Conjugated Aniline Oligomers: More than Just Short Pieces of Conducting Polyaniline

ConspectusPolyaniline, first discovered from coal tar over 150 years ago, is one of the most explored conducting polymers, due to its unique doping/dedoping chemistry, redox behavior, chemical stability, and simple synthesis. About 20 years ago, polyaniline nanofibers were developed through both interfacial polymerization and a rapid mixing method that produced high surface areas and aqueous dispersions, leading to versatile applications. Aniline oligomers, with much shorter and more well-defined chain lengths, gained great attention because they could offer more thorough insights into fundamental properties and perform precise molecular engineering for functionalization and modification. This Account summarizes our recent efforts to probing physical and chemical properties, crystal growth, catalytic effects, and applications of aniline oligomers.With a defined number of doping sites in the phenyl-capped aniline tetramer, we reveal the doping sequence by performing partial doping with characterization using electron paramagnetic resonance. Distinct from polyaniline, aniline tetramers can be dissolved in common organic solvents, thus offering an opportunity to observe the reduction through UV-vis spectroscopy. Acid reductions of the fully oxidized form, known as pernigraniline, were carried out at liquid/liquid and solid/vapor interfaces, as well as by using galvanic and piezoelectric reduction in solid-state reactions. Taking advantage of treating aniline oligomers as small molecules, we demonstrate the growth of organic single crystals through self-assembly. With various acids as dopants, a variety of nanostructured aniline oligomers with well-defined and hierarchical morphologies have been created. Furthermore, by adjustment of the molecule-solvent interactions, organic crystals of the aniline tetramer can be selectively grown on graphene substrates due to strong π-π stacking. The nucleation density, crystal size, and orientation of the organic crystals can be tuned using different solvents and infiltrating nonsolvents in an antisolvent crystallization process, providing an enhanced understanding of directional electrical conductivity in organic crystals. In the aniline polymerization process, it was realized that aniline oligomers serve as both nucleation sites and catalysts to produce ultralong and fibrillar morphologies. Through molecular engineering, the functionalization of the aniline tetramer with perfluorophenyl azide is demonstrated, rendering surface modifications of graphitic materials and antifouling and antibacterial ultrafiltration membranes via a simple UV light exposure. In addition, by covalently grafting aniline oligomers onto reduced graphene oxides and carbon nanotubes as the electrodes, supercapacitors with an ultralong cycle life─5 times longer than their polyaniline composite counterparts─are created. The superior cycling stabilities are due to the successful prevention of detachment of aniline oligomers and the preservation of the microstructure throughout the cycling process.With precise molecular structures and exceptional processability, we have found exciting properties and developed useful applications that outperform their polymer counterparts. Future research on aniline oligomers promises more discoveries of intriguing molecular science properties and practical applications with enhanced performances.