Synthesis and molecular ordering of isoindigo-based organic semiconductors through hydrogen bonding

Abstract

The molecular ordering of semiconducting small molecules comprising a newly designed A(D-A’-D)2 system was successfully achieved. The A(D-A’-D)2 small molecules have two different acceptors based on isoindigo and diketopyrrolopyrrole along with tert-butoxycarbonyl (t-Boc) groups and hexyl chains, which improve the solubility of the molecules. After simple thermal annealing, t-Boc groups were removed to allow strong hydrogen bonds between N-H…C=O hydrogen bonds to form. This resulted in improved molecular ordering of the organic semiconductors. The crystalline morphology was confirmed by X-ray diffraction coupled with high-voltage electron microscopy. The resulting materials showed improved hole mobilities estimated from hole-only devices. Molecular ordering is also achieved using a new highly soluble isoindigo based organic material with cleavable carbamate protecting groups bearing long alkyl chains and thiophene groups. The carbamate protecting groups were easily removed by solvent-vapor thermal annealing using a mixture of trifluoroacetic acid and chloroform. Deprotection of the NH functional groups in the isoindigo backbone in situ facilitated H-bonding that dramatically improved the strength of the intermolecular interactions. X-ray diffraction and high-voltage electron microscopy studies of the resulting organic material showed long-range ordered crystalline structure. High crystallinity due to enhanced π-π stacking and hydrogen bonding significantly improved the charge carrier mobility. The hole mobilities of isoindigo based organic material measured by SCLC method improved from 3.46 x 10-6 cm2 V−1 s−1 to 1.13 x 10-3 cm2 V−1 s−1 upon deprotection of the NH functional groups. In another approach, the synthesis of molecular-scale-ordered isoindigo and bithiophene-based small molecules has been achieved through hydrogen bonding by using ethylenediamine (EDA) as a linking material or by the cleavage of the carbamate protecting groups containing long alkyl chains. From X-Ray diffraction and high-voltage electron microscopy (HVEM) studies, the materials had as body-centered cubic (BCC) or face-centered cubic (FCC) lattice structures. Formation of hydrogen bonded networks reflected the enhanced molecular ordering and charge carrier transport between molecules.