A feasibly synthesized ladder-type conjugated molecule as the novel high mobility n-type organic semiconductor

Abstract

Two ladder-type conjugated molecules, i.e., 2,2′-(2,7-dihexyl-4,9-dihydro-s-indaceno[1,2-b:5,6-b′]dithiophene-4,9-diylidene) dimalononitrile (1) and 2,7-dihexyl-4,9-dihydro-s-indaceno[1,2-b:5,6-b′]dithiophene-4,9-dione (2) were synthesized in high yields, and their electrochemical properties, single crystal structures, thin-film morphology and field-effect mobility were investigated. The LUMO/HOMO levels of 1 and 2 are −4.07/−6.00 and −3.62/−5.82 eV, respectively, as estimated from the thin-film cyclic voltammetry measurements. In the single crystals, both compounds exhibit one dimensional π–π stacking of molecules, and 1 shows the larger π-electron overlap between the neighboring molecules than 2, along with interdigitation of alkyl chains. Thin-film X-ray diffraction (XRD) and atomic force microscopy (AFM) characterizations indicate that 1 can form films with higher molecular order than 2. Field-effect electron mobility up to 0.33 cm² V⁻¹ s⁻¹ in air for 1 and hole mobility up to 0.05 cm² V⁻¹ s⁻¹ for 2 have been demonstrated with the top-contact organic thin-film transistor (OTFT) device structure. Electron mobility of 1 is among the highest value for n-type organic semiconductors.