Fellowship (FC) and the National University of
Malaysia for a studentship (RD). The Durham–Cambridge–
Mons collaboration was supported within the framework
of the European Commission Training and Mobility of
Researchers programme (SELOA Network). The work in
Mons was partly supported by the Belgian Federal
Government ‘‘Service des Affaires Scientifiques, Techniques
et Culturelles’’ (Interuniversity Attraction Pole PAI 4/11) and
by the Belgian National Fund for Scientific Research (FNRS).
DB is an FNRS Senior Research Fellow.
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