The Role Of Aromatic Conjugation On The Charge-Transfer Interactions And Hydrogel Forming Abilities Of Bipyridinium-Based Polyhydrazones

In 1987,French chemist J.M.Lehn won the Nobel Chemistry Award,for his contribution on supramolecular chemistry.The concept of supramolecular chemistry has been defined as “chemistry beyond the molecule”,“chemistry of molecular assemblies and intermolecular bond”.The research of supramolecular chemistry is based on non-covalent intermolecular interaction of so called “host” and “guest”.Supramolecular system is built through self-assembly of host and guest molecules under a variety of interactions including Van der Waals forces,hydrogen bond,ion-ion,dipole-dipole,π-π stacking,hydrophobic forces,and donor-acceptor(DA)charge transfer interactions.Donald J.Cram,Charles John Pedersen,and Jean-Marie Lehn established this field and shared Nobel Prize in 1987.In 2016,supramolecular chemistry won Nobel Prize again,Jean Pierre Sauvage from University of Strasbourg,France,Sir James Fraser Stoddart form Northwestern University,USA,and Bernard L.Feringa from University of Groningen,Netherland shared Nobel Prize in chemistry for the design and synthesis of molecular machine.Now,supramolecular chemistry has been used in many fields including organic chemistry,inorganic chemistry,material chemistry and biological chemistry,and it shows bright future and becoming one of the most evolutionary field.The research of supramolecular chemistry focuses on intermolecular non-covalent interactions,and dynamic control on molecular level by using Van der Waals forces,hydrogen bond,ion-ion,dipole-dipole,π-π stacking,hydrophobic forces,and donoracceptor(DA)charge transfer interactions.Supramolecular chemistry can date back on 1960 s,since that,chemists have been constantly creating and developing unique functional molecules and interactions in this field.Molecular machine,molecular recognition,supramolecular self-assembly,dynamic covalent chemistry,mechanical interlocked molecules are all coming out from basic concept of supramolecular chemistry.Traditional organic chemistry has been changed by molecular recognition and supramolecular self-assembly.In the research of dynamic covalent chemistry,energy gap can be reduced during organic reaction by introduction of non-covalent interactions,and new reaction approach has been established which makes chemist get more desired molecules under moderate condition.In addition,supramolecular recognition can help get given isomers during reactions so that increase stereoselectivity of organic reactions and offer new thinking of asymmetric catalyst.Upon a variety of non-covalent interactions,π-π stacking is one of special electron arrangement which related to two or more aromatic conjugated structure.It exists between π-electron rich and π-electron deficient molecules,and dates back on attraction of different electron density in aromatic system.It usually has two ways: Face-to-face which means two aromatic rings are parallel to each other,and edge-to-face which means two aromatic rings are vertical.π-π stacking is common in nature and wildly used non-covalent interactions when scientists build various of organic materials.Plenty of next generation materials including self-assembly chiral crystal and high efficiency florescence material can be designed and synthesized by using π-π stacking interactions.Aromatic conjugation is a common structure in organic molecules.It comes from a big π bond formed by alternated single bond and double bond.Electron clouds are well-distributed so that the molecule has planar structure,results in the π-electron rich and π-electron deficient parts close to each other and has π-π stacking interactions.Aromatic conjugation is essential in supramolecular chemistry,Through the π-πstacking of aromatic conjugated structure,molecules can achieve specific functions by arranging structure.Aromatic conjugated structure can be divided into two types,π-electron rich andπ-electron deficient.π-electron rich structure includes naphthalene rings,indole rings etc.