One-Pot Stille Coupling Homopolymerization Toward Bithiophene–Quinoxaline Copolymers with Reduced Sequence Defects

To elucidate the effects of main-chain sequence defects in donor-acceptor (D-A) copolymers on their charge-transport behavior, we synthesized bithiophene-quinoxaline (BTQ) copolymers via two complementary polymerization routes: conventional copolymerization by heterocoupling of two monomers and homopolymerization of a single monomer. Matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry confirmed that the BTQ polymers synthesized by one-pot Stille coupling were free from sequence defects, whereas those obtained by conventional heterocoupling contained such defects. UV-vis absorption spectroscopy, photoemission yield spectroscopy (PYS), ultraviolet photoelectron spectroscopy (UPS), and grazing-incidence wide-angle X-ray scattering (GIWAXS) revealed that the absence of these defects led to enhanced backbone ordering, improved crystallinity, and a narrower highest occupied molecular orbital (HOMO) density of states (DOS). Organic field-effect transistors (OFETs) based on the homocoupled BTQ exhibited hole mobilities 3.5 times higher than those based on the heterocoupled BTQ. These results demonstrate that adopting a homocoupling route to minimize sequence defects is an effective strategy for uncovering the intrinsic electronic properties of π-conjugated polymers.