Supramolecular Nanomaterials Constructed Based On Ionic Self-assembly And Their Functions

Supramolecular chemistry has driven the development of various new materials such as nanomaterials, organogels, organic-inorganic hybrid materials and biomolecular materials. Supramolecular self-assembly is widely applied in rationally designing varieties of functionalized materials. Ionic self-assembly (ISA) offers a potent and versatile route to prepare well-defined, discrete supramolecular architectures from simple molecular components under thermodynamic equilibrium. The strategy based on electrostatic interaction has made a great influence due to generalizability, simplicity and cheapness. In this dissertation, we synthesized a series of novel surface active ionic liquids (SAILs) and chose proper oppositely charged molecules. Subsequently, we applied the ionic self-assembly (ISA) strategy to prepare supramolecular materials. There are four main parts in this dissertation as follows:Chapter 1 is a brief introduction of basic knowledge and recent achievements related to the supramolecular materials based on ISA. The ideas for the dissertation were presented.Chapter 2. Based on the strategy of ISA, supramolecular nanomaterials were prepared by surfactants and small organic molecules. (1) Constructing multiple-response smart materials is a great interest and challenge for material 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 oppositely charged COOH-functionalized imidazolium-based SAIL, N-decyl-N-carboxymethyl imidazolium bromide ([N-C10, N’-COOH-Im]Br. One-dimensional (1D) slender fibers were obtained in pH 3.2 aqueous solution 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 solution. Of particular interest is that the transition between 1D and 2D structures are pH reversible. We also found the slender fibers could aggregate into fiber bundle structures with the increasing temperature. In addition, fluorescent switch property of the obtained 1D and 2D materials can be controlled by changing the pH value of aqueous solution between 3.2 and 9.0. 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. (2) Supramolecular structures ranging from nano-to macro-scale were prepared by ISA strategy with commercially available, low-cost dye and surfactant, viz. Rhodamine 6G (R6G) and sodium bis(2-ethylhexylhexyl) sulfosuccinate (NaAOT). Fibers with the length up to two centimeters were fabricated. They exhibit strong strength and high flexibility and could be bent without damaging the original shape. It is noteworthy that this supramolecular complex also displays both remarkable solid-state red light emission and organic semiconducting behaviors. It is extremely rare that the looped nanochains comprised of organic nanoparticles were found during the forming process of the supramolecular structures. A possible mechanism for the formation of the macroscopic fibers is proposed. (3) Multiple-stimulus-responsive smart materials were synthesized via supramolecular self-assembly. One-dimensional nanorods were constructed by the self-aggregation of 4-(phenylazo)benzoic acid (PBA) molecules in aqueous solution at pH 3.2. As the pH of the solution was increased to 6.5, these nanorods transformed into two-dimensional polygons. Upon UV irradiation, the as-prepared nanorods disappeared completely and nanospheres were subsequently obtained. Based on the weak interactions between PBA and additive molecules, e.g. N-alkyl-N’-carboxymethyl imidazolium bromide ([N-Cn, N’-COOH-Im]Br, n=10,12, 14),β-cyclodextrin (β-CD), and cetyltrimethylammonium bromide (CTAB), materials with various morphologies were also fabricated by a surfactant-assistant self-assembly (SAS) strategy. Noteworthy is that salvia officinalis-shaped material is among these materials. To the best of our knowledge, such microstructured material is rarely reported. In addition, slender fibers, sphere-like particles, and the aggregation of spheres were also observed. These results suggest that the rational fabrication of materials with desired shapes and sizes could be achieved by changing external environments during the self-aggregation of PBA molecules. Both cyclic voltammogram experiments and density function theory (DFT) calculations exhibit optoelectronic behavior of the materials, which is expected to be applied for the photoelectronic nanodevices.Chapter 