Boron Ester Dynamic Covalent Bond Molecular Gel: From Gelation Mechanism To Controlled Growth Of Metal Crystals

Over the last half a century supramolecular chemistry has grown into an important field and has brought about numerous innovations of novel materials.Physical gels derived from supramolecular interactions(van der Waals interactions,hydrogen bonds,?-? stacking,electrostatic interactions,hydrophobic interactions,host-guest interactions,dipole-dipole interactions,etc.)are sensitive to some external stimulis and the formation and destruction of the 3D network are generally reversible.Newly developed gels deriving from dynamic covalent bonds were named as dynamic covalent bonding gels(DCB gels)whose features are "dynamic" and“reversible”.Therefore,the DCB gels integrate the stability of chemical gels and the stimuli-responsiveness of supramolecular gels into one system offering an appealing prospect for functional soft materials in sensors,tissue engineering,regenerative medicine,electronic devices and artificial skin,etc.In case of design of molecular gels,urea groups can be employed as potential and efficient low-molecular-mass gelators(LMMGs)because they provide ?-tape motifs of intermolecular hydrogen-bonding interactions affording physical gelation in organic solvents or water.The resulting gels exhibit tunable rheology by balancing interactions between urea-urea and urea-anion,and they are considered as a potential media to grow crystalline nanostructures,pharmaceutical and organic semiconductor.As reported,urea derivatives with tetrahydroxy fragments were proved to be poor gelators and gelation concentration was ony within a small range.Therefore,this dissertation has developed a strategy for triggering supramolecular gelation in solvent by combining dynamic boroester bonds deriving from tetrahydroxy and borate and a-type urea-urea hydrogen bonds.Based on literature study on DCB gels and urea-based molecular gels as well as our works in the fields of molecular gels,this dissertation mainly includes the following three parts:In the first section,a series of novel bisureas bearing tetrahydroxy fragments(compounds a-e)have been designed and synthesized.Their gelation abilities were explored in common pure solvents.Experimentally,almost all these compounds could not form gels in water or organic solvents under heating and cooling treatments or sonication,exhibiting precipitate,insoluble or solution.And the compounds(a-e)also could not form gels even at a higher gelaror concentration in mixed solvent of DMSO and water.However,the introduction of the dynamic boroester bond could trigger the physical gelation in DMSO and water.Interestingly,the b/NaB(OH)4 gels exhibited clear thixotropic properties and the corresponding sol-gel transitions could be repeated for at least 10 cycles without a clear decay.Mechanical strength of the gel was dependent strongly on the concentrations of compound b and the volume ratio of DMSO and water.Morphologies of boric salt-induced gels were different from pristine aggregates of compound b.Dynamic natures of the boronate ester bonds gave rise to gels exhibiting the capability to respond to pH.In addition,1H NMR,FT-IR and rheology measurements have confirmed formation of boronic ester bonds,hydrogen bonding and the dipole-dipole interactions between the ionic groups in the linear polymeric gelators during dynamic covalent triggered gelation.The strategy derived from urea derivatives and boronic ester bonds to trigger physical gelation has not been reported yet.Considering the obtained DCB gels with higher stability and sol-gel temperature,the b/NaB(OH)4 gel was chosen as the preferred matrix for growing Au NPs by in situ reduction of HAuCl4 using N2H4-H2O as the reducing agent.In this case,the addition of HAuCl4 and N2a4·H2O couldn't destroy the networks of the molecular gels.The effects of HAuCl4 and gelator concentrations on growth of Au NPs have been explored.The resulting Au NPs have good stability with about 20-30 nm size distribution.Subsequently,the colorimetric and selective detection of environmentally toxic Hg2+ ions was developed using the Au-gel composites and the DCB-basea gelator was indispensable in this case.The design of this multifunctional gel has provided a new idea for the development of soft materials toward high performance,functional and structural.On the basis of the second work,this dissertation further explored the potential application of the DCB gels.Here,we first demonstrated a simple apporach for crystallization of Cu nanocrystals with controllable shapes and dimension in the DCB gels using CuCl2 as a precursor and hydrazine hydrate as a capping agent.We investigated the effects of copper ion,gelator,temperature,reducing agent concentration and diffusion methods on the growth of copper nanostructures.Interestingly,the dimensional control of the Cu nanocrystals from NPs(0D)to NWs(1D)to NPLs(2D)was tuned by a combination of the reducing agent concentrations and diffusion methods.Copper nanostructures were characterized by TEM,XPS,XRD and other techniques.The results exhibited only the formation of a metallic copper phase in all cases.At present,the growth mechanism of copper nanocrystals triggered by gel matrix and the application of electrochemical reduction of CO2 are under exploration.This method is expected to be novel approach for preparing nanomaterials.