Design and structure-property relationship of benzothienoisoindigo in organic field effect transistors

A novel planar pi-conjugated small molecule, benzothienoisoindigo (BTII), in which additional benzene rings are fused with the thieoisoindigo (TII) unit, has been designed and synthesized. We report the impact of the planar pi-framework and pi-conjugation length on the carrier transport properties using three sets of molecules, BTII, isoindigo (II) and TII, bearing the same hexyl-side chain. The absorption spectra are remarkably red-shifted in the order of II < TII < BTII along with the enhanced molar extinction coefficient in the low-energy region, leading to the reduced bandgap. The single-crystal structure analyses revealed that all molecules have a planar backbone, and II and BTII are packed into a slipped columnar structure showing highly one-dimensional pi-pi interactions, while TII did not form, any noticeable intermolecular overlaps. The carrier transport properties were investigated in field-effect transistors (FETs). All molecules exhibited typical ambipolar properties. Among them, BTII showed the highest FET p-dominant ambipolar performance with the hole mobility of 0.095 cm(2) V-1 s(-1) and electron mobility of 5.8 x 10(-3) cm(2) V-1 s(-1) on the tetratetracontane (TTC)-modified substrate and p-type performance with the hole mobility of 0.18 cm(2) V-1 s(-1) on the octadecyltrimethoxysilane (OTMS)-modified substrate. The microstructure of thin films was characterized by X-ray diffraction (XRD) and atomic force microscopy (AFM) measurements. These results indicated that smooth and densely packed nanorod-like crystalline grains are formed by extension of the pi-conjugation in BTII. Due to the pi-extension of planar organic semiconductors, the novel BTII unit can be extended for the rational design of high performance FET materials.