A versatile synthetic strategy for non-symmetric isoindigo polymers via modular sidechain engineering

Abstract

Sidechain engineering is a powerful strategy for tailoring the intrinsic properties of semiconducting polymers, enabling precise modulation of structure–property relationships critical for organic electronic applications. Breaking symmetry via sidechain engineering—by incorporating two non-identical sidechains onto the conjugated backbone—is an emerging approach to control molecular stacking and polymer chain interactions. However, the fundamental impact of sidechain induced non-symmetry remains underexplored, limiting the full tunability and optimization of this design in soft organic electronics. In this work, we investigate the structure–property relationships of isoindigo-thienothiophene-based semiconducting polymers, employing a novel “lego-like” synthetic strategy for non-symmetric condensation of alkylated precursors. This method significantly enhances specificity, customizability, and reproducibility, facilitating the development of fully tunable systems. Using this approach, we synthesized a series of non-symmetric polymers with linear and branched aliphatic sidechains to evaluate their impact on polymer packing motifs and performance in organic field-effect transistors (OFETs). Multimodal characterization revealed that these non-symmetric polymers exhibit electronic properties comparable to their symmetric counterparts while demonstrating reduced crystallinity and low Young's moduli, as shown by atomic force microscopy. Furthermore, increasing the carbon spacer length from 1 to 4 carbons in the branched chain moiety improved charge transport, achieving average hole mobilities of up to 0.12 cm² V⁻¹ s⁻¹ and 0.10 cm² V⁻¹ s⁻¹ for P[(iITT)(C₁C₁₀C₁₂)(C₁₂)] and P[(iITT)(C₄C₁₀C₁₂)(C₁₂)], respectively. Overall, this work establishes a straightforward and highly tunable approach for designing a library of non-symmetric isoindigo derivatives through sidechain engineering, providing precise control over nanoscale polymer structures in thin films and unlocks new pathways for enhancing the performance of organic electronic materials.