β‑Alkyl substituted Dithieno[2,3‑<i>d</i>;2′,3′<i>-d</i>′]benzo[1,2‑<i>b</i>;4,5‑<i>b</i>′]dithiophene Semiconducting Materials and Their Application to Solution-Processed Organic Transistors

A novel highly π-extended heteroacene with four symmetrically fused thiophene-ring units and solubilizing substituents at the terminal β-positions on the central ring, dithieno­[2,3-<i>d</i>;2′,3′<i>-d</i>′]­benzo­[1,2-<i>b</i>;4,5-<i>b</i>′]­dithiophene (DTBDT) was synthesized via intramolecular electrophilic coupling reaction. The α-positions availability in the DTBDT motif enables the preparation of solution-processable DTBDT-based polymers such as <b>PDTBDT</b>, <b>PDTBDT-BT</b>, <b>PDTBDT-DTBT</b>, and <b>PDTBDT-DTDPP</b>. Even with its highly extended acene-like π-framework, all polymers show fairly good environmental stability of their highest occupied molecular orbitals (HOMOs) from −5.21 to −5.59 eV. In the course of our study to assess a profile of semiconductor properties, field-effect transistor performance of the four DTBDT-containing copolymers via solution-process is characterized, and <b>PDTBDT-DTDPP</b> exhibits the best electrical performance with a hole mobility of 1.70 × 10<sup>–2</sup> cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup>. <b>PDTBDT-DTDPP</b> has a relatively smaller charge injection barrier for a hole from the gold electrodes and maintains good coplanarity of the polymer backbone, indicating the enhanced π–π stacking characteristic and charge carrier transport. The experimental results demonstrate that our molecular design strategy for air-stable, high-performance organic semiconductors is highly promising.