Nucleic Acid Bases Modified Polyoxometalates: Multiple Hydrogen-bonded Self-assembled Supramolecular Polymer

polyoxometalates（POMs）are a kind of inorganic polyanion clusters with various structures.Due to its accurate chemical composition,surface covalent modification,counter cations regulation and rich functions,POMs are frequently used as building blocks to construct ordered assemblies.Meanwhile,the specific structures and properties of POMs can be used for catalysis,optoelectronics and biomedical materials.While during the functionalization process of POMs,researchers found that POMs exist self-aggregation phenomenon in solution.At the same time,due to the inherent structural rigidity of POMs,it is very difficult for POMs to be processed and existing as solid materials.Introducing the POMs with rigid structure into repeating units of supramolecular polymer can enrich the diversity of assembly structure,the specific properties of inorganic components can be cooperated into the final assemblies,which provide new routes for the construction of ordered assemblies and functionalization of final hybrid supramolecular polymers.In the published researches of POMs-containing supramolecular polymers,its driving forces mainly focus on coordination bond,electrostatic interaction and host-guest interaction,but all these driving forces either have weak binding capacity or have poor directivity,stable and ordered assembly structures can’t be directly constructed driven by these interactions.Hence,a kind of driving force with strong binding energy and specific directivity is required for constructing POMs-containing supramolecular polymer,while multiple hydrogen bonds can meet the requirements,which is derived from the maintenance of intrinsic directivity and compensation of relatively low bond energy for individual hydrogen bond.Based on the above-mentioned problems,POMs modified by nucleic acids with covalent bonds were used as building blocks in this dissertation,a series of POMs based supramolecular polymers were constructed driven by multiple hydrogen bonds.Firstly,the guanine covalently-modified Anderson–Evans type POMs（GMn Mo₆）were synthesized as building blocks,then POMs hydrogen-bonded framework（PHF）was constructed with quadruple hydrogen bonds as main driving forces.After that,the performance of framework material in selective adsorption of organic dyes and controllable catalytic performance of methionine were investigated.When the sodium ion with low steric hindrance is utilized as the counter cations of GMn Mo₆,it can form single-layered hydrogen-bonded framework in two-dimensional plane by the formation of G-quartet structure in aqueous solution.By further prolongating the assembly time through anti-ion bonding and hydrophilic/hydrophobic effects,single-layered framework can propagate in perpendicular direction to obtain a three-dimensional PHF with uniform pore size.The resulted PHF has permanent channels after activation in high temperature,and the pore size is about 0.83 nm.Induced by the negative characteristics of inorganic clusters,the dual selective adsorption of ionic dye molecules based on sizes and charges in framework is realized,the adsorption efficiency of methylene blue with smaller volume than pore size can reach 99.7%,while the adsorption efficiency of rhodamine B with larger molecular volume than pore size can only reach 37.7%.Interestingly,when the charge of 4,4’-diaminoazobenzene is regulated from neutral state to protonated state,its adsorption efficiency can be changed from 3.8%to 98.4%.Inspired by the above-mentioned phenomenon and the surface charge of methionine can be regulated by the p H of aqueous solution.When the surface charge of methionine was adjusted to negative charge,neutral charge and positive charge,using PHF as catalyst and H₂O₂ as oxidant,the conversion rate of methionine was 17.3%,41.5%and 99.8%after 2 h,demonstrating the potential capability of PHF as catalyst to controllably oxidize substrates.We herein provide a new strategy to construct hybrid supramolecular polymers using organic-inorganic compounds as repeating units,and further using the specific characteristics of inorganic components to enrich the functional properties of supramolecular polymers.Secondly,on the basis of above work,the building blocks with specific hydrogen-bonded recognition mode is simplified,ureidopyrimidone（UPy）modified Anderson–Evans type POMs（UPy-Mn Mo₆）were synthesized as building blocks,the hydrogen-bonded interaction patterns of building blocks are calculated with modulation of cations’species.The rigid structure of inorganic cluster not only restricts the folding of Upy units on both sides,but also inhibitsπ-πstacking of UPy dimers due to its large molecular volume.Furthermore,by taking advantages of POMs with controllable counter cations,tetrabutylammonium or tetraethylammonium were selected as counter cations of UPy-Mn Mo₆,and 1D crystalline hybrid supramolecular polymers based on quadruple hydrogen bonds were constructed.After that,when tetramethylammonium was used as the counter cations of UPy-Mn Mo₆,1D crystalline hybrid supramolecular polymer based onπ-πinteraction of pyrimidine ring was constructed.The size of cations was further reduced to calculate the interaction modes of UPy-Mn Mo₆.When trimethylamine and dimethylamine were utilized as counterions of Upy-Mn Mo₆,the discrete structure among adjacent UPy units were observed.and the three interaction modes of Upy in crystals were further verified by infrared spectroscopy.When sodium ion is used as the counter cation of Upy-Mn Mo₆,the existence of fibrous structure could be observed after self-assembly in aqueous solution after 3 h,and the infrared spectra results of assemblies also indicated the formation of quadruple hydrogen-bonded structure.In this work,three interaction modes of UPy are reported for the first time.The results of infrared spectroscopy results also provide a reference for the interaction model of UPy units in amorphous states.Thirdly,based on the previous work,the inorganic clusters of UPy-Mn Mo₆ are electrostatically encapsulated by DODA·Br to improve its compatibility and processability,then the resulting complex is used as the rigid building blocks for hybrid supramolecular polymer.The UPy double-modified polyether ammonia（PEU-400）is used as the flexible construction units of hybrid supramolecular polymer.Through the ratio regulation between rigid and flexible building blocks,a series of supramolecular adhesives with good processing performance and regulable rigidity were obtained.The results of transmission electron microscopy and powder X-ray diffraction confirmed that DODA-UPy-Mn Mo₆ maintained the initial packing mode during complexation process,existing as fibrous aggregates in hybrid supramolecular polymers,rigid and flexible building blocks are cross-linked by the exposed UPy units driven by quadruple hydrogen bonds.Differential scanning calorimetry（DSC）confirmed that the introduction of DODA-UPy-Mn Mo₆ could significantly increase the glass transition temperature（Tg）of PEU-400,which is resulted from the limitation of chains movement of PEU-400,and Tg increased with the augment of DODA-UPy-Mn Mo₆ contents.With glass as substrates,the shear strength of adhesive increases with the content increase of DODA-UPy-Mn Mo₆,the shear strength can reach 5.6 Mpa when the content of DODA-UPy-Mn Mo₆ is 50%.and no obvious changes of shear strength was observed for fractured adhesives after reprocessing.Contrast experiments demonstrate the cross-linking of alkyl chains between DODA⁺and the hydrogen-bonded interactions between exposed UPys of two construction units play the dominant roles in bonding properties of adhesives.This work provides a new method for developing POMs based adhesives.Based on the results of this researches,different nucleic acid bases derivatives were connected to the surface of Anderson-Evans POMs by covalent modification,further utilizing it as building blocks,ordered and stable POMs based hybrid supramolecular polymers were successfully constructed driven by multiple hydrogen bonds.Induced by the rigid and functional properties of POMs,hybrid supramolecular polymers with stable structures exhibit selective adsorption and catalytic properties.By adjusting the types of cations on the surface of POMs,the hydrogen-bonded interaction patterns in crystals and crosslinking degree of hybrid supramolecular polymers can be easily adjusted,further influencing their materials performance.The research results of this paper provide new examples for the expansion of supramolecular polymers,also present a new strategy and method for the development of POMs based hybrid supramolecular polymers.