Dialkoxybithiazole: A New Building Block for Head-to-Head Polymer Semiconductors

Polymer semiconductors have received great attention for organic electronics due to the low fabrication cost offered by solution-based printing techniques. To enable the desired solubility/processability and carrier mobility, polymers are functionalized with hydrocarbon chains by strategically manipulating the alkylation patterns. Note that head-to-head (HH) linkages have traditionally been avoided because the induced backbone torsion leads to poor π–π overlap and amorphous film microstructures, and hence to low carrier mobilities. We report here the synthesis of a new building block for HH linkages, 4,4′-dialkoxy-5,5′-bithiazole (<b>BTzOR</b>), and its incorporation into polymers for high performance organic thin-film transistors. The small oxygen van der Waals radius and intramolecular S­(thiazolyl)···O­(alkoxy) attraction promote HH macromolecular architectures with extensive π-conjugation, low bandgaps (1.40–1.63 eV), and high crystallinity. In comparison to previously reported 3,3′-dialkoxy-2,2′-bithiophene (<b>BTOR</b>), <b>BTzOR</b> is a promising building block in view of thiazole geometric and electronic properties: (a) replacing (thiophene)­C–H with (thiazole)N reduces steric encumbrance in <b>–BTzOR–Ar–</b> dyads by eliminating repulsive C–H···H–C interactions with neighboring arene units, thereby enhancing π–π overlap and film crystallinity; and (b) thiazole electron-deficiency compensates alkoxy electron-donating characteristics, thereby lowering the <b>BTzOR</b> polymer HOMO versus that of the <b>BTOR</b> analogues. Thus, the new <b>BTzOR</b> polymers show substantial hole mobilities (0.06–0.25 cm²/(V s)) in organic thin-film transistors, as well as enhanced <i>I</i>_on:<i>I</i>_off ratios and greater ambient stability than the <b>BTOR</b> analogues. These geometric and electronic properties make <b>BTzOR</b> a promising building block for new classes of polymer semiconductors, and the synthetic route to <b>BTzOR</b> reported here should be adaptable to many other bithiazole-based building blocks