| Metal-organic frameworks（MOFs）are a class of porous crystalline materials assembled by metal ions and organic ligands.These materials possess rigidity of metal and flexibility of organic,and are widely applicable in areas such as gas storage,catalysis and sensing.In the field of optics,MOFs have the advantages of diverse emission centers and emission mechanisms as well as tunable emission properties.Combined with the features of tunable pore size,easy functionalization and excellent biocompatibility,luminescent MOFs have excellent development prospects in the fields of lighting display,bio-imaging,anti-counterfeiting and sensing,etc.Aromatic carboxylic acid organic molecules are widely used in coordination chemistry owing to their strong coordination ability and diverse coordination modes.In this regard,1,4-Benzenedicarboxylic acid（BDC）with a simple and strictly symmetric planar structure is the primary choice for the construction of MOFs with novel structures,regular pore channels and excellent properties.However,linear organic ligands are prone to vibrate and rotate in the topological network structure,resulting in MOFs constructed based on those ligands facing the problem of low photoluminescence quantum yield（PLQY）or even nonemissive.This severely limits the application of such materials,and improving the PLQY is a key scientific problem to be solved in this field.We are dedicated to finding an effective way to inhibit the rotation and vibration of organic ligands to enhance PLQY without changing the chemical components and framework structure of MOFs and simultaneously study the emission mechanism.It is of great significance to expand the application of MOFs based on para-substituted linear carboxylic acid ligands in condensed matter physics.As an independent thermodynamic parameter,pressure can effectively modulate the crystal structure and electronic structure of materials to obtain new structures and properties that are difficult to obtain under conventional conditions.Particularly,nonemissive materials could realize efficient emission under high pressure,i.e.,pressure-induced emission（PIE）,in the enhancement of optoelectronic properties.However,the high-pressure reversibility leads to the difficulty of retaining the luminescence properties of nonemissive materials obtained at high pressure to ambient conditions.Given abundant and easily regulated hydrogen bonding interactions in the structure of MOFs,the multiple hydrogen bondings constitute a hydrogen bonding network with strong intermolecular interactions,i.e.,hydrogen bonding synergies.We selecte three BDC-based MOFs materials for the study.Combining pressure treatment strategies and hydrogen bonding synergetic effects,we elevate the rotational barriers of the organic ligands to achieve efficient emission of the MOFs.Meanwhile,a high rotational barrier is retained at ambient conditions,allowing initially nonemissive or very weakly emissive materials to be highly emissive at ambient conditions.Meanwhile,the conformational relationships of hydrogen bonding,structure and optical properties under high pressure are explored by in situ high-pressure measurement techniques and theoretical calculations.It provides a new route to realize efficient emission of nonemissive or weakly emissive MOFs materials at ambient conditions.Firstly,Zn（BDC）（DMF）（H2O）（DMF=N,N-Dimethylformamide）,which is initially nonemissive,is selected for the study of optical properties under high pressure to investigate the pressure-induced emission of nonemissive MOFs materials.Below7.0 GPa,Zn（BDC）（DMF）（H2O）exhibits the pressure-induced blue light emission.After full pressure release from 16.3 GPa,Zn（BDC）（DMF）（H2O）exhibits bright blue light emission at ambient conditions.Detailed in situ high-pressure structural analysis show that the interlayer compression and square-frame deformation of Zn（BDC）（DMF）（H2O）under pressure effectively modulate the hydrogen bonding interactions in the framework,break the old hydrogen bondings and form new,stronger ones.After complete release of pressure,the strong hydrogen bonding network formed under high pressure lock the distorted frame structure and retain the higher rotational potential barrier to ambient conditions.Furthermore,theoretical calculations show that the hydrogen bonding synergistic effect enhances the rotational barrier of the rotator BDC from the initial 0.91 e V/mol to 3.87 e V/mol after full release of pressure.Restriction of organic ligand rotation after the pressure treatment leads to Zn（BDC）（DMF）（H2O）exhibiting bright blue light emission.In this work,the pressure treatment strategy is employed to endow nonemissive materials with luminescence properties at ambient conditions.Secondly,considering that the luminescence properties of chromophores are very sensitive to their molecular configurations,Cd（BDC）（DMF）with multiconfiguration ligands is selected in this study for the high-pressure optical property study.So as to further investigate the pressure-induced multicolor tunable emission of MOFs.At pressure up to 7.2 GPa,Cd（BDC）（DMF）exhibits pressure-induced blue light emission enhancement.After full release pressure from 18.3 GPa,26.1 GPa,and 30.0 GPa,Cd（BDC）（DMF）shows bright blue,green,and white light emission at ambient conditions.In situ high-pressure structural analysis shows that negative compression of the framework under pressure leads to the enhancement of hydrogen bonding between the BDC units on the acute edges of the rhombic framework.After complete release of pressure,the hydrogen bonding synergistic effect limits the rotation of the organic ligands and intercepts the highly fluorescent states with higher rotational barriers to ambient conditions.In addition,higher pressure treatments lead to greater negative compression of the framework and more efficient charge transfer between planar BDCs and twisted BDCs,facilitating phosphorescence emission of the triplet state.Therefore,the pressure-treated Cd（BDC）（DMF）shows a conversion from fluorescence-dominated blue light emission to phosphorescence-dominated green and white light emission when the pressure-treated values is increased from 18.3 GPa to 26.1 GPa and 30.0 GPa.Finally,ZrO（BDC）as well as isostructured ZrO（NDC）（NDC=2,6-Naphthalenedicarboxylic acid）and ZrO（BPDC）（BPDC=4,4’-Biphenyldicarboxylic acid）are selected for the comparative study of high-pressure optical properties.Comparative study is based on the consideration that MOFs with different pore sizes have different structural response behaviors under pressure and aims to further explore the high-pressure optical properties of BDC-based MOFs.ZrO（BDC）exhibits the pressure-induced blue light enhancement under pressure,the bright blue light emission is intercepted to ambient conditions after full release the pressure.In Situ high-pressure structural analyses show that the narrowed aperture structure under pressure leads to enhanced hydrogen bondings between DMF and BDC.After full release of pressure,hydrogen bonding synergistic effect limits the rotation of the BDC rotor and intercepts the highly emissive states with higher rotational barriers to ambient conditions.As for ZrO（NDC）constructed with NDC featuring a larger conjugated structure,the distortion of pore channel under pressure leads to the weakening of emission.The restoration of the structure leads to the restoration of emission to the initial state after the complete release of pressure.In the case of ZrO（BPDC）constructed with BPDC possessing a longer organic chain length,the bending of the BPDC under pressure leads to the weakening of the emission.After a complete release of pressure,the bent BPDC could not be restored resulting in weaker luminescence than the initial state. |
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