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

Abstract

A ladder-type dithieno[3,2-b: 6,7-b'] 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 PDTFDPP16, PDTFDPP20, and PDTFDPP32 incorporating different aliphatic side chains (R-1 group at DTF; R-2 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, PDTFDPP16 with octyl (R-1) and 2-ethylhexyl (R-2) side chains tends to align in an edge-on pi-stacking orientation, whereas PDTFDPP20 using 2-butyloctyl (R-1) and 2-ethylhexyl (R-2) adopts a predominately face-on orientation. PDTFDPP32 with the bulkiest 2-butyloctyl (R-1) and 2-octyldodecyl (R-2) side chains shows a less ordered amorphous character. The OFET device using PDTFDPP20 with a face-on orientation determined by GIXS measurements achieved a high hole-mobility of up to 5 cm(2) 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.