| Single molecule magnets(SMMs)have been of intense interest due to their fundamental importance and unique potential applications in the information storage,molecular spintronics,and quantum computing devices etc.The lanthanide-based single molecule magnets are appealing in this aspect because of their high anisotropy energy barriers,the complex relaxation mechanisms,tunable magnetic dynamics and potential applications in multifunctional materials.With the aim of exploring lanthanide-based SMMs with magnetic and multifunctional properties,we have prepared mononuclear,binuclear and 2D layer lanthanide compounds based on the Salen,cucurbit[6]uril(CB[6])and cyclic polyaminephosphonate ligands,most of them show slow magnetic relaxation processes associated with SMM behavior.We focus on four aspects:1)studying the magnetic anisotropies based on their crystal field and magnetic interactions;2)tuning the magnetic properties through the diamagnetic dilution,solvent degree and the solid states;3)investigating the multiple magnetic relaxation processes and mechanisms;4)exploring the multifunctional SMMs combing photoluminescence and optical activity into magnetism.The main achievements of this thesis work are listed as below:1.The mononuclear lanthanide-Salen compounds with multifunctionsEight mononuclear lanthanide compounds(Et3NH)[Ln(3-NO2salen)2]ˇSolvent[Ln= Eu(1),Tb(2),Dy(3),Ho(4),Er(5),Yb(6)],(Et3NH)[Dy((R,R)-3-NO2salcy)2](7R)and(Et3NH)[Dy((S,S)-3-NO2salcy)2](7S)are obtained based on N,N'-3-nitrosalcylaldehydoethylenediamine(3-NO2salenH2)or N,N'-(1,2-cyclohexa-nediylethylene)bis(3-nitrosalicylideneiminato)(3-NO2salcyH2).Compounds 1-5 are isostructural,crystallizing in triclinic P-1 space group while 7R crystallizes in monoclinic C2 chiral space group.The circular dichroism(CD)spectra confirm that 7R and 7S are enantiomers.All these compounds display mononuclear structures containing a[LnN4O4]core,the lanthanide ions are chelated by two salen-type ligands without any other terminal ligands.The two ligands locate in the perpendicular planes,unlike many other reported sandwiched complexes.The magnetic measurements reveal that the field-induced slow magnetic relaxation processes are observed in 3,4,5 and 7R(S).Under 1.5 kOe external dc field,3 and 7R show similar Orbach relaxation processes above 6.5 K.The spin-reversal energy barriers are 39.7 K(?0= 3.90×10-6 s)for 3 and 39.9 K(?0 = 3.62 × 10-6 s)for 7R according to the Arrhenius law,indicating that the magnetic anisotropies remain almost unchanged with different substituent ligands.However,the quantum tunneling magnetization processes at low temperature(below 6.5 K)are dramatically different between 3 and 7R due to their different intermolecular interactions.The relaxation times at 2 K are 24 ms for 3 and 198 ms for 7R,respectively.Moreover,7R exhibits dual relaxation processes under low dc fields(0.2-0.8 kOe)while 3 show only single relaxation process.The results demonstrate that the intermolecular interactions play an important role in multiple relaxation mechanisms.Compounds 1,5 and 6 exhibit characteristic lanthanide luminescence with sharp and well-separated emission bands in UV-VIS(1)and NIR(5,6)regions.The 3-NO2 salen ligand provides efficient sensitization for the lanthanide ions.For 1,the excitation range is dramatically broadened(200-500 nm with the maximum of 460 nm)by ligand sensitization,which has the potential for application in optical devices.2.The mononuclear dysprosium-CB[6]complex showing multiple relaxation processesA mononuclear dysprosium complex[DyCB[6](NO3)(H2O)4](NO3)2ˇ8H2O(8)has been synthesized(CB[6]= cucurbit[6]uril).The Dy3+ center is coordinated with eight O atoms from one CB[6]ligand,one NO3-anion,and four crystal water.The[DyO8]polyhedron has a low crystal field symmetry.The Dy-CB[6]units are connected with each other through the network of hydrogen bonds.Compound 8 is stable in water.However,when taken out of the mother liquor,compound 8 could lose crystallization water easily and fall apart.The magnetic dynamics of 8 is investigated in three different states,and all show field-induced dual relaxation processes but the relaxation time and ratio