Studies Of Luminescent Lanthanide Single Molecule Magnets Based On Photodimerized 9-Diethylphosphonomethylanthracene Ligand

Given the importance of known lanthanide compounds behaving as singlemolecule magnets(SMMs),which are potential candidates for high-density information storage,ultra-fast quantum computing and spintronics,adding other functions to these properties should extend their range of applications.The lanthanide compounds can also exhibit characteristic luminescence with well resolved emission bands and long lifetimes,therefore a number of lanthanide complexes have been constructed showing magnetic and luminescent bifunctions.However,luminescent Ln-SMMs with their properties manipulated by external stimulus such as light and thermal treatment have been rarely reported.Anthracene and derivatives can experience[4+4]photodimerization in the solid state when the stacking of the neighbouring anthracene groups meets the Schmidt criteria,e.g.face-to-face ?-? stacking with the distance less than 4.2 (?).Noting that the Schmidt criteria is not satisfied in most of the known metalorganic complexes containing anthracene groups,herein we take an alternative route by using a pre-photodimerized anthracene derivative to assemble new Ln-SMMs and study the impact of de-dimerization of the ligand on the magnetic and optical properties of these materials.In this thesis work,a pre-photodimerized ligand depma2 and a Schiff base ligand H2L(see the Chart below)were used to react with the lanthanide salts in solution.The successful elaboration of eight new dinuclear or one-dimensional chain compounds allowed for a thorough study of their thermal induced and photoinduced structural change,and regulation of optical and magnetic properties.The achievements are obtained as follows:1.Synergetic switching of magnetic and luminescent properties in dinuclear lanthanide complexes containing photodimerized anthracene phosphonate Using the already photodimerized 9-diethylphosphonomethylanthracene(depma2)and a Schiff base blocking auxiliary ligand(H2L),four isostructural dinuclear compounds,Ln2L2(depma2)Cl2[Ln=Dy(1Dy),Gd(2Gd),Er(3Er),Yb(4Yb)],were obtained.The two lanthanide ions,adopting pentagonal-bipyramid seven coordination geometry,coordinated by three N and two O from L2-in the equatorial plane respectively,bridged by two O of one depma2 and covered by two Cl-anion in apical position.Neighbouring dimers are stacked through strong intermolecular N-H…C1 hydrogen bonds to form layers which are then stacked though weak C-H…Cl hydrogen bonds in an ABAB fashion.Taking compounds 1Dy and 2 Gd as examples,their thermal induced and photoinduced structural changes were studied thoroughly and applied to regulate their optical and magnetic properties.The measurements of TG.DSC and TG-MS indicated that thermal annealing from room temperature to 350? induced consecutive two-step structural transformation,de-dimerization of the bridged depma2 to depma at ca.170? with DyL(depma)Cl(1Dy-1)and GdL(depma)Cl(2Gd-1)obtained,and elimination of one chloroethane at ca.250? giving DyL(epma)(1Dy-2,epma=9ethylphosphono-methylanthracene)and GdL(epma)(2Gd-2).After removing the coordinated chloride ion,there has no change of coordination number through EXAFS research.Therefore,it is presumed that the coordination vacancy is occupied by oxygen atom from phosphonate of adjacent molecules.Structural changes inevitably have an impact on its properties.The magnetic researches indicated that all compounds 1Dy,1Dy-1 and 1Dy-2 exhibited field-induced SMM behaviors.However,the effective energy barrier of 1Dy(Ueff=39.0 K,?0=2.19×10-10 s)is higher than that of 1Dy-1(Ueff=14.6 K,?0=1.49 ×10-7 s)and 1Dy-2(Ueff=16.4 K,?0=1.02 × 10-8 s).Fluorescent spectra revealed that 1Dy is non-luminescent while 1Dy-1 exhibits blue emission peaking at 480 nm with quantum yield QY=2.53%and average lifetime ?av=4.72 ns,attributed to slipped ?-? interaction of anthracene rings,and 1Dy-2 emits green(?em=530 nm)from face-to-face ?