Side-chain modulation of dithienofluorene-based copolymers to achieve high field-effect mobilities

A ladder-type dithieno[3,2-<i>b</i>:6,7-<i>b</i>']fluorene (DTF), where the central fluorene is fused with two outer thiophene rings at its 2,3- and 6,7-junctions, is developed. The pentacyclic DTF monomers were polymerized with dithienodiketopyrrolopyrrole (DPP) acceptors to afford three alternating donor-acceptor copolymers <b>PDTFDPP16</b>, <b>PDTFDPP20</b>, and <b>PDTFDPP32</b> incorporating different aliphatic side chains (R₁ group at DTF; R₂ group at the DPP moieties). The side-chain variations in the polymers play a significant role in determining not only the intrinsic molecular properties but also the intermolecular packing. As evidenced by the 2-dimensional GIXS measurements, <b>PDTFDPP16</b> with octyl (R₁) and 2-ethylhexyl (R₂) side chains tends to align in an edge-on π-stacking orientation, whereas <b>PDTFDPP20</b> using 2-butyloctyl (R₁) and 2-ethylhexyl (R₂) adopts a predominately face-on orientation. <b>PDTFDPP32</b> with the bulkiest 2-butyloctyl (R₁) and 2-octyldodecyl (R₂) side chains shows a less ordered amorphous character. The OFET device using <b>PDTFDPP20</b> with a face-on orientation determined by GIXS measurements achieved a high hole-mobility of up to 5 cm² V^-1 s^-1. The high rigidity and coplanarity of the DTF motifs play an important role in facilitating intramolecular 1-dimensional charge transport within the polymer backbones. The implementation of main-chain coplanarity and side-chain engineering strategies in this research provides in-depth insights into structure-property relationships for guiding development of high-mobility OFET polymers.