韓永芳 蔡麗珍 郭國聰

(1中國科學(xué)院福建物質(zhì)結(jié)構(gòu)研究所，結(jié)構(gòu)化學(xué)國家重點實驗室，福州 350002)(2中國科學(xué)院大學(xué)，北京 100039)

0 Introduction

Bi-functional materials combining luminescence with magnetic properties have become a research focus due to their various applications,such as information storage,sensing (optical detection of magnetic compounds)and bio-imaging (combining for instance magnetic relaxation imaging (MRI)and fluorescence labelling)[1-3].However,because of the quenching effect of paramagnetic transition metal (TM)ions on the luminescence of organic dyes,it is difficult to obtain magneto-optical materials from TM-based complexes[4].On the contrary,lanthanide-based complexes are well known for their magnetic properties with large angular momentum and crystal-field effect[5],as well as their exceptional luminescent properties with narrow band and long-lived emission,large Stokes shifts,high luminescence efficiency,and ligand-dependent luminescence sensitization[6-7].In this regard,lanthanide ions have been incrementally used to design bi-functional magneto-optical materials by the rational choice of suitable ligands,which should hold not only efficient energy-transfer properties for the luminescent materials but also the proper orbitals for the construction of lanthanide-based magnetic materials[8-11].

Aromatic carboxylate ligands are usually used to constructlanthanide-based materialsbecausecarboxylic groups can clip fit well with the oxytropic lanthanide ions,and mediate strong magnetic interactions between paramagnetic centers[9].Simultaneously,aromatic substituent groups can act as sensitive antennas to promote the luminescence of lanthanide ions and lead toπ stacking[12-13].Previously,we chose the N-succinylpyridine ligand to achieve two d10-based compounds[14]and several lanthanide-based compounds[15],revealing the flexible coordination modes,rich fluorescent properties,and well magnetic mediums of N-succinylpyridine ligand.These results are very helpful for our further design and investigation on the bi-functional magnetooptical lanthanide-based materials.

In this work,we reported the preparation,structures,magnetic and fluorescent properties of two new complexes,[Ln(HL)2(H2O)4]Cl3(Ln=Pr(1),Eu(2),HL=N-succinopyridine).

1 Experimental

1.1 Materials and instrumentation

N-succinopyridine was obtained according to the reported procedure[16],and other chemicals were commercially available analytical grade and used without further purification.The elemental analyses were carried out on an Elementar Vario ELⅢmicroanalyzer 9.Powdered X-ray diffraction (PXRD)patterns were tested on a Rigaku MiniFlexⅡdiffractometer using Cu Kα radiation(λ=0.154 2 nm)at 30 kV and 15 mA in a range of 5°～50°.The simulated patterns were finished by the Mercury software.The FT-IR spectra were measured on a Perkin-Elmer spectrum using KBr pellets as the matrices within 4 000～400 cm-1.Optical diffuse reflectance spectra were achieved at room temperature on a PE Lambda 900 UV-Vis spectrophotometer used BaSO4plate as a standard(100%reflectance). Thermogravimetric analysis (TGA)experiments were recorded on a Netzsch STA449CQMS403C thermogravimetric analyzer under N2atmosphere with the sample heated in an Al2O3crucible at a heating rate of 10 K·min-1.The temperature dependence of magnetic susceptibility was done with a Quantum Design MPMS-XL superconducting quantum interference device (SQUID)magnetometer in a temperature range of 2～300 K at a field of 1 kOe and diamagnetic corrections were made using Pascal′s constants.The determination of PL was conducted on a double excitation monochromator Edinburgh FL920 fluorescence spectrometer equipped with a R928P PMT detector.

