Comparative Evaluation of Pyrrole Fused Donor Moieties: 1H-Indole and Pyrrolo[2,3-b] Pyridine in Benzothiadiazole-Based D–A–D Type Conjugated Small Molecules for Organic Field-Effect Transistors

Pyrrolic units have been utilized as building blocks for organic semiconducting small molecules and polymers in the recent past. Even though pyrrole-based materials have shown promising semiconducting properties, they have been challenging due to their lower stability under ambient conditions. In this study, we synthesized two pyrrole-fused moieties: 1H-indole (IN) and pyrrolo[2,3-b] pyridine (PPy), which were then explored for their potential as effective donor moieties in organic semiconducting materials. Each donor block was employed to synthesize two donor–acceptor–donor-type small molecules. Thiophene-flanked benzo[c][1,2,5]thiadiazole was used as an acceptor to generate diethyl 6,6'-(benzo[c][1,2,5]thiadiazole-4,7-diylbis(thiophene-5,2-diyl))bis(1-dodecyl-1H-indole-2-carboxylate (IN-BT2T-IN) and diethyl 6,6'-(benzo[c][1,2,5]thiadiazole-4,7-diylbis(thiophene-5,2-diyl))bis(1-dodecyl-1H-pyrrolo[2,3-b]pyridine-2-carboxylate (PPy-BT2T-PPy) donor–acceptor–donor molecules. These novel donor–acceptor–donor molecules were tested for their hole-transport properties by fabricating and testing organic field-effect transistors (OFETs). Both molecules exhibited moderate hole-transporting properties with maximum hole mobilities of 0.00483 and 0.00381 cm2 V–1 s–1 for IN-BT2T-IN and PPy-BT2T-PPy, measured under annealing conditions. The enhanced hole mobilities measured in the annealed OFET devices were attributed to thermally induced crystallinity, as demonstrated by atomic force microscopy and grazing incidence X-ray diffraction measurements.