Solution-Processed Isoindigo- and Thienoisoindigo-Based Donor–Acceptor–Donor π-Conjugated Small Molecules: Synthesis, Morphology, Molecular Packing, and Field-Effect Transistor Characterization

Abstract

Molecular design and precise control of thin-film morphology and crystallinity of solution-processed small molecules are important for enhancing charge transport mobility of organic field-effect transistors and gaining more insight into the structure-property relationship. Here, two donor-acceptor-donor (D-A-D) architecture small molecules <b>TRA-IID-TRA</b> and <b>TRA-TIID-TRA</b> comprising an electron-donating triarylamine (<b>TRA</b>) and two different electron-withdrawing cores, isoindigo (<b>IID</b>) and thienoisoindigo (<b>TIID</b>), respectively, were synthesized and characterized. Replacing the phenylene rings of central <b>IID</b> A with thiophene gives a <b>TIID</b> core, which reduces the optical band gap and upshifts the energy levels of frontier molecular orbitals. The single-crystal structures and grazing-incidence wide-angle X-ray scattering (GIWAXS) analysis revealed that <b>TRA-TIID-TRA</b> exhibits the relatively tighter π-π stacking packing with preferential edge-on orientation, larger coherence length, and higher crystallinity due to the noncovalent S···O/S···π intermolecular interactions. The distinctly oriented and connected ribbon-like <b>TRA-TIID-TRA</b> crystalline film by the solution-shearing process achieved a superior hole mobility of 0.89 cm² V^-1 s^-1 in the organic field-effect transistor (OFET) device, which is at least five times higher than that (0.17 cm² V^-1 s^-1) of <b>TRA-IID-TRA</b> with clear cracks. Eventually, rational modulation of fused core in the π-conjugated D-A-D small molecule provides a new understanding of structural design for enhancing the performance of solution-processed organic semiconductors.