-? interaction(QY=1.1%,?av=6.89 ns).Obviously,thermal treatments could induce de-dimerization of the bridged depma2 to depma,and elimination of one chloroethane,and then regulate the magnetic properties and luminescence.Unfortunately,the thermal transformation from 1Dy to 1Dy-1 mentioned above cannot be reversed by UV irradiation.Thermal treatment of 1Dy at 170? for 2 minutes gave intermediate 1Dy-170-2 min which could be returned to 1Dy by 365 nm UV light irradiation.As a result,synergetic and reversible magneto-optic switching is realized.The structural transformation had little effect on the magnetic properties of Gd compounds,but the fluorescence properties change obviously.2Gd exhibits weak luminescence(QY=0.67%)while 2Gd-1 exhibits blue emission peaking at 480 nm(QY=10.67%,?av=17.9 ns),2Gd-2 emits green light at 530 nm(QY=4.7%,?av=31.6 ns).Compared with series of Dy compounds,corresponding Gd compounds show almost the same emission peaks but with higher quantum yield and longer lifetime which suggests that luminescence of ligand could be quenched by low-energy excited states of Dy?.Field-induced SMM behaviors were also observed in 3Er and 4Yb with energy barriers of 29.7 and 26.0 K and ?0 of 3.34 × 10-10 s and 6.82 × 10-7 s for 3Er(750 Oe)and 4Yb(1000 Oe),respectively.2.Synergetic switching of magnetic and luminescent properties in onedimensional lanthanide complexes containing photodimerized anthracene phosphonate ligandSimilar reaction of depma2,H2L with LnBr3 provides two isostructural onedimensional chains,[LnL(depma2)]Br·nH2O[Ln=Dy(5Dy),Gd(6Gd)].The lanthanide retains the seven pentagonal-bipyramid coordination geometry with the bridging dimerized phosphonate ligand and the capping Schiff base but the Br is now not involved in the coordination and acts as a charge balancing anion.A thorough study is conducted by thermal-induced structural change for the regulation of optical and magnetic properties.The analyses by TG,DSC and microscopy with hot-stage suggest that thermal annealing at 140? results in complicated structural transformation containing de-dimerization of depma2 and elimination of CH3CH2Br in melting state for 5Dy and 6Gd.In consequence,new compounds DyL(epma)(depma)(5Dy-1)and GdL(epma)(depma)(6Gd-1)were obtained.Magnetic studies showed that both 5Dy and 5Dy-1 exhibit field-induced single molecule magnet behavior.Compound 5Dy showed an enrgy barrier of 27.0 K(?0=1.83 × 10-6 s)under 500 Oe bias field,but compound 5Dy-1 shows no peak of the outof-phase signal.As for luminescence,5Dy is silent while 5Dy-1 exhibits blue emission at 460 nm(QY=48.96%,?av=13.0 ns),originated from slipped ?-? interaction of anthracene rings,achieving an OFF/ON switch.Also,the structural transformation had obvious effect on the fluorescence properties of Gd compounds.6Gd exhibits relative weak luminescence(QY=2.05%)while 6Gd-1 exhibits strong blue emission peaking at 460 nm(QY=43.95%,?av=12.4 ns).3.Thermally induced SC-SC transformation of a one-dimensional chainThe reaction of depma2 with Dy(NO3)3 in methanol solution gives two onedimensional chains,Dy(NO3)3(depma2)(CH3OH)·CH3OH(7Dy)and Dy(NO3)3(depma2)(H2O)H2O(8Dy).By increasing reaction temperature and reaction time,thermodynamically controlled product of 7Dy can be isolated as a pure phase.But a pure phase of 8Dy was not obtained.The main difference of the two complexes in composition is that the coordinated and lattice solvent are methanol for the former,and water molecule for the latter compound.Single crystal structural analyses revealed that compounds 7Dy and 8Dy crystallize in P21/c and P-1 space groups,respectively.Compound 7Dy contains one crystallographically independent Dy?,whereas two independent Dy? are found in compound 8Dy.In both cases,the Dy? can be described as having a geometry of distorted spherical capped square antiprism,bridged by bidentate depma2 ligand forming chain structure.Thermal induced structural change was preliminarily performed on compound 7Dy.Thermal annealing from 130 to 170? led to complicated structural transformations,combining the de-dimerization of depma2,the melting of the complex or ligand,and the migration of depma ligand.A single crystal to single crystal transformation was achieved by the thermal treatment of 7Dy at 140? for 30 min,forming an intermediate with the same composition Dy(NO3)3(depma2)(CH3OH)·CH3OH(7Dy-1).Although 7Dy-1 maintains the chain structure,there appear two kinds of depma2 in the structure.The middle C-C bonds of one depma2 are elongated evidently while those of the other keep the same bond length.A single crystal to single crystal transformation was also achieved by thermal treatment of 7Dy at 160? for 10 min,leading to a mononuclear complex Dy(NO3)3(depma)3(7Dy-2).In 7Dy-2,the Dy? ion is coordinated by six oxygen atoms from three chelated NO3-and three oxygen atoms from three depma,forming a distorted spherical capped square antiprism geometry.Obviously,migration of the depma ligand occurred during the phase transition.Preliminary study on magnetic property indicated that compound 7Dy did not exhibit single molecule magnet behaviour.After thermal treatments for 30 min in the temperature range of 100-155 ?,a red-shift of the emission peak was witnessed.The degree of red-shift increased as the temperature increased to 145 ?,originated from the ?-? interaction of anthracene rings.