| Synergistic effects of various building blocks can be realized and novelself-assemblied structures can be fabricated by controlling intermolecular interactions,guided by the concept of supramolecular chemistry, and fascinating functionalproperties which the building blocks themselves did not possess can be achieved even.In order to construct functional materials bearing novel assembled structure, variousattempts have been made to design different types of molecular unit. Ionic amphiphilicmolecules （especially ionic liquid crystals） exhibit excellent properties in anisotropicion conductor and nanomaterials synthesis template. Due to the strong ionic interactionsbetween ionic amphiphiled, most ionic liquid crystals exhibit simple structures, such assmectic or columnar phase. As liquid crystalline properties are directly related to theassemblied structure, therefore, how to develop a common approach to achieve newtypes of liquid crystal structure is still needed.Polyoxometalates （POMs） are a class of nanoscaled functional inorganic clusterswhich have become excellent functional inorganic building blocks in supramolecular self-assembly. POM supramolecular self-assembly has made great strides, the orderedstructures of the POM and functional properties have been constructed through varioussupramolecular methods, however, building novel supramolecular structure is still thechallenge. It is difficult to form chiral structure for POMs duo to their highly symmetricstructures. Even chiral POM compound are formed, most of which can only be stable incrystals. The chiralities are unstable in solutions, as the racemic products are easilyformd. When the chiral moieties are modified to POM, the chirality can be inducedalthough. How to realize the chiral assembly for achiral POM complex in soft materialis still the challenge.The assemblied propreties of POMs could be improved evidently when modifiedby organic moieties, which are beneficial to construct novel structure and functionalassemblies. The organically modified POM complexes are composed by hydrophobicshell and hydrophilic core, are a typical class of amphiphilic building blocks, theirassembly structures are not only affected by the cluster size and the charge of the POMs,the structure of the organic components and the self-assembled environment, but alsorealized the dynamically controll through by external stimuli. In this paper, we havesuccessfully prepared building blocks which has dynamic assembly capacity, andrealized the novel quasi-2D lamellar structure and helical assembly by changing the thelateral alkyl chain length or the solvent environment, and achieved the dynamicself-assembly of the helical structures by redox controlling.First, in order to construct ionic liquid crystal materials which have novel structureand to study the mechanism of structure formation and evolution, four ammoniumscontaining carboxylic acid groups with different lateral alkyl chains A-n （n=6,8,10,and12） was designed and synthesized, and characterized by1H NMR, IR andMALDI-TOF spectra, and elemental analysis. We investigated the intermolecularH-bonding interactions by temperature-dependent IR spectra, which showed thatbranched Bola form complex was constructed at both room temperature and liquid crystal ranges. The thermal properties of the H-bonding complex were characterized byTGA, DSC, POM and XRD. The branched Bola amphiphiles with different lateral alkylchains assembled into different structure. A-6with the shorted lateral alkyl chainsorganize into simple layered structure at low temperature and smectic A phase at hightemperature, and benzoic acid dimers act as the mesogens. A-8with longer alkyl chainsexhibits liquid crystalline property at room temperature, the long axis of the rodlikemesogen is parallel to the normal direction of the layered structure. The enhancedflexibility derived from lengthening the lateral alkyl chains lead to the lower transitiontemperature and wide liquid crystal temperature range, which is benifical for thepontential application. Interestingly, novel assemblied structure has been constructed byfurther lengthening the lateral alkyl chains, A-10and A-12organize into quasi-2Dlamellar structure, their long axes of the rodlike mesogens a perpendicular to the normaldirection of the layered structure. The microphase separation between the lateral alkylchains modified ionic groups and H-bonding benzoic acid dimer leads to the layeredstructure along the short axis of the dimer, and the aggregation of the nonpolar alkylchains yield to the layered structure along the long axis of the dimer, which are rare inthe ionic molecules. More interestingly, the quasi-2D lamellar structures aretemperature-dependent, which