1.2 Syntheses of 1 and 2

The preparation procedures and methods of the two complexes are similar.The lanthanide halide(2 mmol)and N-succinylpyridine (195.2 mg,1 mmol)were dissolved in 10.0 mL of distilled water and stirred under an atmosphere for 3 hours.Then the resulting solution was filtered into a glass beaker and slowly evaporated to crystallize.After about half a month,prismatic crystals which can be used for X-ray analysis were obtained.The crystal samples used for all testing and characterization were carefully handpicked with the help of a microscope.The purity of the obtained crystalline samples was measured PXRD(Fig.1).Elemental analysis data of C,H,N was also used to prove the purity of test samples.Anal.Calcd.for C18H26Cl3N2O12Pr(1)(%):C,30.44;H,3.66;N,3.95.Found(%):C,29.31;H,3.84;N,3.74.Anal.Calcd.for C18H26Cl3N2O12Eu(2)(%):C,29.97;H,3.61;N,3.89.Found(%):C,29.39;H,3.88;N,3.60.

Fig.1 PXRD patterns of 1(a)and 2(b)

Fig.2 (a)TG curves of 1 and 2;(b)IR spectra of 1 and 2

TG analysis data (Fig.2a)indicates that 1 and 2 are stable up to 109 and 114℃,then show a weight loss between 155 and 162℃,in accordance with the removal of four coordinated water molecules(Obsd.13.86%and 9.52%;Calcd.10.15%and 10.00%for 1 and 2,respectively).Yield based on HL:49%for 1;18%for 2.IR (KBr,cm-1)(Fig.2b)for 1:3 430(s),1 735(s),1 632(s),1 559(s),1 488(s),1 409(s),1 272(w),1 238(w),1 210(s),1 175(m),1 159(m),1 109(w),984(w),971(w),896(m),785(w),764(w),694(m),637(m),613(m);2:3 447～3 393(s),1 737(s),1 630(s),1 489(s),1 409(s),1 338(m),1 274(m),1 238(m),1 177(s),1 156(s),1 111(m),1 082(m),1 050(m),984(m),969(m),899(w),764(m),693(s),640(m),613(s),529(m),477(w),420(w).

1.3 Single-crystal diffraction analysis

The single-crystal X-ray diffraction measurement was collected on a Rigaku AFC7R diffractometer for 1 and Rigaku SCX mini for 2,respectively,which were equipped with Mo Kα radiation (λ=0.071 07 nm).Evidence during data collection indicated that the crystal did not decay,indicating that the obtained polymers were stable at ambient temperature.The intensity data sets were collected with theωscan technique and corrected for Lp effects.The primitive structures were solved by the direct method and reduced by the CrystalClear software[17].The subsequent successive difference Fourier syntheses yielded the other non-hydrogen atoms.The final structure was refined using a fullmatrix least-squares refinement on F2.All non-hydrogen atomswererefined anisotropically.The hydrogen atoms of HL molecule were added geometrically and refined using the riding model.The hydrogen atoms of all water molecules were located in the idealized positions and refined with O-H distances restrained to a target value of 0.085 nm,the H-H distance to 0.134 nm,and Uiso(H)=1.5Ueq(O).All of the calculations were performed by the Siemens SHELXTL version 5 package of crystallographic software[18].The crystal data and structure refinement results for 1 and 2 are given in Table 1.The selected bond lengths(nm)and angles(°)are given in Table 2 and Table 3.

CCDC:885100,1;902937,2.

2 Results and discussion

2.1 Crystal structure description

The single-crystal X-ray diffraction analyses indicated that 1 and 2 belong to isostructural phases.Hence,compound 1 is taken as an example to depictthe crystal structure.Compound 1 crystallizes in the monoclinic system P2/c.(No.147),and the asymmetric unit consists of one Pr（Ⅲ）ion,one neutral HL ligand,two coordinated water molecules,one and half a Clanions.

