Metal-centered Luminescence Research Articles

Improved Energy Transfer in the Sensitization of Americium Enables Observation of Circularly Polarized Luminescence.

The first example of circularly polarized luminescence (CPL) from a molecular americium (Am) complex is reported. Coordination of Am(III) by a combination of thenoyltrifluoroacetonate and chiral diphosphine oxide ligands yielded a complex with strong sensitized metal-centered luminescence. The energy transfer process for sensitization appears to occur via a unique resonant pathway, which results in the removal of the overlap between ligand phosphorescence and sensitized Am luminescence that has always been observed. Owing to this feature, and despite the limited amount of material that could be used due to the radioactivity of 241Am, CPL could be measured. The collected luminescence and CPL spectra provide insight into the crystal field splitting of the 5D1 → 7F1 transition. These results pave the way for future studies of Am(III) luminescence to investigate electronic structure effects in this and other 5f elements.

Ligand Chirality Transfer from Solution State to the Crystalline Self-Assemblies in Circularly Polarized Luminescence (CPL) Active Lanthanide Systems.

The synthesis of a family of chiral and enantiomerically pure pyridyl-diamide (pda) ligands that upon complexation with europium [Eu(CF3SO3)3] result in chiral complexes with metal centered luminescence is reported; the sets of enantiomers giving rise to both circular dichroism (CD) and circularly polarized luminescence (CPL) signatures. The solid-state structures of these chiral metallosupramolecular systems are determined using X-ray diffraction showing that the ligand chirality is transferred from solution to the solid state. This optically favorable helical packing arrangement is confirmed by recording the CPL spectra from the crystalline assembly by using steady state and enantioselective differential chiral contrast (EDCC) CPL Laser Scanning Confocal Microscopy (CPL-LSCM) where the two enantiomers can be clearly distinguished.

Tridentate Nitrogen Ligand as a Tool for the Construction of Well-Defined Rare Earth Trichloride Complexes.

LnCl3(THF)3 (Ln = Y, La ÷ Nd, Sm ÷ Lu) readily react with the tridentate 1,3,5-trimethyl-1,3,5-triazacyclohexane (Me3tach) ligand to form mono- or binuclear lanthanide trichloride complexes, depending on the stoichiometry of the reaction and the ionic radius of the metal: mononuclear pseudosandwich [LnCl3(Me3tach)2], (Ln = Y, La ÷ Ho) or binuclear complexes [Ln2Cl6(Me3tach)3], or [LnCl3(Me3tach)(THF)]2 (Ln = Sm, Tb). Detailed analysis of the NMR data of [LnCl3(Me3tach)2] complexes with paramagnetic lanthanide ions showed that their structures remained unchanged in the toluene solution. A series of isomorphous complexes [LnCl3(Me3tach)(Py)2] (Ln = La, Sm, Tb, Er, Lu; Py = pyridine) have been obtained by the recrystallization of either mononuclear or binuclear complexes from pyridine. Complexes of terbium and europium ions with the Me3tach ligand exhibit relatively high quantum yields of metal-centered luminescence (0.39 and 0.32, respectively).

Synthesis and Characterization of Coordination Compound [Eu(µ2-OC2H5)(btfa)(NO3)(phen)]2phen with High Luminescence Efficiency.

A high-luminescent, blue-light excitable europium(III) coordination complex, [Eu(µ2-OC2H5)(btfa)(NO3)(phen)]2phen (1) {btfa = benzoyl trifluoroacetone, phen = 1,10-phenantroline}, has been synthesized and investigated. The complex was characterized by infrared (IR) and photoluminescence (PL) spectroscopy. The PL emission spectra of powder samples registered in a range of 10.7–300 K exhibit characteristic metal-centered luminescence bands, assigned to internal radiative transitions of the Eu3+ ion, 5D1→7Fj and 5D0→7Fj (j = 0–4). The high-resolution spectrum of the transition 5D0→7F0 shows that it consists of two narrow components, separated by 0.96 meV, which indicates the presence in the matrix of two different sites of the Eu3+ ion. The splitting pattern of 5D0→7Fj (j = 0–4) transitions indicates that europium ions are located in a low-symmetry environment. The absolute quantum yield and the sensitization efficiency were determined to be 49.2% and 89.3%, respectively. The complex can be excited with low-cost lasers at around 405 nm and is attractive for potential applications in optoelectronics and biochemistry.