transform into simple layered structures for A-10andA-12when increasing the temperature. In the present work, the main driving forces forthe branched structure are explained from the microphase separation of incompatiblemoieties, aggregation of compatible moieties and steric effects, which indicated thatcompetition between steric effects and van der Waals interactions plays the importantrole for the assembly of the H-bonding complex. Changing the length of the lateral alkylchains of the branched Bola amphiphiles is an effective strategy to construct novelorganized structure. We believed that other novel assemblied structure could beconstructed by changing the lateral alkyl chains.Second, in order to construct stimula-responsive controllable self-assembly system based on the novel assemblied structures of POM, redox-responsive POMsupramolecular complex （DODA）2[Mo6O19] was prepared through simple ion exchangereaction, which are obtained by replacing the cation of Lindqvist-type POM（TBA）2[Mo6O19] with DODA containing double long alkyl chains. The results of1HNMR, IR and MALDI-TOF spectra, TGA and elemental analysis demonstrate that thePOM complex （DODA）2[Mo6O19] with the rodlike symmetric structure was obtainedeasily. And the self-assemblies of the complex were studied in the mixed solvent ofdichloromethane and propanol with the volume ratio4/1. SEM, TEM and POM imagesshow that helical assemblies are constructed in the mixed solvent, EDX results displaythat the helical strips are composed by （DODA）2[Mo6O19] complex indeed. Helicalassemblies built by achiral building blocks are explained by spontaneous spontaneoussymmetry breaking, which are proved by the results of XRD patterns and IR spectra.Controllable assembly can be realized by redox-stimula, the inorganic POM cluster arereduced to polyblue after UV light irradiation, and the helical assemblies transformedinto spherical aggregates accompanied by the photochromism. And after oxidized byH2O2, the spherical aggregates transform into helical assemblies again, indicating thatreversibly dynamic self-assemblies between helical and spherical structure can berealized through controlling the redox of POM. After three redox cycles, the structureare unchanged proved by UV-visible,1H NMR and IR spectra, and elemental analysis.All the present results demonstates that the assemblied structure of the POM aredynamic. The morphology evolution between helical and spherical structure aremonitored by controlling the irradiation time, and the structural and morphologicaltransformation between helical and spherical assemblies are driven by the alternation ofthe electrostatic interactions of organic surfactant cation and inorganic POM anion,confirmed by XPS and1H NMR spectra.Third, in order to study the effects of solvent environment on the self-assembly ofPOM complex, and to realize the dynamically controlling the assembly by adjusting the solvent environment, the long alkyl chains-containing surfactant encapsulated LindqvistPOM with linear symmetric structure are chosen as the building blocks, and variousvolume ratios of CH2Cl2and CH3OH were selected as solvent environment. SEMimages show that （DODA）2[Mo6O19] self-assemblied into straight strips in singleCH2Cl2solvent. When methanol was added （the volume ratio of CH2Cl2and CH3OH is30/1）, the helical assemblies formed. We think that the dielectric constant is increasedupon adding methanol, and the Culomb interactions between cation and anion woulddecrease. It is expected that the electrostatic repulsion between polyanions woulddecrease at the meanwhile, which will reduce the distance between polyanions. Thesymmetric breaking derived from the volume mismatch lead to the formation of helicalassembly. When further changing the volume ratio of CH2Cl2and CH3OH, differentmorphology, such as peonylike and ice ball architectures have been constructed. Themorphology evolution is driven by the alternation of the electrostatic interactionsbetween organic cations and inorganic anions and hydrophobic interactions betweenalkyl chains.In conclusion, the amphiphiles containing carboxylic acid groups and organicallymodified POM complex are chosen as the building blocks, novel lamellar structure andhelical assembly have been constructed by changing the lateral alkyl chains and solventenvironment, and the dynamically reversible self-assemblies have been realized bycontrolling the redox of the inorganic cluster. We believe that the present results willprovide new insights for constructing novel structure and controllable assembly basedon the amphiphiles and POMs supramolecular complex, and provide new matetrials forthe functional applications of the amphiphiles and POMs. |
PDF Full Text Request