Molecular engineering of acetylenic dithieno[3,2-b:2′,3′-d]thiophene derivatives for organic field-effect transistors

In this study, we report a series of dithieno[3,2-b:2′,3′-d]thiophene (DTT)-based small molecules featuring five different end-capping substituents: 2-octyl-6-(5-(phenylethynyl)thiophen-2-yl)dithieno[3,2-b:2′,3′-d]thiophene (compound 1), triisopropyl((5-(6-octyldithieno[3,2-b:2′,3′-d]thiophen-2-yl)thiophen-2-yl)ethynyl)silane (compound 2), 2-octyl-6-(5-(thiophen-2-ylethynyl)thiophen-2-yl)dithieno[3,2-b:2′,3′-d]thiophene (compound 3), 2-octyl-6-(5-((5-octylthiophen-2-yl)ethynyl)thiophen-2-yl)dithieno[3,2-b:2′,3′-d]thiophene (compound 4), and 2-(5-((5-(2-ethylhexyl)thiophen-2-yl)ethynyl)thiophen-2-yl)-6-octyldithieno[3,2-b:2′,3′-d]thiophene (compound 5). To enhance solubility, one-sided linear octyl chain was introduced on the DTT core, while extended π-conjugation was achieved through the incorporation of additional thiophene rings and acetylenic linkages to promote effective charge transport. The thermal stability, optical absorption, and redox characteristics of the synthesized molecules were studied via thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), UV–vis spectroscopy, and cyclic voltammetry (CV). The DTT thin films formed via solution shearing were characterized by atomic force microscopy (AFM) and X-ray diffraction (XRD) to examine surface topology and molecular arrangement. When implemented as active materials in organic field-effect transistors (OFETs), all five compounds displayed p-type semiconducting behavior. Among them, compound 3, which possesses a thienyl terminal group without bulky alkyl side chains, exhibited the most favorable charge transport characteristics under ambient atmosphere, attaining a carrier mobility of 0.036 cm² V⁻¹ s⁻¹ and an on/off current ratio above 10⁶. These observations underscore the importance of rational side-chain modification in optimizing molecular assembly and film crystallinity to enhance OFET performance.