Design,Synthesis,and Photoresponsive Properties Investigation Of Porous Organic Cages

Porous organic molecular materials are a kind of porous materials.They have discrete molecular structures and could assemble into solid porous structures by intermolecular supramolecular interactions.Porous organic cages,as one of them,are also a kind of discrete molecules.Due to three-dimensional molecular structure,permanent cavity and windows,open channel assembled by adjacent cage molecules,considerable specific surface area,post-synthetic modification,solubility in many solvents,recycling and reprocessing,these features endow them with many applications in absorption and separation,nanoparticle carrier,sensor,energy storage,and so on.They have drawn more and more attention.In order to increase the diversity and function of porous organic cages,the basic building units with photoresponsive property were incorporated into porous organic cages applied to the different fields,such as selective fluorescence response,heterogeneous catalysis,and selective gas adsorption,which are of great significance to the research of porous organic cages.Specific research contents are as follows:(1)The well-defined molecular cavity endows porous organic cages as excellent research objects to selectively accommodate various guests and ultrafine metal nanoparticles exhibiting excellent catalytic capability for various organic reactions.Thus far,utilizing the synergistic effect of ultrafine metal nanoparticles and porous organic cages to boost organic reactions via two different catalytically active sites has rarely been explored,due to the lack of functionality for most cages.It is therefore of significant importance to develop new functional porous organic cages towards prominent cage-based synergistic/sequential catalysts.In this work,the imine condensation reaction of 5,5'-(benzo[c][1,2,5]thiadiazole-4,7-diyl)diisophthalaldehyde with cyclohexanediamine resulted in a tubular porous organic cage(MTC1).Various spectroscopics and single crystal X-ray diffraction investigation revealed the structure of the compound.MTC1 exhibits selective fluorescence sensing towards divalent palladium ions(Pd2+)with a very low detection limit of 38 ppb.X-ray photoelectron spectroscopy(XPS)also suggested the effective complexation between these two species.The subsequent reduction of MTC1 and palladium acetate with NaBH4 afforded a cage-supported catalyst with highly dispersed ultrafine palladium nanoparticles(Pd NPs)in a broad size of 1.9-3.1 nm.It is worth noting that increasing in the mole ratio of Pd2+and MTC1 gradually enlarges the size of Pd NPs.The mole number of Pd2+and MTC1 is 1:5,affording a cage-supported catalyst with highly dispersed ultrafine palladium nanoparticles in a narrow size of 1.9±0.4 nm,denoted as Pd@MTC 1-1/5.The catalytic results showed that MTC1 could promote the evolution of superoxide radical anion under light irradiation and visible light-induced aerobic hydroxylation reaction between phenylboronic acid derivatives and superoxide radical anion producing phenol derivatives.In the presence of sodium borohydride,ultrafine palladium nanoparticles could catalyze hydride reduction reaction of 4-nitrophenol to 4-aminophenol.Such ultrafine Pd NPs in Pd@MTC1-1/5,in cooperation with photocatalytically active MTC1,enables efficient sequential reactions involving visible light-induced aerobic hydroxylation of 4-nitrophenylboronic acid to 4-nitrophenol and the following hydride reduction reaction of 4-nitrophenol to 4-aminophenol.With the increase of the size of nanoparticles,the catalytic performance of the catalyst was obviously different with obtaining 4,4'-dinitro-1,1'-biphenyl as by-product,indicating that there was a synergistic catalytic effect between Pd NPs and organic cage.The present result represents the first example of a multifunctional organic cage being capable of sensing,directing nanoparticle growth,and thus synergistically catalyzing sequential reactions.(2)Special porous nature of porous organic cages has been revealed to be capable of overcoming the issue of the aggregation-induced deactivation of homogeneous photocatalysts in heterogeneous catalysis.Polycyclic aromatic hydrocarbons(PAHs)with high redox potentials could drive many photoredox reactions.In this work,a novel porous pyrene-based organic cage(PyTC1)has been constructed by the imine condensation reaction of cyclohexanediamine with 5,5'-(pyrene-1,6-diyl)diisophthalaldehyde.Various spectroscopics and single crystal X-ray diffraction investigation revealed the structure of the compound.Single crystal X-ray diffraction analysis revealed the effective intercage C-H…?interaction between cyclohexanediimine segments and pyrene segments.Such a soft intercage C-H…? interaction,rather than a classic J-aggregation with slipped?