| Single-molecule magnets is a kind of nano-sized molecular magnetic materials in the true sense of magnetism.Its magnetism comes from the magnetic moment of a single molecule with magnetic bistability.Under blocking temperature,magnetization and magnetic order can be preserved without external magnetic field,and slow relaxation can be observed in ac magnetic field.Due to its characteristics of easy processing,low density and chemcical solubility,it has attracted much attention for its potential application in ultra-high density information storage,quantum computer and spintronics.Among all kinds of single-molecule magnets,such molecular magnets constructed with transition metal Co(?)and rare earth metal Dy(?)are favored by researchers on account of their unquenched orbital angular momentum and strong magnetic anisotropy.Because of its reversible photochromic effect under the external stimulation of illumination and heating,the molecules of diarylethene,viologen and derivates of anthracene can be used for information storage of optical switches.With these in mind,we synthesized a series of Co(?)and Dy(?)based single-molecule magnets with special structures constructed by diarylethene,viologen,and derivates of anthracene and systematically studied their optical,magnetic and magneto-structural relations.The research contents are as follows:1.Modulated Six-coordinated Co(?)single-ion magnets by ligands and latticeguestsTwo ligands,that is 2,6-bis(4,5-bis(2,5-dimethylthiophen-3-yl)-1H-imidazol-2-yl)pyridine(BTIP)and 2,6-bis(4,5-diphenyl-1H-imidazol-2-yl)pyridine(BPIP),were synthesized by one-step method,in which BTIP with the diarylethene moiety cannot achieve reversible photochromic effect under uv irradiation.Three mononuclear Co(?)complexes 1,2 and 3 were synthesized by reacting them with Co(?)salts at room temperature.All three complexes are twisted six-coordinated octahedral configurations.There are multiple intramolecular and three-dimensional?...?interactions in complex1.However,in complexes 2 and 3,since the counter anion is the large tetraphenylborate,the intermolecular?...?interaction was absent,and only intramolecular?...?interaction can be found The magnetic test showed that all three complexes were field-induced single-ion magnets.And apparently,the relaxation time of 1 was shorter than 2 and 3,which may result from the vibration of multiple methyl group in 1.The effective barrier of 3 was significantly higher than the isostructural 2,which may be caused by distinction of lattice guests.2.Modulated single-ion magnets of mononuclear?-diketone Dy(?)complexesby ligand and intermolecular hydrogen bondsFour?-diketone based Dy(?)single-ion magnets(SIMs)manipulated by auxiliary ligands of 2-(anthracen-9-yl)-1H-imidazo[4,5-f][1,10]phenanthroline(AIP),2-(4-(anthracen-9-yl)phenyl)-1H-imidazo[4,5-f][1,10]phenanthroline(APIP),2,3-diphenylpyrazino[2,3-f][1,10]phenanthroline(DPP)and 2,3-bis(2,5-dimethylthiophen-3-yl)pyrazino[2,3-f][1,10]phenanthroline(BPP),thatis,[Dy(?)(TTA)3(AIP)]·0.5CH3CH2OH·0.5H2O(4),[Dy(?)(TTA)3(APIP)]·2CH3OH·H2O(5),[Dy(?)(TTA)3(DPP)](6)and[Dy(?)(TTA)3(BPP)]·0.5CH3CH2OH(7),have been fully synthesized through alteration of the phenanthroline derivates.The ligand of BPP,AIP,and APIP containing the diarylethene and anthracene moieties showed no photochromic effect.Magnetic investigations reveal that all complexes perform as SIMs with notably different effective barriers of 69.4 K(4),147.3 K(5),122.1 K(6)and 234.2 K(7)in zero direct current(dc)field.What is noteworthy is that such complexes of 5 and 7 exhibiting relatively lower local symmetry are in possession of almost twofold higher effective barriers than the comparative 4 and 6 who possess higher symmetry.By structurally analyzing among four complexes,the significant difference of magnetic dynamic stems from the varying degree of intermolecular hydrogen bonds interaction and charge delocalization of auxiliary ligands that bridged with protonated five-membered imidazole ring or non-protonated six-membered pyrazine ring.The ab initio calculation provides the corresponding support in theory,which the conspicuous variation of auxiliary ligand brings about varying intensity of quantum tunnelling magnetization(QTM).With the combination of experimental and theoretical analyses,this work provides a visual and instructive perspective in understanding the authenticity of fine tuning of auxiliary ligand for designing such structurally modulated SIMs of mononuclear?