Functionalization strategies for Benzo[1,2-b:5,4-b']dithiophene derivatives with acetylenic bonds for solution-processable organic field-effect transistors

This study presents the synthesis and characterization of six small-molecule benzo[1,2-b:5,4-b']dithiophene (BDT) derivatives incorporating acetylenic bonds and various thienyl-based substituents, designed for application in solution-processable organic field-effect transistors (OFETs). Structural variations included pristine, linear alkylated, and branched alkylated thiophene and bithiophene units. Thermal, optical, electrochemical, and theoretical properties of these molecules were systematically analyzed, along with characterization of thin-film morphologies and microstructure via AFM and XRD measurements. All derivatives demonstrated sufficient solubility in toluene, affirming compatibility with solution process. Among the compounds, the unsubstituted molecule (compound 1) exhibited the highest hole mobility (up to 0.16 cm2/Vs) with average hole mobility (0.086 cm2/Vs) and marked film crystallinity. This enhancement is attributed to its high molecular planarity and rigidity, induced by the acetylenic linkage. In contrast, compounds functionalized with alkyl chains or bithiophene units exhibited increased molecular distortion, poor crystallinity, and significantly reduced OFET performance. These results underline the critical role of structural planarity and minimal steric hindrance in achieving high-performance OSCs, simultaneously highlighting the potential performance degradation from improper functionalization.