Low-bandgap conjugated polymers (CPs)
with strong absorption in
the short-wave infrared region usually show ambipolar characteristics.
Herein, three low-bandgap CPs, that is, PQF, PQT, and PQS, with unipolar
n-type transport behavior were synthesized via direct arylation polycondensation (DArP) with quinoids
based on terchalcogenophene-flanked diketopyrrolopyrrole (DPP) derivatives
as C−Br monomers and less electron-rich (E)-1,2-bis (3,4-difluorothien-2-yl) ethene (4FTVT) as a C−H
monomer, and the chalcogen effects on the isomerism of the quinoids
and the properties of the resultant CPs were investigated. Thiophene-flanked
DPP-containing quinoids are the mixtures of (E, E), (E, Z), and (Z, Z) isomers, while the sole (E, E) isomer is found in furan-flanked
DPP-based quinoids and (Z, Z) isomer
is dominant in the quinoids based on selenophene-flanked DPP. The
quinoidal monomers are stable to undergo DArP. All three CPs show
lower bandgaps of ∼1.1 eV and unipolar n-type transport behavior
with field-effect electron mobility (μe) above 0.1
cm2 V−1 s−1. Among
them, the polymer PQF, comprising furan-flanked DPP, exhibits the
highest μe of 0.21 cm2 V−1 s−1, possibly attributed to the simple chain conformation.
This study provides a strategy to design low-bandgap unipolar n-type
polymer semiconductors.