Syntheses,Small Molecules Adsorption And Recognition Properties Of Porous Organic Polymers And Hydrogen-bonded Organic Frameworks Based On Carbazole Derivatives

Porous organic materials（POMs）have important applications in the fields of industry,energy,environment and biosafety.Over the past few decades,several different POMs have been studied with advances in chemistry and materials science,including the widely studied metal-organic frameworks（MOFs）,porous organic polymers（POPs）,and the emerging hydrogen-bonded organic frameworks（HOFs）and boron←nitrogen frameworks（BNFs）.Among them,POPs and HOFs formed from small organic molecules by covalent and hydrogen bonds are two important branches of POMs,which have developed rapidly in recent years,and have attracted extensive attention of scientists and become a research hotspot in the field of materials and chemistry.POPs and HOFs have become two important platforms for the design and development of multifunctional porous materials,due to the advantages of no metal ions,low skeleton density,simple synthesis,permanent porosity,high specific surface area,modular design,and easy functional modification.Most organic molecules used to build POPs and HOFs materials are aromatic,and it is easy to integrate organic molecules with specific functions into POPs and HOFs materials at the atomic level,thus building porous materials with target functions such as adsorption,catalysis,and optical sensing.In addition,carbazole and its derivatives have huge advantages in the construction and design of multifunctional porous materials due to their rigid plane,largeπconjugation system,rich N sites,multiple reactive active sites,easy functionalization and good optical properties.In this paper,we selected carbazole derivatives as organic precursors to construct a series of microporous polycarbazole frameworks and hydrogen-bonded organic frameworks and explored their potential applications in gas/vapor capture and separation and small molecule recognition.Firstly,carbazole derivatives with largeπconjugation and rigid plane were introduced into the polymer framework,and two microporous polycarbazole frameworks FJU-P6 and FJU-P7 with continuous largeπconjugation systems were successfully constructed.Due to the different saturations of C6 molecules such as benzene（C₆H₆）,cyclohexene(C₆H₁₀)and cyclohexane(C₆H₁₂),the binding energy between them and the microporous polycarbazole framework is different,which finally leads to the obvious differences in the adsorption of C6 vapor by these two polymers.In addition,IAST calculation results indicated that FJU-P6 and FJU-P7 could be promising candidates for separating C₆H₆/C₆H1₂ and C₆H₁₀/C₆H₁₂.This study not only provides promising candidate materials for the low energy separation of C₆H₆/C₆H₁₂ and C₆H₁₀/C₆H₁₂ in industry,but also provides a strategy for the challenging separation of C6 mixtures based on POPs separation.To overcome the trade-off between adsorption capacity and selectivity of porous adsorbents,we improved CO₂ capture capacity and CO₂/N₂ separation selectivity by increasing the number of external linkers to increase the microporosity of the microporous polycarbazole frameworks.This is mainly due to the fact that the three-connected linker is easier to pack carbazole monomers tightly than the two-connected linker,thus improving the porosity of the three-connected polymer.This strategy is simple to synthesize,does not require the involvement of noble metals,broadens the selectivity of organic monomers,provides a new method for the preparation of microporous adsorbents,and overcomes the serious challenges faced by porous adsorbents in achieving high CO₂ adsorption capacity and selectivity,the so-called trade-off.The building unit 4,4’,4’’,4’’’-（9H-carbazol-1,3,6,8-tetrayl）tetrabenzaldehyde（CTBA）molecule of HOF-FJU-2 was designed,and the tetrabenzaldehyde on the molecule provides hydrogen bonding,and the Lewis acid imine（N-H）site on the carbazole is used for the specific recognition of small molecules.HOF-FJU-2 can exhibit a reversible structural transition between open and closed pore phases.More importantly,the functional carbazole N–H sites on the pore surface showed a preferential recognition performance for acetone over methanol,so HOF-FJU-2 can easily separate acetone from acetone/methanol mixture through column separation.Owing to the large aromatic nature of the donor（D）-π-acceptor（A）organic building blocks,this novel HOF material exhibits solvent-dependent color-changing characteristics,which makes the acetone/methanol column separation visible.To the best of our knowledge,this is the first reported organic porous adsorbent with visual column separation performance.The molecular recognition performance of HOF-FJU-2 was studied using HOF-FJU-2 with available functional sites and its luminescent properties of carbazole molecules.The non-porous and yellow HOF-FJU-2a can be quickly transformed back to the porous and white HOF-FJU-2·Acetone by direct acetone fumigation or immersion,and with the fluorescence turned off/on and the color change observed,it exhibits the dual-mode behavior of fluorescence change and color change visible to the naked eye.These unique phenomena are mainly attributed to the reversible switching of D-Aπ-πinteractions and the rearrangement of weak C-H···O hydrogen bonds caused by hydrogen bond competition,leading to thermal vibration limitation/relaxation of the luminescent groups and the reduction of non-radiative transitions.In addition,HOF-FJU-2a can also specifically recognize organic vapors with Lewis base sites through color change and different discoloration rates.In summary,carbazole derivatives were used as organic precursors to construct a series of novel POPs and HOFs materials in this paper.According to the structural characteristics of these materials,C6 vapor capture,CO₂/N₂ separation,acetone/methanol separation and small molecule recognition were studied in depth.