Modulation Of Supramolecular Nanostructures Formed From Oligeother Dendron Grafted Bent-shape Aromatic Segments

Supramolecular chemistry has been defined by one of its leading proponents,Jean-Marie Lehn,who won the Nobel Prize for his work in the area in 1987,as the ‘chemistry of molecular assemblies and of the intermolecular bond'.Supramolecular self-assemble is based on intermolecular interactions,such as host-guest interactions,hydrogen bonding,hydrophobic-hydrophobic interaction,?-? interaction and electrostatic interaction.Much recent work has focused on the development of self-assembling synthetic pathways towards large molecules or molecular arrays.These systems often self-assemble using a variety of interactions,some of which are clearly non-covalent(e.g.hydrogen bonds)and some of which possess a significant covalent component(e.g.metal–ligand interactions).During the past decade,supramolecular nanostructures produced via self-assembly processes have received considerable attention because these structures can lead to dynamic materials.Among these diverse self-assembly systems,the aqueous assemblies that result from the sophisticated design of molecular building blocks offer many potential applications for producing biocompatible materials that can be used for tissue regeneration,drug delivery,and ion channel regulation.Along this line,researchers have synthesized self-assembling molecules based on ethylene oxide chains and peptide building blocks to exploit water-soluble supramolecular structures.Another important issue in the development of systems that self-assemble is the introduction of stimuli-responsive functions into the nanostructures.Recently,major efforts have been undertaken to develop responsive nanostructures that respond to applied stimuli and dynamically undergo defined changes,thereby producing switchable properties.As a result,the introduction of stimuli-responsive functions into aqueous self-assembly provides an attractive approach for the creation of novel nanomaterials that are capable of responding to environmental changes.In Professor Lee's group,they mostly focus on developing responsive nanostructures via the self-assembly of small block molecules based on rigid-flexible building blocks in aqueous solution.Because the rigid-flexible molecules self-assemble into nanoscale aggregates through subtle anisometric interactions,the small variations in local environments trigger rapid transformation of the equilibrium features.For example,the aqueous nanofibers formed through the self-assembly of the rod amphiphiles respond to external triggers by changing their shape into nanostructures such as hollow capsules,planar sheets,helical coils,and 3D networks.When an external trigger is applied,supramolecular rings laterally associate and merge to form 2D networks and porous capsules with gated lateral pores.We expect that the combination of self-assembly principles and responsive properties will lead to a new class of responsive nanomaterials with many applications.Aqueous assembly of amphiphilic molecules has great advantages to the creation of desired materials in terms of biological applications and environmentally friendly processability.Examples of molecular building blocks for aqueous assembly include block copolymers,surfactants,peptide derivatives,and lipid molecules.Depending on the external environments,molecular structures and shapes,and relative volume fraction of hydrophilic and hydrophobic parts,these molecules self-assemble into diverse supramolecular architectures,such as spherical or cylindrical micelles,vesicles,ribbons,and tubules.Besides the formation of interesting structures at nanoscale dimensions,the molecular assembly of the amphiphilic molecules through weak noncovalent interactions including hydrogen bonding,electrostatic interaction,and hydrophobic effect is ideally suitable for the construction of the responsive materials since the dynamic and reversible conformational changes can be triggered by external environments.Normally,this change is fully reversible once the stimulus has been removed.Numerous possible applications in the fields of environmental sciences,biomedical sciences,and nanodevices have been described for stimulus-responsive.A number of dynamic supramolecular systems have been developed and can be changed in their topologies and properties upon exposure to external triggers such as temperature,light,p H,and redox potential.For example,microgel particles based on thermal responsive poly(N-isopropylacrylamide)(PNIPAm)exhibit a conformational transition depending on temperature variation,which has been widely explored for a variety of material applications including drug delivery and sol-gel interconversion.The photoresponsive azobenzene derivatives have been employed for another strategy to construct responsive self-assemblies.The azobenzene-functionalized amphiphiles form helical assemblies with photo tunable helical pitches and chiroptical properties by conformational change between trans-azobenzene and its cis-isomer.The applied magnetic field can also trigger the deformation of supramolecular assemblies.The capsule-like structures formed from bola amphiphilic oligothiophenes deform into oblate spheroids resulting from the orientation of anisotropic thiophene segments along the applied magnetic field.Among a variety of self-assembling building blocks,rod amphiphiles,consisting of rigid rod and flexible coil segments,are excellent candidates for creating well-defined supramolecular structures in selective solvents for flexible side chains.For the aqueous self-assembly of rigid-flexible block molecules,the amphiphilic combination of hydrophilic coil and hydrophobic rod segments leads to the formation of a well-defined nanostructure with rigid hydrophobic core surrounded by flexible hydrophilic chains.In contrast to coil-coil systems,the rod-coils can form well-ordered structures even at very low molecular weights of each block because a stiff rod-like conformation of the rod segments imparts orientational organization.The packing arrangements of these small aniso-tropic rod segments are able to rapidly transform into their equilibrium states when faced with very small environmental changes,which is an essential prerequisite for construction of responsive nanostructures.Supramolecular molecular self-assemble is a very useful tool to create new organic materials and devices.Most importantly,we can construct various kinds of supramolecular nanostructures such as tubular structures,toroid structures,sheet structures,network structures.The construction of supramolecular nanoscopic architectures has been intensively pursued because of their unique features for applications in nanoscience and biomimetic chemistry.Molecular self-assemblies of aromatic rigid-flexible amphiphiles consisting of rigid segments and hydrophilic flexible chains in aqueous solution provide a facile avenue