| Metal-organic frameworks(MOFs)are a class of topological extended network structures formed by the connection of metal ions and organic linkers.MOFs present widely prospects in gas storage,magnetism,catalysis,drug delivery,and optics due to their ultra-high porosity and large internal specific surface area.Synergy arises when linkers and inorganic nodes are combined,and one coordination system can provide multiple emitting centers while metal-ligand energy/charge transfer related emission can endow MOFs with dimensional fluorescent functionality.As a new type of photoluminescence(PL)materials,rare earth-based MOFs have become“star materials”in PL MOFs because of their high color purity,large Stokes shifts and high photoluminescence quantum yield(PLQY).Depending on the electronic transition type of rare earth ions,improving metal-to-ligand charge-transfer(MLCT)and/or ligand-to-metal energy-transfer(LMET)is an efficient method to obtain high PLQY.Pressure engineering as an effective means of modifying the crystal and electronic structures,has been utilized to regulate the optoelectronic properties of various types of materials.Given the MLCT and LMET are governed by the crystal and electronic structures,we therefore speculate that the modulation of metal-ligand charge/energy transfer by pressure treatment may enhance PL performance of MOFs.Therefore,we selected three rare earth MOFs based on trimesic acid(H3BTC)as research objects to explore the relationship between optical properties and structure under high pressure,thus opening up an alternative avenue for the structural design of MOFs materials with high PLQY.Firstly,we conducted the systematic high pressure investigation of Y(BTC)(H2O)6with the characteristic of MLCT.According to the in situ high-pressure PL measurement,we found that Y(BTC)(H2O)6 obtained a bright deep blue emission based on the initial PL color after a pressure treatment,and the PLQY increased from 2.8%to 75%.The deep-blue emission provided a colour coordinate of(0.1514,0.035),approaching the standard colour Rec.2020(0.131,0.046)specification.In addition,we observed a pressure induced emission enhancement within 5.1 GPa,which was highly associated with the increasement of the overall planarization trend between the metal Y atoms and three benzene rings under pressure.Based on the in situ high-pressure infrared(IR)measurements,we found that the hydrogen bond interaction was enhanced during the compression process and upon complete release of the pressure,which effectively limited the crystal structure recovery.Combined with in situ high-pressure angle dispersive synchrotron X-ray diffraction(ADXRD)measurements and first-principles calculations,the pressure-treated engineering could enhance electronic transition diversity of MLCT and oscillator strength,accounting for the intriguing high-efficiency PL in Y(BTC)(H2O)6.We have optimized MLCT by pressure-treated engineering to achieve PL enhancement of Y(BTC)(H2O)6.Then,we selected Tb(BTC)(H2O)6 with the characteristic of LMET as a representative system to further explored its high-pressure PL properties.Tb(BTC)(H2O)6 exhibited PL enhancement from 1 atm to 2.5 GPa and the PL intensity started to decrease with pressure above 2.5 GPa.The PLQY of pressure-treated Tb(BTC)(H2O)6 was measured to be 90.4%,which was much higher than the initial value(50.6%).Based on the in situ high-pressure IR measurements,we found that the hydrogen bond interaction was enhanced during the compression process and upon complete release of the pressure,which effectively limited the crystal structure recovery.According to the in situ high-pressure ADXRD measurements,we found that the pressure-treated process can optimize the ligand singlet and triplet states closer to the Tb3+energy level,facilitating the LMET process,thus obtaining the efficient green PL emission of Tb(BTC)(H2O)6 with high PLQY.In view of Eu-based MOFs could emit characteristic red emission based on central metal ions and possessed the same LMET process as Tb-based MOFs.We selected Eu(BTC)(H2O)6,which has the same structure as Tb(BTC)(H2O)6,as the research object to explore its PL behavior under high pressure.A phenomenal pressure-induced emission enhancement is monitored by in situ high-pressure PL measurement,where the brilliant red emission was achieved at 9.2 GPa.Comprehensive analysis on the experimental and calculated results indicated that optimizing the ligand triplet states closer to the Eu3+energy level was beneficial to facilitate the LMET process.In addition,restricting the vibration of O-H bonds could also reduce the non-radiative transition of Eu3+excited state.Finally,the red emission enhancement was achieved under the synergy of the above two aspects. |
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