Table 1 Crystal and structure refinement data for 1 and 2

Table 2 Selected bond lengths(nm)and angles(°)for 1

Table 3 Selected bond lengths(nm)and angles(°)for 2

As shown in Fig.3a,every Pr（Ⅲ）ion coordinates eight oxygen atoms and exhibits distorted square antiprismatic geometry.One basal aspect of the reverse prism composes of O1,O1W,O2#2 and O2W atoms,and the other base builds up with O2W#3,O2#1,O1#3 and O1W#3 atoms.The Pr-O bond distances ranging from 0.237 8(3)to 0.252 5(4)nm are normal as compared with Pr carboxylates[15,19-21].Note that Cl2 atom splits up into Cl2 and Cl21 with the SOF value of 0.35(0)and 0.15(0).Adjacent Pr（Ⅲ）ions are doubly bridged by syn-syn carboxylate groups forming linear 1D chains.Neighboring chains form 2D layered structure by O3…Cl1…O1W hydrogen bonds along b axis (Fig.3b).Finally,these layers are further expanded into a 3D network through the hydrogen bonding of O1W…Cl2…O2W (O3…Cl1 0.304 0(4)nm,Cl1…O1W 0.306 3(5)nm,O1W…Cl2 0.314 5(5)nm,Cl2…O2W0.313 5(5)nm,Fig.3c).Thecoordination mode of ligand in 1 is different to that in our previous work[15],revealingtheflexiblecoordination modesof HL.The intrachain distanceof Pr…Pr is 0.564 0(2)nmand the nearest interchain distance of Pr…Pr is 0.961 9(5)nm.It is worth noting that although the lanthanide ions has a good affinity for oxygen,only one carboxylate unit is coordinated to Pr（Ⅲ）ions in the HL ligand.

Fig.3 (a)Coordination environment of Pr（Ⅲ）in 1;(b)View of the hydrogen bonding 2D framework of 1 along b axis;(c)View of the hydrogen bonding 3D framework of 1 along b axis

2.2 Magnetic properties

2.2.1 Magnetic property of 1

The temperature dependence of magnetic susceptibility for 1 is investigated and presented in Fig.4.Compound 1 exhibited a χMT value of 1.62 cm3·K·mol-1at room temperature,which is in excellent agreement with the theoretical value of 1.60 cm3·K·mol-1for an uncoupled Pr（Ⅲ）ion (3H4,g=4/5)in the ground state.As the temperature was lowered from 300 to 2 K,theχMT value decreased gradually and reached a minimum of 0.08 cm3·K·mol-1,which could be caused by a selective depopulation of the excited crystal field state and antiferromagnetic interaction between Pr（Ⅲ）ions.There are no available expressions to estimate the magnetic susceptibility because of the large anisotropy in this 1D system.In order to get a rough quantitative estimation of the magnetic interaction between Pr（Ⅲ）ions,it can be assumed that the Pr（Ⅲ）ion exhibits a splitting of the mjenergy levels(2)in an axial crystal field.Therefore,χPrandχMcan be described as equation(1)and(2),respectively.

Fig.4 Plots ofχM T and χMvs T for 1

In these formulas,Δis the zero-field splitting parameter,and N,g,β,and k represent their basic meanings.By using the above equation and considering the molecular field approximation with zJ′as the total exchange parameter between Pr（Ⅲ）ions,we can fit our experimental data with Eq.(2).The best fitting of the susceptibility data in a temperature range of 8～300 K gave zJ′=-2.71 cm-1, Δ=0.99 cm-1,g=0.89,and R=∑(χobsd-χ′cacld)2/∑(χobsd)2=1.82×10-4.The negative value of zJ′demonstrates that an overall antiferromagnetic interaction between Pr（Ⅲ）ions is operative.

2.2.2 Magnetic property of 2

About Eu（Ⅲ）,due to the weak energy separation and spin-orbit coupling,the crystal field effects and the possible thermal population of higher states should be accounted.The χMT and χMvs T plots for 2 are shown in Fig.5.2 had a χMT valueof 1.29 cm3·K·mol-1,which is much higher than the expected value of 0 cm3·K·mol-1for one isolated Eu（Ⅲ） ion in the ground state.Upon cooling,χMT gradually reached a value close to zero (0.016 cm3·K·mol-1)at 2 K because of the depopulation of Stake levels,which corresponds to a nonmagnetic ground state of7F0for Eu（Ⅲ）ions.The shape of the curve is a typical characteristic occurrence of thermally populated excited states.As for Eu（Ⅲ）,the7F ground term is split by the spin-orbit coupling(H^=λLS)into seven states7FJ,with J ranging from 0 to 6.The7F0ground state is taken as the origin,andλis small enough for the first excited states to be thermally populated (around 300 cm-1),the magnetic susceptibility follows the numerical expression given in Eq.(3):

