Synthesis and Properties of Two Cationic Narrow Band
Gap Conjugated Polyelectrolytes
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Abstract
We report the design, synthesis,
and optical and electronic properties
of two novel narrow band gap conjugated polyelectrolytes (NBGCPEs)
based on a poly[2,6-(4,4-bis-alkyl-4<i>H</i>-cyclopenta[2,1-<i>b</i>;3,4-<i>b</i>′]dithiophene)-<i>alt</i>-4,7-(2,1,3-benzothiadiazole)] donor/acceptor backbone.
Comparison with the properties of the neutral precursor material shows
that the ionic component in these cationic NBGCPEs leads to a red-shift
in the absorption spectra and to a modification of the polymer electronic
energy levels. Both the HOMO and the LUMO are lowered in energy, with
the net effect being dependent on the choice of counterion, i.e. bromide
vs tetrakis(1-imidazolyl)borate. Moreover, we unexpectedly
find n-type transport in thin-film transistors, as opposed to the
widely studied p-type transport in neutral systems with isoelectronic
backbones. From these observations we conclude that introduction of
ionic functionalities adjacent to semiconducting polymers that exhibit
charge-transfer excitations offers unique opportunities for materials
design