Exploring the effect of the spacer structure in the heterocyclic ring-fused isoindigo-based conjugated polymer on the charge-transporting property

Isoindigo (IID) has been developed as an electron deficient moiety for high-performance conjugated organic materials, especially for constituting donor–acceptor (D-A) type conjugated polymers. To further enhance the planarity of IID unit to promote the associated intramolecular charge transfer (ICT) effect of the derived polymers, various of structural modifications have been performed in recent years. Herein, two heterocyclic ring-fused IID core structures, tetrafluorobenzene-centered IID (FBIID) and pyrazine-centered IID (PzIID), were developed and copolymerized with two common electron-rich moieties, 2,2′-bithiophene (2 T) and (E)-1,2-bis(thiophen-2-yl)ethene (TVT). The properties and structure-mobility relationship of the prepared four D-A type polymers, PFBIID-2 T, PFBIID-TVT, PPzIID-2 T, and PPzIID-TVT are systematically studied to elucidate the effect of the spacer structure in the heterocyclic ring-fused IID units. Both the simulation results and experimental analyses show that the PzIID-based polymers possess much higher planarity and mobility than the FBIID-based ones because of the less substituents on the pyrazine spacer, which reduces the steric hindrance within the structure. Finally, we show that the distinctly different planarity between FBIID and PzIID results in significantly different field-effect transistor (FET) mobility. Owing to the higher planarity and improved stacking and crystallinity of the polymer chains, the PzIID-derived polymers deliver much higher hole mobilities (μ_hs) of > 3 × 10 ⁻⁴ (cm² V⁻¹ s⁻¹) than the FBIID-derived polymers that show no conspicuous charge-transporting properties. This result clearly highlights the crucial role of the spacer structure in designing the heterocyclic ring-fused IID core structures.