Fig.5 Plots ofχM T and χMvs T for 2

with A=24+[(27x-3)/2]e-x+[(135x-5)/2]e-3x+(189x-7/2)e-6x+(405x-9/2)e-10x+[(1 485x-11)/2]e-15x+[(2 457x-13)/2]e-21xand B=1+3e-x+5e-3x+7e-6x+9e-10x+11e-15x+13e-21xand where N stands for Avogadro′s number,β for the Bohr magneton,k the Boltzmann constant,T the temperature,θthe Weiss constant,x=λ/(kT)andλ the spin-orbit coupling parameter.The best fitting of the χMT vs T curve in a temperature range of 20～300 K gave the spin-coupling parameter,λ=373 cm-1,a value in the expected range,andθ=3.17 K,R2=0.999 41.The positiveθvalue reveals the presence of weak ferromagnetic interactions between Eu（Ⅲ）ions.

2.3 Optical properties

The reflectance diffusion spectra (Fig.6)reveals that 1 and 2 display intense π-π*transition absorption bands at 274 and 285 nm,respectively.In addition,the characteristic f-f electronic transition bands at 444,470,482,and 594 nm attributed to3H4→3PJ(J=2,1,0)and1D2for Pr（Ⅲ） ion in 1,318 nm (7F0→5H5),363 nm(7F0→5D4),375 nm(7F0→5G4),383 nm(7F0→5G3),393 nm (7F0→5L6),465 nm (7F0→5D2),525 nm(7F0→5D1),534 nm (7F1→5D1)for Eu（Ⅲ） ion in 2 were also observed.

Fig.6 Optical diffuse reflectance spectra of 1 and 2

The solid-state luminescence properties of these two complexes are also investigated at room temperature.Upon excitation at 396 nm,2 emitted intense red luminescence (Fig.7)and exhibited four characteristic peaks at 590,612,650,and 697 nm,respectively,corresponding to the transitions of Eu（Ⅲ）from5D0to7FJ(J=1,2,3 and 4).The quite weak emission band for5D0→7F0in the position of 575 nm is due to the symmetry-forbidden transition of the Eu（Ⅲ） ion in 2.It is well known that the5D0→7F1transition is magnetic dipole in nature and less sensitive to its environment,while5D0→7F2is electric dipole in origin and its intensity is strongly influenced by the crystal field[22-23].The intensity ratio of D0→7F2to5D0→7F1is about 2.5,implying relatively lowsymmetric crystal field for the center Eu（Ⅲ）in 2,which is consistent with the single-crystal structure.Luminescence lifetime measurements in solid monitored at 614 nm of 2 was 0.233 ms.Regrettably,because of the large energy gap of the Pr（Ⅲ）ion,no characteristic peak for Pr（Ⅲ）ion was observed in the luminescent spectra of 1.These results suggest that energy transfer from the ligands to the resonance level of lanthanide ions is efficient for Eu（Ⅲ）ion but not for Pr（Ⅲ）ion.

Fig.7 Solid-state emission spectra of 2 at room temperature

3 Conclusions

In conclusion,two new 3D isostructural polymers based on N-succinopyridine ligand have been synthesized in aqueous solution at room temperature and characterized in terms of structure,magnetic properties and luminescence.The magnetic studies show that theχMT value of 1 decreases as the temperature drops,which could arise from a selective depopulation of the excited crystal field state and antiferromagnetic interaction between Pr（Ⅲ）ions.And 2 is a typical characteristic occurrence of thermally populated excited states because of the weak energy separation and spin-orbit coupling.The efficient energy transfer from the HL ligand to the resonance level of lanthanide ions results in that 2 exhibited intense characteristic luminescent property of Eu（Ⅲ）.

Conflicts of interest:There are no conflicts to declare.

Acknowledgements:This work was financially supported by the National Natural Science Foundation of China(Grants No.91545201,21601185),the Key Research Program of Frontier Sciences,CAS (Grants No.XDB20010000,QYZDBSSW-SLH020),the Youth Innovation Promotion Association of CAS,the Natural Science Foundation of Fujian Province(Grants No.2017J01033,2018J01028).