Comparative Evaluation of Pyrrole Fused Donor Moieties: 1H-Indole and Pyrrolo[2,3-<i>b</i>] 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: 1<i>H</i>-indole (IN) and pyrrolo[2,3-<i>b</i>] 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[<i>c</i>][1,2,5]thiadiazole was used as an acceptor to generate diethyl 6,6'-(benzo[<i>c</i>][1,2,5]thiadiazole-4,7-diylbis(thiophene-5,2-diyl))bis(1-dodecyl-1H-indole-2-carboxylate (<b>IN</b>-<b>BT2T</b>-<b>IN</b>) and diethyl 6,6'-(benzo[c][1,2,5]thiadiazole-4,7-diylbis(thiophene-5,2-diyl))bis(1-dodecyl-1H-pyrrolo[2,3-<i>b</i>]pyridine-2-carboxylate (<b>PPy</b>-<b>BT2T</b>-<b>PPy</b>) 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 <b>IN-BT2T-IN</b> and <b>PPy-BT2T-PPy</b>, 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.