Length-Dependent Discrete Oligomers and Their Assembly Pathways

For the past couple of decades, the organic electronics field has been gaining attention and momentum in anticipation of becoming a counterpart for traditional inorganic electronics. Devices fabricated from organic materials like organic light-emitting diodes (OLEDs), organic field-effect transistors (OFETs), organic photovoltaics (OPVs), benefit from their tunable chemistry, solution processability, cost-effectiveness, lightweight nature, flexibility, and low working temperatures. Historically, research has focused on small molecules and dispersed polymers, with a prevailing belief that high molecular weight materials exhibit superior charge transport properties. Challenging this notion, a recent study stood against this standard with mesopolymers synthesized from direct arylation polymerization (DArP), which demonstrated high charge carrier mobilities. This suggests a potential breakthrough for 5~20k Da oligomers and mesopolymers, materials that were often left overshadowed. Oligomers and mesopolymers are also promising in their monodispersity and emergent properties, ensuring consistent, reproducible device performances – a challenging feat with polydispersed materials. In this study, we explore assembly pathways of pyridine-flanked diketopyrrolopyrrole (Py-DPP) based oligomers precisely synthesized from DArP. Solution studies allowed us to classify oligomers’ aggregation states and liquid crystalline phases. Validation methods involved ultraviolet-visible spectroscopy (UV-vis) for conjugation lengths and aggregation states, cross-polarized microscopy (CPOM), scanning electron microscopy (SEM), and atomic force microscopy (AFM) for surface morphology comparison, circular dichroism (CD) for chirality, and small-angle x-ray scattering (SAXS) for molecular packing. Based on these results, our findings revealed that even a monomer-length difference in molecules could widely alter orientational order and assembly pathways, leading to unexpected liquid crystalline phases. Blade-coated films doped with Magic Blue, a p-type dopant, demonstrated a clear trend in doping efficiency with increasing oligomer length, but it is worth noting that heptamer had a comparable conductivity to that of polymer’s (Mn ~14k). We anticipate this fundamental understanding of length-dependent oligomer assembly pathways could pave way for more efficient and versatile organic electronic devices. These insights could also open up to new possibilities for tailoring material properties at a molecular level, pushing boundaries of what organic electronics can achieve.