Supramolecular helical nanofibers formed by an achiral monopyrrolotetrathiafulvalene derivative: water-triggered gelation and chiral evolution

Abstract

Chiral nanostructures have attracted increasing attention, which would provide not only a better understanding of the bio self-assembly process, but also open a route for new applications. In this research study, the organogel formation and self-assembly of an achiral MPTTF-based molecule were investigated. It was found that the compound could not gelate any single solvent either through a heating and cooling process or by ultrasonic treatment, but formed certain ordered supramolecular self-assembled aggregates in pure DMF with left- (M) and right- (P) helicities. Interestingly, supramolecular gelation was triggered by adding a small amount of water in its DMF solution, and the structure of the fibers was almost entirely turned into right-handed helicity. The twisted superstructure was visualised by AFM, and CD was used to observe chirality within the assembly. Also, the investigation of UV-Vis spectra, FTIR and SAXS indicated that these supramolecular assemblies were induced by hydrogen bonding, π–π stacking and S⋯S interaction between the molecules. Furthermore, the formed organogel underwent a reversible gel–sol phase transition via a chemical redox reaction. This work sheds new light on the hierarchical transformation of chiral structures from achiral molecules by controlling the solvent composition and polarity.