Synthesis And Luminescence Properties Of Lanthanide Metal–organic Coordination Polymers Based On Carboxylate Ligands

As a new valuable type of luminescence materials, lanthanide coordinationpolymer can efficiently convert to characteristics transition of lanthanide ion byabsorption ultraviolet light of ligand and have good fluorescence monochromaticityand high purity. In this paper, we have prepared32lanthanide coordinationpolymers using different rigid ligands as linear chain oxalic acid （H₂ox）,five-member ring imidazole-4,5-dicarboxylic acid （H₃IMDC）, six-member ringligands1,3-benzenedicarboxylate （1,3-H₂BDC） and1,4-benzenedicarboxylate（1,4-H₂BDC）, benzimidazole ring ligand benzimidazole-5,6-dicarboxylic acid（H₃BIDC） and N-containing heterocyclic ligand pyridine-2,6-dicarboxylic acid（H₂PDA） under hydrothermal/solvothermal conditions with different molar ratio,temperature, reaction time and pH values. The empirical formulas are as following:{[Ce（ox）_1.5（H₂O）3]2H₂O}_n（Ceox），{[Pr（ox）_1.5（H₂O）₃]3H₂O}_n（Prox）,{[Ln（ox）₂（H₃O）] EtOH3H₂O}_n[Ln=Nd（Ndox）, Sm（Smox）, Eu（Euox）, Gd（Gdox）, Tb（Tbox）,Dy（Dyox）, Ho（Hoox）, Er（Erox）, Yb（Ybox）],{[Ln₂（IMDC）₂（H₂O）3]2.25H₂O}_n[Ln=Sm（SmIMDC）, Eu（EuIMDC）, Gd（GdIMDC）, Dy（DyIMDC）],{[Tb2（IMDC）₂（H₂O）3]1.75H₂O}_n（TbIMDC）,{[Ln4（1,3-BDC）6（DMF）（H₂O）4]·DMF·2H₂O}_n[Ln=Tb （TbBDC）, Ho （HoBDC）, Er （ErBDC）],[Tb（1,4-BDC）_1.5（H₂O）（DMF）]_n（TbBDC1）,{[Pr（HBIDC）（ox）0.5（H₂O）]·H₂O}_n（PrBIDC）,[Yb（HBIDC）（ox）_0.5（H₂O）₂]_n（YbBIDC）,[Ln（HBIDC）（ox）0.5（H₂O）3]_n[Ln=Ho（HoBIDC）, Tb（TbBIDC）],{[Ln（H₂BIDC）（HBIDC）（H₂O）₃]·3H₂O}_n[Ln=Tb（TbBIDC1）, Sm（SmBIDC）,Dy（DyBIDC）, Gd（GdBIDC）],{[Ln（HPDA）（PDA）（H₂O）₂]·4H₂O}_n[Ln=Eu （EuPDA）, Tb（EuPDA）, Ce（CePDA）, Nd（NdPDA）]. The structures of all coordinationpolymers have were characterized by elemental analysis, infrared spectra andsingle-crystal X-ray diffraction. We have a systematic investigation of luminescentand thermal properties for coordination polymers.The2-D structure of Ceox, Prox, HoBIDC andTbBIDC are （6,3）-connectedhcb network, Lnox（Ln=Nd^Yb） appear3-D diamond structure with66-networktopology, and have large porosity （45.9-44.8%）; LnIMDC（Ln=Sm^Dy） are3-Dstructure conformed with double-helix chain and the triple-helix chain by thebridged carboxyl group, resulting in a novel （3,4）-connected net with symbol of{4·63·82}{63}; LnBDC（Ln=Tb, Ho and Er） display a （3,4）-connected net with thepoint symbol of {42.63.8}{42.6}, TbBIDC1possesses a two-fold interpenetrating3-D pcu network, which has a very large pore, and the percent of effective freevolume is27.8%. Topological analysis has been known as3-D pcu network, withthe point symbol of {412·63} in PrBIDC. YbBIDC exhibits a4-connected44 topology, and LnBIDC（Ln=Ho and Tb） appear2-D （6,3）-connected hcb networktopology. The1-D helical infinite chain of LnBIDC（Ln=Sm, Gd, Tb and Dy）around the crystallographic21axis spread along the b axis direction.Luminescence and NIR emission spectra analyses that all coordinationpolymers exhibited the characteristics transitions of corresponding lanthanide ions.Coordination polymers of europium （Euox, EuIMDC and EuPDA） and terbium（Tbox, TbIMDC, TbBDC, TbBDC1, TbBIDC, TbBIDC1and TbPDAy） exhibitedthe characteristics transitions at ca.616nm and ca.544nm of correspondinglanthanide ions, and have potential applications in red and green emitting material.In samarium （Smox, SmIMDC and SmBIDC） and dysprosium （Dyox, DyIMDCand DyBIDC） complexes, we not only measured emission spectra in the visibleregion, but also detected the infrequent NIR emission spectra of samarium anddysprosium ions. Europium and terbium polymers have long fluorescence lifetimes,and the longest lifetimes can reach1103.11μs and1149.83μs, respectively. Thereason of fluorescence lifetime enhancement are summarized: Optimalligand-to-metal energy transfer process need2000–4500cm^–1and π–π stackinginteractions between two-dimensional layered structure could facilitate the transferof electrons. Good match conducive to the sensitization of rare earth ions. Even ifthe content is less coordinated water molecules in polymers can increase theradiationless transition and result in the increased fluorescence lifetimes.Based on above discussion, a model for the indirect excitation mechanism isproposed: By calculated, the singlet excited state of H₂ox, H₃IMDC and H₃BIDCligand were30842cm^–1,32258cm^–1and30303cm^–1, and the lowest triplet stateenergy level were23753cm^–1,22371cm^–1and24390cm^–1, respectively. Effectiveextent of energy transfer from H₃BIDC ligand to lanthanide ions follows thesequence of Tb³⁺, Dy³⁺> Sm³⁺. For H₂ox ligand, the energy transfer follows thesequence of Tb³⁺, Dy³⁺> Sm³⁺, Eu³⁺. As to H₃IMDC, transition from the tripletenergy level is only effective to samarium. Accordingly, the ability from ligand tolanthanide ions follows the sequence of H₂ox>H₃BIDC>H₃IMDC.Complexes LnPDA（Ln=Eu andTb） were dispersed in mesoporous materialsSBA-15in DMF solution （denoted as ML-SBA-15, ML=EuPDA, TbPDA）, whichwere characterized by XRD, IR, and fluorescence spectra. The complexesencapsulated in mesoporous materials SBA-15exhibited stronger luminescenceintensity and longer fluorescence lifetime because the photoluminescence efficiencyof hybrid material was improved by the energy transition between mesoporousmaterials and the complexes.