Effect of Non-Covalent Interactions on the 2,4- and 3,5-Dinitrobenzoate Eu-Cd Complex Structures

Heterometallic {Eu2Cd2} complexes [Eu2(NO3)2Cd2(Phen)2(2,4-Nbz)8]n·2nMeCN (I) and [Eu2(MeCN)2Cd2(Phen)2(3,5-Nbz)10] (II) with the 2,4-dinitrobenzoate (2,4-Nbz) and 3,5-dinitrobenzoate (3,5-Nbz) anions and 1,10-phenanthroline were synthesized. The compounds obtained were characterized by X-ray single-crystal analysis, powder X-ray diffraction analysis, IR spectroscopy, and elemental analysis. Moreover, the thermal stability of the complexes was also studied. Analysis of the crystal packing showed that where 1,10-phenanthroline is combined with various isomers of dinitrobenzoate anions, different arrangements of non-covalent interactions are observed in the complex structures. In the case of the compound with the 2,4-dinitrobenzoate anion, these interactions lead to a significant distortion of the metal core geometry and formation of a polymeric structure, while the complex with the 3,5-dinitrobenzoate anion has a structure that is typical of similar systems. The absence of europium metal-centered luminescence at 270 nm wavelength was shown. For all the reported compounds, a thermal stability study was carried out that showed that the compounds decomposed with a significant thermal effect.

Slow Magnetic Relaxation and Luminescent Properties of Mononuclear Lanthanide-Substituted Keggin-Type Polyoxotungstates with Compartmental Organic Ligands.

The reaction of mid to late lanthanide ions with the N,N′-dimethyl-N,N′-bis(2-hydroxy-3-formyl-5-bromobenzyl)ethylene-diamine organic ligand and monolacunary Keggin type [α-SiW11O39]8– anion affords a series of isostructural compounds, namely, K5[LnIII(α-SiW11O39)(C20H22Br2N2O4)]·14H2O (1-Ln, Ln = Sm to Lu). The molecular structure of these sandwich-type complexes is formed by the LnIII ion in a biaugmented trigonal prismatic geometry, which occupies the external O4 site of the organic ligand and the vacant site of the lacunary polyoxometalate (POM) unit. The empty N2O2 coordination site of the organic ligand allows its unprecedented folding, which displays a relative perpendicular arrangement of aromatic groups. Weak Br···Br and π–π interactions established between adjacent molecular units govern the crystal packing, which results in the formation of assemblies containing six hybrid species assembled in a chairlike conformation. 1-Gd and 1-Yb display slow relaxation of the magnetization after the application of an external magnetic field with maxima in the out-of-phase magnetic susceptibility plots below ∼5–6 K, which is ascribed to the presence of various relaxation mechanisms. Moreover, photoluminescent emission is sensitized for 1-Sm and 1-Eu in the visible region and 1-Er and 1-Yb in the NIR. In contrast, the quenching of metal-centered luminescence in the 1-Tb derivative has been attributed to the out-of-pocket coordination mode of the lanthanide center within the POM fragment. It is demonstrated that the 1-Yb dual magneto-luminescent material represents the first lanthanide-containing POM reported to date with simultaneous slow magnetic relaxation and NIR emission. Solution stability of the hybrid molecular species in water is also confirmed by ESI-mass spectrometry experiments carried out for 1-Tb and 1-Tm.

Facile solvothermal syntheses of isostructural lanthanide(III) formates: Photocatalytic, photoluminescent chemosensing properties, and proficient precursors for metal oxide nanoparticles

Three isostructural lanthanide coordination polymers (CPs) with the formula C3H3LnO6 [Ln = Pr (1), Ce (2), and Dy (3)] were solvothermally synthesized by self-assembly of lanthanide(III) and in-situ generated methanoic acid by the hydrolysis of N,N-dimethyl formamide. These CPs were characterized by FTIR, elemental analysis, PXRD, and single crystal analysis. The metal centered luminescence properties of these coordination polymers revealed that only the dysprosium based CP exhibited significant luminescence, and it was found to be selective for chemosensing of picric acid. 3 showed quenching efficiency of 89.7% with KSV value of 2.89 × 104 M−1. These coordination polymers were also exploited as photocatalysts for degradation of methylene blue, and the dysprosium based CP proved to be a fantastic photocatalyst with the degradation activity of 92% owing to its effective electronic transitions. The solvent-free thermolysis of these LnCPs fabricated CeO2 and Pr6O11 nanoparticles characterized by PXRD, FTIR, particle size analyzer, and elemental analysis.

A series of bis(2-phenethyl)dithiophosphinate-based Ln(III) complexes: Synthesis, magnetic and photoluminescent properties

A series of the bis(2-phenethyl)dithiophosphinate-based lanthanide(III) complexes, namely [Ln(phen)L3] (Ln = Sm (1), Eu (2)), Ln(phen)L2(NO3) (Ln = Sm (3), Eu (4), Tb (5)), and Et4N[LnL4] (Ln = Sm (6), Eu (7), Gd (8), Tb (9)), where L− = (PhCH2CH2)2PS2−, and phen = 1,10-phenanthroline, has been designed and synthesized. The magnetic properties of complexes 1–9 have been investigated in the temperature range of 2–300 K. The μeff(T) and 1/χ(T) dependences of complexes show paramagnetic behavior, which is determined by the Ln3+ ions. For complexes 5 and 9, significant anisotropy results in nonlinear field dependencies of magnetization at 2 K. Complexes 1–7 and 9 exhibit metal-centered luminescence in the solid state at room temperature, whereas complex 8 emits ligand-centered phosphorescence. The maximal quantum efficiency (φf = 53% at 300 K) is observed for the complex Tb(phen)L2(NO3).