-diketone Dysprosium(?)complexes.3.Mononuclear Dy(?)single-ion magnets with reversible photochromic effectA ligand,4,5-bis(2,5-dimethylthiophen-3-yl)-1,3-dimethyl-1,3-dihydro-2H-imidazol-2-one(Lo)with a rapid reversible photochromic effect was synthesized by one-step method.The white ligand Lo underwent a ring-closed reaction at 254 nm uv light accompanying with rapidly color change to orange-red,and then recovered to white when exposed to 365 nm uv light.Two mononuclear Dy(?)complexes,8-o and9-o,were obtained by reacting with dysprosium nitrate pentahydrate in different molar quantities at room temperature.Both complexes showed reversible photochromic effects under 254 nm and 365 nm uv light:colorless to orange-red for 8-o and colorless to saffron for 9-o.The crystal structures of the complexes show that there are two Lo ligands in 8-o and three Lo ligands in 9-o.It is this distinction that makes the complex8-o behave as a field-induced single-ion magnet,while the single-ion magnet behaviour of complex 9-o was off.Upon irradiating with 254 nm light,the SMM property of 8-o deteriorates with the energy barriers decreasing from 68.1 K to 44.3 K while the magnetic dynamic of complex 9-o shows no response to irradiation.The ab initio calculations reveal the quantum tunnelling of magnetization between the ground Kramers doublets in 9-o is quite strong,leading to the disappearance of SMM behavior.For complex 8-o,the strong magnetic anisotropy and moderate quantum tunnelling of magnetization relative to 9-o give rise to the appropriate energy barrier for SMM.On the other side,the decrease of energy barriers for 8-o after irradiation result from the weakening of magnetic anisotropy,which was caused by stronger charge delocalization of coordinated carbonyl oxygen atom of the ring-closed DTE ligand.In this part,by regulating the reactant ratio,two photochromic DTE-based Dy(?)complexes were obtained and the SMM property of complex 8-o can be tuned by the light-induced isomerization of dithienylethene ligand.4.Dinuclear Dy(?)single-molecule magnets modulated by ligand and latticeguestFive Dy(?)complexes with similar dinuclear unit in the centre constructed by multidentate ligands,that is,2,6-dimethoxyphenol(HL)and 4-hydroxy-3,5-dimethoxybenzaldehyde(HL?),withgeneralformulasof[Dy2(NO3)4(L)2(H2O)2]·2Me CN(10),[Dy2(NO3)4(L)2(H2O)2]·2(NO3)·DMBD·2Me OH(11),[Dy2(NO3)4(L)2(TPO)2]·2Me CN(12),[Dy2(L?)6(H2O)2]·4Me CN(13)and[Dy2(NO3)6(L)2(Cl)2(H2O)2]·DMBD(14)(DMBD=1,1'-dimethyl-[4,4'-bipyridine]-1,1'-diium and TPO=phosphine triphenyl oxide),were elaborately synthesized,structurally characterized and magnetically investigated.In complexes 11 and 14,the viologen molecules were successfully introduced into the lattice.However,no photochromism was observed in the stimulus of light.The complexes 10-12 display slightly discrepant coordination geometries around Dy(?)ion with different endmost ligands(Oend:H2O for 10 and?11,TPO for 12)while complex 13 adopts distinct coordination model as a result of the introduction of aldehyde group on HL?ligand.The nearly planar core structures in 10-12 that consist of dinuclear Dy(?)ions and two L ligand are almost perpendicular to four nitrate ligands,make it accessible to introduce endmost ligands in the terminal along the Dy···Dy orientation.The magnetic studies reveal that they all display significant zero-field single-molecule magnet(SMM)behaviour and their magnetic dynamic can be modulated by different endmost ligands and lattice guests with variant energy barriers of 107.5(10),127.1(11),168.7(12)and251.9 K(13)respectively.In complexes 10-12,the not such small transverse anisotropy accounts for the non-negligible QTM(quantum tunnelling of magnetization)process.However,such an under-barrier process was effectively suppressed in complex 13.The intramolecular and intermolecular magnetic interactions between complexes 10 and 11are distinguishable due to different hydrogen bond interactions by lattice guests and thus result in the variation of Dy-OH2O bond distances,which are responsible for the improvement of SMM property of 11.By feat of theoretical calculation,the main magnetic axis orientation of ground state gradually verging on the relatively short Dy-Oend bond are attributed to the stronger ligand field of TPO and phenoxy anion than H2O,as a result of which gives rise to the increase of uniaxial anisotropy and superior SMM properties of 12 and 13 to 10 and 11.The ferromagnetic interaction between Dy(?)ions mostly originate from dipole-dipole interaction.This build strategy drastically enhances the SMM properties and these complexes of 12 and 13 possess higher energy barriers than those reported dinuclear Dy(?)SMMs constructed by 2,6-dimethoxyphenol and 4-hydroxy-3,5-dimethoxybenzaldehyde.As a novel concept and design for dinuclear Dy(?)complexes,it provides an alternative model to investigate the influence of endmost ligands and lattice guests on the single-ion anisotropy and magnetic interaction. |
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