β-alkyl substituted dithieno[2,3-d;2′,3′-d′]benzo[1, 2-b;4,5-b′]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-d;2′,3′-d′] benzo[1,2-b;4,5-b′]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 PDTBDT, PDTBDT-BT, PDTBDT-DTBT, and PDTBDT-DTDPP. 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 PDTBDT-DTDPP exhibits the best electrical performance with a hole mobility of 1.70 × 10 ^-2 cm ² V ^-1 s ^-1. PDTBDT-DTDPP 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.