Impact of Carbosilane Side-Chain Branching Positions on Aggregation Behavior and Semiconducting Properties of Isoindigo-Based n-Type Conjugated Polymers

Side chain structures have a significant influence on the solution preaggregation and thin-film microstructures of conjugated polymers (CPs) and are thus one of the key structural parameters determining the charge transport properties of CPs. Herein, a series of isoindigo-based n-type CPs appending carbosilane side chains with varying Si-branching positions, named B<i>m</i>SiC12 (<i>m</i> = 3-8, representing the number of CH₂ units between the Si atom and the conjugated backbone), were synthesized via direct arylation polymerization. The effect of branching positions of carbosilane side chains on solution preaggregation, thin-film microstructures, and semiconducting properties of B<i>m</i>SiC12 was systematically investigated. These properties of B<i>m</i>SiC12 exhibit an obvious odd-even dependence on <i>m</i> as <i>m</i> ≤ 6. The polymers with even <i>m</i> tend to form larger one-dimensional (1D) fibrous aggregate in solution and higher long-range order in thin film, and thus display more efficient charge transport in organic thin-film transistors (OTFTs). The odd-even effect gradually diminishes as the <i>m</i> value increases to above 6. Among the polymers, B4SiC12 and B6SiC12 form the largest solution preaggregates and highly aligned thin films, achieving maximum OTFT electron mobilities up to 3.65 and 3.95 cm² V^-1 s^-1, respectively.