into this area.We are also able to control the shape and sizes of these nanostructures.Molecular self-assemble is depending on the connection of designing appropriate building blocks,then self-assemble materials form well-ordered nanostructures.The development of supramolecular self-assembly has been mostly focused on conjugated aromatic molecules,in our group,we have initiated our research work in the field of supramolecular chemistry to control self-assembled nanostructures by introducing rod-coil self-assembling concept.We have demonstrated that incorporation of flexible coils into a rod blocks leads to controllable self-assembly of molecular rods in aqueous solution depending on their molecular architecture.Based on this,we have also extended our self-assembling concept to construct switchable dynamic nanostructures in aqueous solution in response to external forces.We has employed rigid-flexible block systems consisting of oligo(para-phenylene)rod units as a rigid segment and flexible chains such as poly(propylene oxide)(PPO),poly(ethylene oxide)(PEO),or alkyl chains for the construction of various self-assembled structures in bulk.As an extension of the bulk-state supramolecular structures,we have pioneered diverse aqueous assemblies such as tubules,toroids,porous capsules,and helical fibers.These nanostructures are able to respond to applied stimuli such as certain guests,p H,solvent,and temperature by changing their shape or macroscopic properties.We have focused on two features in these aqueous self-assembling systems to construct stimuli-responsive nanostructures.The first feature is that the rigid aromatic rods have a strong tendency to arrange with an anisotropic orientation.Small environmental changes are expected to significantly affect the details of aromatic rod packing and thus the structural nature of supramolecular assemblies.The second feature is that ethylene oxide segments are well-known for a lower critical solution temperature(LCST)property,exhibiting a hydration-dehydration transition in response to relatively small changes in temperature.Combination of these two features in one system allows the self-assembled nanostructures to exhibit stimuli-responsive properties.This thesis mainly contains 3 parts,including:1)Guest-Driven Inflation of Self-Assembled Nanofibers through Hollow Channel Formation:In the first part we have construct bent-shaped aromatic building blocks containing an oligoether dendron at its apex which assemble to smart nanostructures with switchable pores.The highlight of self-assembly is the reversibility of various types of noncovalent interactions which leads to construct smart nanostructures with switchable pores.Here,we report the spontaneous formation of inflatable nanofibers through the formation of hollow internal channels triggered by guest encapsulation.The molecules that form this unique nanofibers consist of a bent-shaped aromatic segment connected by a m-pyridine unit and a hydrophilic dendron at its apex.The aromatic segments self-assemble into paired dimers which stack on top of one another to form thin nanofibers with pyridine-functionalized aromatic cores.Notably,the nanofibers reversibly inflate into helical tubules through the formation of hollow cavities triggered by p-phenylphenol,a hydrogen-bonding guest.The reversible inflation of the nanofibers arises from the packing rearrangements in the aromatic cores from transoid dimers to cisoid macrocycles driven by the reversible hydrogen-bonding interactions between the pyridine units of the aromatic cores and the p-phenylphenol guest molecules.2)Solvent-Switchable Breathing Motion of Self-Assembled TubularOne of the great challenges in molecular self-assembly is how to construct dynamic supramolecular nanostructures in response to external triggers.Nanostructures can display significantly different properties depending on their physicochemical parameters such as size,morphology,and stability.Supramolecular nanostructures based on self-assembly processes have received considerable attention because these structures can lead to dynamic nanomaterials.Among the diverse self-assembled nanostructures,nanofibers and tubulars are interesting nanostructures with the potential for many applications such as channel formation because of their unique symmetrical and annular shape.Notably,a further stacking of the nanofibers can produce well-defined one dimensional nanotubules with responsive properties triggered by external stimuli such as temperature,solvent,and guest molecules.In this part,we have designed and synthesised bent-shape macrocycle molecule based on anthracene unit.The self-assembled nanostructures of this macrocycle molecule were investigated by transmission electron microscopy(TEM)in different solvent condition.In the pure Me OH solution,we find it is nanofiber structure.However,if we addition of H2 O to this Me OH solution,fluorescence red-shift and quench,this represent the hydrophobic interaction more strong in mixture solution compare to pure Me OH solution,and nanofibers structure transform to nanotubulars structure.The nanotubules based on the stacking of fibers nanostructures show responsive properties triggered by external stimuli such as solvent polarity.Although only a few studies related to fibers and nanotubules have been reported,it is obvious that more intensive research efforts will be achieved because of the fascinating functionalities and dynamic properties.Therefore,the development of dynamic nanotubules through self-assembly will be widely applicable to advanced research areas ranging from material chemistry and nanotechnology all the way up to biological science.3)Static and Dynamic Nanosheets from Selective Assembly of Geometric Macrocycle Isomers:We have designed and synthesised amphiphilic macrocycle isomers.The cis and trans isomers were synthesized and characterized by NMR spectroscopy and MALDI-TOF mass spectroscopy.Both trans and cis isomers self-assemble into 2D sheet structures through lateral association of primary nanofibers in aqueous solution.In contrast to the significant advances that have been made in the construction of two-dimensional(2D)nanostructures,the rational modification from static to dynamic 2D sheets remains a great challenge.Static and dynamic sheets formed from selective self-assembly of geometric macrocycle isomers based on anthracene units are presented.The self-assembly of the cis isomer generates static planar sheets,whereas the trans isomer forms dynamic rolled sheets which are reversibly unrolled upon stimulation by a thermal signal.Furthermore,the mixed solution of the two isomers exhibits self-sorting behavior,generating the coexistence of the two independent self-assembled structures,the planar sheets and the folded scrolls.The self-sorted supramolecular objects with considerable shape and size differences are able to be readily separated,one isomer from the other.