π-electron deficient part includes pyridine and bipyridine rings.In π-π stacking interactions,two parts are close to each other which arise charge-transfer(CT)interactions,and color change can be seen.Meanwhile,π-π stacking are weak and reversible so that microstructure of material can be influenced by changing different host and guest molecules.Polyhydrazones can be synthesized by polycondensation of dihydrazide and dialdehyde.When adding aromatic conjugated structure into dialdehyde,morphology of polyhydrazones is altered in water.The conjugation can help to form a hydrogel,and when the conjugated structure breaks,the hydrogel collapses.Surfactant is a kind of molecule which can reduce surface tension of liquid,which usually contains two parts,hydrophilic head and hydrophobic tail.Surfactant can arrange on the surface of solution and reduce surface tension of solution.Critical Micelle Concentrations(CMCs)is one of the most important parameters of surfactant.Micelle cannot form under CMCs,while upon CMCs,micelle can be formed by self-assembly.Surfactant with conjugated structure can change CMC by altering assembled microstructure in solution.Meanwhile,charge transfer interactions happen when different guest molecules are added into surfactant solutions so that different colors form.This research can give a better understanding of the role of aromatic conjugation on hydrogel forming ability and charge transfer interactions.Among various aromatic conjugation molecular system,bipyridinium-based structure has recently emerged as π-electron deficient group which has donor-acceptor(DA)and charge-transfer(CT)interactions with other π-electron-rich molecules.Since bipyridinium based aromatic conjugated structures have extensive applications in organic and supramolecular chemistry,it is of importance to define structure-property relationship of bipyridinium based units.Recently,our group developed a series of bisbipyridinium based gemini surfactants which are capable of forming hydrogel with π-electron rich guest,due to CT interaction between donor and acceptor.These gemini surfactants showed hydrogel forming ability in the presence of π-electron rich guest and performed different color at different temperature because of the competition of n-π* CT and π-π* CT absorption.Another work of our group is the development of bipyridinium based polyhydrazones which are capable of forming hydrogel in the absence of π-electron rich guest,and having charge transfer absorption band after addition of different guests.However,despite of numerous examples of self-assembly and hydrogel forming of bipyridinium units,defining a structure-property relationship of aromatic conjugated bipyridinium based materials on CT interactions and hydrogel forming abilities is necessary.Motivated by previous work of our group on bipyridinium based surfactant and hydrogel,we endeavored to determine structure-property relationship of bipyridinium based aromatic conjugation on CT interactions and hydrogel forming.4,4’-pyridine derivatives are wildly used to form DA complex due to the aromatic conjugation property of two pyridine rings,however,it is essential to break down conjugated structure of 4,4-pyridine to find out how they affect CT interactions and hydrogel forming abilities.Hence building on what we learned from interactions between bipyridinium units and π-electron donor,we chose two kind of bipyridine derivatives as π-electron deficient units which one of them contains C=C bond between two pyridine causing a larger aromatic conjugated system while another undergoing removing of one saturation on C=C bond thereby destroying conjugation system.Based on that,two bipyridinium surfactants were synthesized by Menshutkin reaction with one methyl group connecting with N of pyridine and another undecylenic long chain connecting as hydrophobic tail.We compared critical micelle concentrations(CMCs)in the presence and absence of π-electron rich donor,di(ethylene glycol)-disubstituted 1,5-diaminonaphthalene(DAN-TEG)of two surfactants by variable-concentration surface tension and conductivity measurements to show that conjugated S1 undergoes larger CMCs reduction after addition of donor than S2 due to aromatic conjugation which facilitates donor-acceptor interactions.Furthermore,two polyhydrazones were synthesized(P1·n2Br and P2·n2Br)with bipyridinium dialdehyde derivatives and terephthalic dihydrazide as monomer units by step-growth condensation polymerization.Vial-inverted test and oscillatory amplitude stress plots test revealed that conjugated P1·n2Br formed hydrogel in aqueous solution.In addition,the noncovalent DA interactions brought by the presence of donor,DAN-DEG,effectively augmented mechanical property and thermal stability of hydrogel P1·n2Br while P2·n2Br has less change.This confirmed aromatic conjugation can help DA interactions and hydrogel forming abilities of bipyridinium based polyhydrazones.This work gives a new way to understand aromatic conjugation on hydrogel forming and charge-transfer interactions.We believe that such work can help others to have better ideas on the development of hydrogel and other materials.