Enhanced Crystallinity and Electrical Conductivity in Conjugated Polymers via Dynamic Covalent Bond Exchange

Abstract

Conjugated polymers have been extensively used for organic electronics applications due to their tunable optical, electrical, and mechanical properties. Traditional solution-based processing methods for conjugated polymers often require toxic solvents and are limited by polymer solubility. In contrast, solid-state hot pressing offers a straightforward, solvent-free processing method, but subjecting conjugated polymers to temperatures above their melting temperatures Tm can lead to thermal oxidative degradation. Here, we investigate the effect of hot-pressing and acid doping on the crystallinity and electron transport properties of conjugated polymers with dynamic covalent imine bonds between quaterthiophene units (DQT-DP) compared to nondynamic analogs (DQT-NP). Polymers were processed at temperatures below Tm, and enhanced crystallinity was observed using differential scanning calorimetry (DSC) and wide-angle X-ray scattering (WAXS), which is attributed to thermally activated dynamic bond exchange promoting polymer subsegment crystallization and flow-induced interchain π–π stacking. In contrast, negligible impacts on crystallinity were observed when hot-pressing the nondynamic polymer analog DQT-NP. Enhanced crystallinity in hot-pressed DQT-DP led to higher electrical conductivity after iodine vapor doping. Acid doping was further investigated to catalyze imine bond exchange and was found to promote backbone planarity, leading to enhanced crystallinity and conductivity. Density functional theory (DFT) calculations showed that protonation of imine bonds significantly enhances molecular planarity, consistent with experimental observations. Overall, this work shows that dynamic covalent bond exchange provides a useful strategy to enhance the structural and electronic properties of semiconducting polymers.