Synthesis and characterization of diketopyrrolopyrrole-based conjugated molecules flanked by indenothiophene and benzoindenothiophene derivatives

Four indeno[1,2-b]thiophene- or benzo[5,6]indeno[1,2-b]thiophene-flanked diketopyrrolopyrrole (DPP) derivatives, i.e., DPP-PhCO, DPP-PhCN, DPP-NaCO and DPP-NaCN in which phenyl or naphthyl and thiophene units were bridged with a carbonyl (DPP-PhCO and DPP-NaCO) group or a malononitrile-substituted carbon atom (DPP-PhCN and DPP-NaCN), were synthesized via intramolecular Friedel-Crafts acylation and Knoevenagel condensation. Compared to 5-phenylthiophen-2-yl substituted DPP (DPP-Ph) which has no bridge between the phenyl and thiophene rings, DPP-PhCO, DPP-PhCN, DPP-NaCO and DPP-NaCN have more planar structures and exhibit deeper highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels. The semiconducting properties of the compounds were studied using organic thin film transistors (OTFTs). DPP-Ph, DPP-PhCO, DPP-NaCO and DPP-NaCN exhibited p-type transport behavior and DPP-PhCO showed the highest mobility (up to 0.052 cm(2) V(-1)s(-1)), while DPP-PhCN displayed ambipolar transport properties with hole and electron mobilities of 0.017 and 8.6 x 10(-4) cm(2) V-1 s(-1), respectively.