Cathodoluminescence of organo-lanthanide complexes

The cathodoluminescence (CL) of organo-lanthanide complexes is studied as a modeling of beta luminescence. It is found that complexes of Sc, La, and lanthanides with anionic ÔO, ÔN bidentate and O-monodentate ligands under excitation by fast electrons display luminescence of various intensities. In the spectra of Sc, La, Nd, Er, and Tm compounds, only broadened bands of the ligands peaked about 450 nm are observed. A similar spectrum gives anthracene, which is used as a reference compound. In contrast, the emission spectra of Eu and Tb compounds contain only the narrow bands of f–f transitions characteristic of these metals. The spectra of Sm, Dy, and Yb derivatives exhibit both the bands of ligands and weak metal-centered luminescence. X-ray luminescence (RL) of several terbium complexes is also investigated. A comparison of the photoluminescence, CL and RL spectra of the studied compounds reveals a similarity of the shape of the spectra but a significant difference in their intensities. Terbium compounds show record scintillating activity. Possible mechanisms of luminescence of organo-lanthanide complexes upon different types of excitation are discussed.

Synthesis, crystal structure and luminescent properties of novel lanthanide complexes with CAPh type ligand diphenyl-N-benzoylamidophosphate and bidentate nitrogen donors

Herein we report the synthesis and investigations of the new lanthanide coordination compounds [LnL3bipy] (Ln1) and [LnL3phen] (Ln2) with carbacylamidophosphate type ligand diphenyl-N-benzoylamidophosphate (HL = [PhCONHPO(OPh)2]) and additional ligands 2,2′-bipyridine and 1,10-phenantroline. The obtained complexes were characterized by means of X-Ray, thermal gravimetric analysis, IR, 1H NMR, UV absorption spectroscopy and luminescence spectroscopy at 298 and 77 K. The X-ray diffraction analysis of Eu1 and Eu2 shows that the complexes crystalize in monoclinic crystal system, P 21/n space group. Efficient energy transfer from organic ligands after their excitation sensitizes the lanthanide interior emission from 5D0 or 5D4 levels of EuIII or TbIII respectively, leading to a strong metal-centered luminescence with decay time 1.5–2.0 ms. Intrinsic quantum yields (QLnLn) for europium complexes found to be equal 78 (Eu1) and 70% (Eu2). Overall QY equals 46 and 42% respectively.

Lanthanide(III) (Eu, Gd, Tb, Dy) Complexes Derived from 4-(Pyridin-2-yl)methyleneamino-1,2,4-triazole: Crystal Structure, Magnetic Properties, and Photoluminescence.

The reaction of lanthanide(III) nitrates with 4-(pyridin-2-yl)methyleneamino-1,2,4-triazole (L) was studied. The compounds [Ln(NO3 )3 (H2 O)3 ]⋅2 L, in which Ln=Eu (1), Gd (2), Tb (3), or Dy (4), obtained in a mixture of MeCN/EtOH have the same structure, as shown by XRD. In the crystals of these compounds, the mononuclear complex units [Ln(NO3 )3 (H2 O)3 ] are linked to L molecules through intermolecular hydrogen-bonding interactions to form a 2D polymeric supramolecular architecture. An investigation into the optical characteristics of the Eu3+ -, Tb3+ -, and Dy3+ -containing compounds (1, 3, and 4) showed that these complexes displayed metal-centered luminescence. According to magnetic measurements, compound 4 exhibits single-ion magnet behavior, with ΔEeff /kB =86 K in a field of 1500 Oe.

Six isostructural lanthanide-containing MOFs built on a semi-rigid tripodal organic ligand

By using a semi-rigid tripodal ligand 1,1′,1′′-(2,4,6-trimethylbenzene-1,3,5-triyl)tris(methylene)tripyridinium-4-olate) (L), six isomorphous lanthanum metal-organic frameworks (MOFs), namely, [Pr(L)2(Cl)2]·(Cl)(DMF)0.5·(H2O)·(1), [Nd(L)2(Cl)2]·(Cl)(2), [Tb(L)2(Cl)2]·(Cl)·(DMF)·(H2O)(3), [Eu(L)2(Cl)2]·(Cl)(DMF)·(H2O)(4), [La(L)2(Cl)2]·(Cl)(5), [Sm(L)2(Cl)2]·(Cl) (DMF)·(H2O) (6), were obtained and characterized by single crystal X-ray diffraction, IR and elemental analysis. Six compounds are isostructural with Ln3+ (Ln=Pr, Nd, Tb, Eu, La, