Multifunctional, Polymorphic, Ionic Fullerene Supramolecular Materials: Self-Assembly and Thermotropic Properties
An N-methylfulleropyrrolidine (2) bearing three eicosyloxy chains on the laterally substituted phenyl group can be further functionalized to give the ionic fullerene derivative, i.e., N,N-dimethylfulleropyrrolidinium iodide (1). The spectroscopic, electrochemical, self-assembly, and liquid crystalline properties of 1 have been investigated and compared to its neutral precursor 2. Changes in electronic structure upon ionization are observed in the UV spectra. Additionally, a positive potential shift of electrochemical reductions for 1 compared to those of 2 is noted in both homogeneous solution and film state. Driven by the π–π, van der Waals, and electrostatic interactions, the ionic compound 1 is able to form a variety of functional and polymorphic self-assembled structures both from solution and on substrates, including hierarchically organized flakelike microparticles with high water repellency, doughnut-shaped objects with rough surfaces, and long one-dimensional C60 nanowires (>1 μm). The thermotropic behavior of 1 has also been investigated, and a smectic liquid crystalline phase was observed at elevated temperatures. Further investigations of the thermotropic behavior of 1 revealed that a deionization back-reaction from 1 to the neutral precursor 2 gradually occurred. The mechanism of this deionization reaction is presented and discussed. These investigations provide insight into the effects of added ionicity to alkylated fullerene derivatives, in particular on their self-assembly features and functionality.