Fabrication of pH- and temperature-directed supramolecular materials from 1D fibers to exclusively 2D planar structures using an ionic self-assembly approach

Constructing multiple-response smart materials is a very interesting and challenging task in materials science. Here we employed an ionic self-assembly (ISA) strategy to fabricate pH- and temperature-responsive supramolecular materials with controllable fluorescence emission properties by using charged Congo red (CR) and an oppositely charged COOH-functionalized imidazolium-based surface active ionic liquid (SAIL), N-decyl-N′-carboxymethyl imidazolium bromide ([N-C10, N′-COOH-Im]Br). One-dimensional (1D) slender fibers were obtained in aqueous solution at pH 3.2 by the self-assembly of CR/[N-C10, N′-COOH-Im]Br (molar ratio = 1 : 2) at room temperature. Noteworthy is that two-dimensional (2D) planar structures, viz. bamboo leaf-like, spindly, discoid and rectangular structures, were then formed only by further changing the pH of the solution. Of particular interest is that the transition between 1D and 2D structures is pH reversible. We also found that the slender fibers could aggregate into fiber bundle structures with increasing temperature. In addition, fluorescence emission of the obtained 1D and 2D materials can be controlled by adjusting the morphologies of the aggregates. The electrostatic and hydrophobic interactions, in concert with π–π stacking between Congo red and [N-C10, N′-COOH-Im]Br molecules, were regarded as the main driving forces. The dimer-type π–π stacking existing among CR molecules was testified by DFT calculations.