3. With the ISA method, supramolecular nanomaterials were constructed by SAILs and POM, and their applications in the fields of catalysis and biology were investigated. (1) Via ionic self-assembly (ISA) strategy, in water, an oppositely charged COOH-functionalized SAIL, N-decyl-N-carboxymethyl imidazolium bromide ([N-C10, N’-COOH-Im]Br) was used to encapsulate Keggin-type structure polyoxometalate (POM), phosphotungstic acid (H3PW12O40), resulting in polyoxometalate/ionic liquid hybrid nanomaterials with pH-responsive, electrochemical and catalytic properties. The POM-based hybrid materials show great catalytic enhancement when employed in photodegradation of methyl orange (MO) in aqueous solution. Of particular interest is that due to the electrostatic repulsions between POM and the deprotonated SAIL, pH-induced disassembly and assembly of the hybrids makes it possible to recover the catalytic efficiency. Compared with traditional catalyst, noteworthy is that the catalytic reaction using this new type of hybrid material is not dependent on light. The insolubility in water for the solid-state catalyst leads to the settlement of the catalyst poisoning issue that is common to heterogeneous catalysis in industry. (2) In aqueous solution, [N-C12, N’-COOH-Im]Br/ Na9EuWioO36·32H20 (Eu-POM) supramolecular spheres were fabricated based on ISA strategy. These supramolecular spheres have highly ordered structures and show excellent reversible self-assembly and tunable photoluminescence properties, which can be manipulated by adjusting pH of the aqueous solution. Specifically, the formation of Eu-POM/[N-C12, N’-COOH-Im]Br supramolecular spheres results in quenching of fluorescence emitted by Eu-POM because hopping of d1 electron in the POM molecule is blocked by hydrogen bond existing between the oxygen atom of POM and the carboxylic acid group of [N-C12, N’-COOH-Im]. However, the fluorescence can be completely recovered by gradually increasing pH of the aqueous solution due to the pH-induced deprotonation of the carboxylic acid group of [N-C12, N’-COOH-Im]Br, which results in disassembly of the fabricated supramolecular spheres. Applications of these stimuli-responsive photoluminescent POM-based supramolecular materials are demonstrated in biological media. Dual signaling responses of turbidity and fluorescence are observed simultaneously in the detection of urease and heavy metals based on pH-induced disassembly of the supramolecular spheres during the biochemical events in aqueous solution. In addition, guest molecules are encapsulated into the supramolecular spheres and controlled release of these entrapped molecules is demonstrated in the presence of external stimuli. This study shows potential of stimuli-responsive Eu-POM/[N-C12, N’-COOH-Im]Br supramolecular materials in biological applications. (3) In water, Eu-containing polyoxometalate/Gemini Surfactant hybrid spheres with long emission timescale (3.758 ms) and high fluorescence quantum yield (25.17%) behaviors were synthesized. The hybrid spheres can be used as a bioprobe to image living cells.Chapter 4. We used ISA method to build supramolecular catalysts using polyoxomylatates (POM) and a common commercial surfactant with oppositely charge, hexadecyltrimethylammonium bromide (CTAB). The supramolecular catalysts can be dissolved in temperature-sensitive ionic liquid, tetraburylphosphonium trifluoroacetate ([P4444][CF3COO]) but becomes immiscible in water. Based on these properties, a new-style catalytic system was constructed by supramolecular catalysts, [P4444][CF3COO] and water. [P4444][CF3COO] aqueous solution molecules can form microemulsion at 25℃ and supramolecular catalysts can exist in microemulsion phase. Phase separation occurs when the temperature is increased to 35℃. After 30 min under UV-light, POM/CTAB supramolecular complex can catalyze potassium dichromate (Cr(VI) to Cr(III)). When the temperature increased to 35 ℃, ionic liquid phase with supramolecular complex was separated from water phase. So the catalysts can be separated via the change of temperature. After manifold recycles, the activity of new-style catalytic system is confirmed to be quite stable. Moreover, supramolecular catalysts in ionic liquid can avoid poison in the catalytic process. This new-style catalytic system is probably of great significance for cost saving in industry.