Two novel conjugated polymers incorporating quinoidal thiophene are successfully synthesized. By combining 1D nuclear magnetic resonance (NMR) and 2D nuclear Overhauser effect spectroscopy analyses, the isomeric form of the major quinoid monomer is clearly identified as the asymmetric Z, E-configuration. The quinoidal polymers are synthesized via Stille polymerization with thiophene or bithiophene. Both quinoidal polymers exhibit the low band gap of 1.45 eV and amphoteric redox behavior, indicating extended conjugation owing to the quinoidal backbone. These quinoidal polymers show ambipolar behaviors with high charge carrier mobilities when applied in organic field-effect transistors. In addition, the radial alignment of polymer chains achieved by off-center spin-coating leads to further improvement of device performance, with poly(quinoidal thiophene-bithiophene) exhibiting a high hole mobility of 8.09 cm V s , which is the highest value among the quinoidal polymers up to now. Microstructural alteration via thermal annealing or off-center spin-coating is found to beneficially affect charge transport. The enhancement of crystallinity with strong π-π interactions and the nanofibrillar structure arising from planar well-delocalized quinoid units is considered to be responsible for the high charge carrier mobility.
Since
quinoidal molecules have double-bond linkages between aromatic
rings, they have many advantages for efficient charge transport resulting
from high planarity and extended π-conjugation length. However,
they unavoidably generate some isomers, which cause difficulty in
purification and characterization. In this study, sulfur–oxygen
conformation locking and steric repulsion approach is introduced to
manipulate syn- and anti-isomerization of a quinoidal building block
(bis-QEDOT). As a result, isomer-free bis-QEDOT is synthesized by
introducing the 3,4-ethylenedioxy group, and the geometrical structure
of bis-QEDOT is identified by thin-layer chromatography, 1H NMR, and density functional theory calculation. Furthermore, thiophene
(T), bithiophene (2T), and thienylene vinylene (TV) as π-conjugated
building blocks are polymerized with bis-QEDOT. Due to the quinoid
structure, PQEDOT-T, PQEDOT-2T, and PQEDOT-TV show an intensified
near-IR absorption and a low band gap around ∼1.16 eV. Grazing
incidence wide-angle X-ray diffraction reveals that three quinoidal
polymers show in the (h00) diffraction peaks up to
third order after thermal annealing at 250 °C, demonstrating
high crystallinity of the films. Finally, the electrical properties
of the three polymers are investigated as an active layer in organic
field-effect transistors showing hole mobilities of 4.3 × 10–2 (PQEDOT-T), 1.8 × 10–2 (PQEDOT-2T),
and 7.8 × 10–3 cm2 V–1 s–1 (PQEDOT-TV).
The morphology of conjugated polymer thin films, determined by the kinetics of film drying, is closely correlated with their electrical properties. Herein, we focused on dramatic changes in thin film morphology of blade-coated poly{[N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)} caused by the effect of solvent and coating temperature. Through in situ measurements the evolution of polymer aggregates and crystallites, which plays a decisive role in the formation of the charge transporting pathway, was observed in real-time. By combining in situ ultraviolet-visible spectroscopy and in situ grazing-incidence wide-angle X-ray scattering analysis, we could identify five distinct stages during the blade-coating process; these stages were observed irrespective of the solvent and coating temperature used. The five stages are described in detail with a proposed model of film formation. This insight is an important step in understanding the relationship between the morphology of thin polymer films and their charge-transport properties as well as optimizing the structural evolution of thin films.
In this study, we obtained a new structural insight into the charge-transporting properties in TPD-based polymers that cannot be solely explained in terms of the type of orientation. We synthesized two types of copolymers comprising mono-TPD or bis-TPD as the accepting unit. Although the planarity and energy levels are similar with the mono-TPD unit, the aggregation state is quite different, and the X-aggregation tendency seems to be stronger when the bis-TPD unit is incorporated. In the case of TPD1, an effective π−π orbital overlap is found to originate from the Haggregates, and 3D charge transport pathways are formed with a bimodal orientation of edge-on and face-on, resulting in an efficient charge transportation (1.84 cm 2 •V −1 •s −1 of hole and 0.31 cm 2 •V −1 •s −1 of electron). In contrast, despite the well-aligned edge-on orientation of TPD2, it exhibited a relatively very low mobility and splitted emission characteristics in photoluminescence spectra because of the tilted intermolecular stacking pattern with an X-shape (0.015 cm 2 •V −1 •s −1 for hole and 0.16 cm 2 •V −1 •s −1 for electron). An overall characterization of the semiconducting polymers was performed, and it was found that the type of aggregation in the final thin films, such as H-or X-aggregation, is indeed important and perhaps more important than the orientation to obtain polymers with a high charge carrier mobility.
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