are different:For the sample transferred directly from the mother liquor to pure water(measured in water,8a),the ratio of relaxation A and B are comparable,relaxation A shows the Orbach process in the high temperature range with the energy barrier 38.4 K(?0 = 4.78×10-9 s);For the sample separated by filtration(measured as dry powder,8b),relaxation A is dominated in the low temperature;For the sample separated by filtration,dried,and then immersed in water(measured in water,8c),relaxation B becomes the main process.The results demonstrate that magnetic relaxation behavior of 8 is very sensitive to the structural changes induced by the loss/gain of the solvent of crystallization.3.Binuclear dysprosium phosphonate showing tunable magnetic behaviorBased on the 1,4,7-triazacyclononane-1,4,7-triyl-tris(methylenephosphonic acid)(notpH6),a binuclear compound {Dy(notpH4)(NO3)(H2O)?2ˇ8H2O(9)has been synthesized.It contains two equivalent Dy3+ ions with a distored D3h three-capped trigonal prism coordinated environment.The equivalent Dy atoms are bridged by the phosphonate oxygen(O1)as well as two pairs of O-P-O units into a dimer.Hydrogen bond interactions are found with the dimer,forming a three dimensional supramolecular network structure.Complex 9 can be transformed into{Dy(notpH4)(NO3)(H2O)}2(9-de)in a reversible manner by desorption and absorption of solvent water at ambient temperature.This process is accompanied by a large dielectric response.PXRD measurement reveal that the structures of 9 and 9-de are different.9 exhibits the thermally activated magnetization relaxation under 2.0 kOe with energy barrier of 49.0 K(?0= 6.75 ×1 0-9 s),while 9-de shows temperature dependent relaxation process in the absence of dc field,the energy barrier of 81.8 K(?0=1.19 × 10-7 s)is nearly two times larger than that for 9.Both 9 and 9-de clearly show the magnetic hysterisis loops at 1.8 K.Further,the ratio of the two relaxation processes corresponding to compounds 9 and 9-de,respectively,is extremely sensitive to the degree of dehydration.Taking advantage of this sensitive property,a reversible change of the magnetic properties is achieved by desorption and absorption cycles of solvent water.Such a process can be fine-tuned in a simple but useful way at room temperature.This discovery provides new insight for designing responsive functional materials based on lanthanide complexes.4.Layered lanthanide phosphonates exhibiting field-tunable magnetic relaxation and optical correlationA series of lanthanide phosphonates[Ln(notpH4)(H2O)]ClO4ˇ3H2O[Ln =Dy(10),Ho(11),Er(12),Y(13)]in which the lanthanide ion has a pseudo-D5h symmetry have been prepared.Complexes 10-13 are isostructural,displaying layer structures where the neighbouring lanthanide atoms are connected by a pair of O-P-O bridges.The ClO4-counterions and the lattice water molecules are between the adjacent layers.Magnetic studies reveal that complexes 10,11 and 12 exhibit the field-induced slow magnetic relaxation processes associated with SMM behavior.Complexes 10 and 12 are of particular interests because they show field-tunable dual relaxation processes:The faster relaxations are single ion anisotropies in origin,showing thermally activated relaxation processes with energy barriers 41.4 K(?0 ?1.42×10-9 s)for 10 and 34.9 K(?0 = 3.07 × 10-9 s)for 12,respectively.The slower relaxations are originating from the collective spin relaxation brought about by short-range intermolecular ordering,showing weak temperature dependence.The latter disappears in the diamagnetic diluted sample 10a and 12a.As far as we know,12 is the first Er-based SMM in five-fold crystal field symmetry.It is also the first observation of the collective spin effect in a 2D layered coordinated polymer.Moreover,complex 10 shows simultaneous emissions from the Dy3+,the ligand and the radiative energy transfer from ligand to Dy3+ in the UV-ViS region,while 12 exhibits the characteristic Er3+ luminescence in the NIR range.The high-resolution emission spectra of 10 and 12 provide a direct probe of the crystal field splitting of Dy3+ and Er3+ correlated to the magnetic data. |
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