-?-stacking configuration,induces a counterintuitive bathochromic-shift of the pyrene-based chromophore absorption from ultraviolet region for PyTC1 in solution to visible light range for the solid sample.In addition,the effective intermolecular C-H…? interaction was also found in the single crystal structures of a reference for one third structure of the organic cage(PyTM)and pyrene.The bathochromic-shift of the electron absorption spectra of PyTM and pyrene was also observed for solid materials.Various experimental evidences combined with theoretical simulation revealed weak intercage C-H…? interaction could induce absorption bathochromic-shift for solid materials.This study is different from the previous conclusion that bathochromic-shift phenomenon of electron absorption spectra for solid materials was only attributed to J-aggregation.The electron absorption spectrum of PyTC1 solid material in the visible region enables the heterogeneous visible light-induced aerobic hydroxylation of benzeneboronic acid derivatives.(3)Post-synthetic modification of porous organic cages through diverse chemical techniques has been extensively employed for the purpose of increasing the function-added value.However,post-synthetic modification over porous organic cages by external stimuli has not yet been reported,to the best of our knowledge.In this work,a photoresponsive porous organic cage(PPOC-1)has been constructed by the imine condensation reaction of cyclohexanediamine with photochromic dithienylethene derivative.Various spectroscopics and single crystal X-ray diffraction investigation revealed the structure of the compound.PPOC-1 could assemble into supramolecular structure depending on the rare multiple C-F…H-C hydrogen bonds and van der Waals forces between the neighbour cage racemates according to single crystal X-ray diffraction analysis.Electronic absorption measurement revealed the reversible photochromic behaviors of both the solution and film for PPOC-1 upon UV and visible light irradiation due to the photoresponsive property of dithienylethene units.Upon UV light irradiation,nitrogen gas sorption experiment revealed a reduced BET surface area of crystalline racemic PPOC-1 from 495.0 m2 g-1 to 379.0 m2 g-1.Interestingly,heating the irradiated sample of racemic PPOC-1 dispersed in 1,4-dioxane led to the recovered porosity according to gas sorption data due to the recovered cage structure confirmed by NMR spectroscopy.X-ray photoelectron spectroscopy(XPS),in combination NMR and theoretical simulation data,disclosed the quantitative photoisomerization of photochromic dithienylethene moieties in PPOC-1 racemate.The present results establish a non-invasive and precise post-synthetic modification method for porous organic cages by introduction of molecular photoswitches and external stimuli.(4)Porous organic cages have been widely used in selective gas adsorption due to their inherent cavities and intermolecular assembled pore structures.In view of the issues for porous organic cages in improving gas adsorption capacity and selective adsorption,in this work,two kinds of tubular porous organic cages(TPTC1 and NC1)have been constructed by imine condensation reaction between cyclohexanediamine and biphenyltetraaldehyde precursors with different chain lengths,respectively.Various spectroscopics and single crystal X-ray diffraction investigation revealed the structure of these two compounds.Single crystal X-ray diffraction analysis revealed that these two racemic organic cages had the same number and type of N and O atoms,but their pores had the different sizes.According to the gas adsorption experiments and the Ideal Adsorbed Solution Theory(IAST),both TPTC1 and NC1 have enough active sites to selectively adsorb methane,acetylene,and carbon dioxide gas mixtures.Moreover,compared to TPTC1,NC1 with shorter length of biphenyl chain has a smaller pore size,larger specific surface area and total pore volume,and shows much higher gas adsorption capacity and adsorption selection,as well as lower isosteric enthalpy of adsorption.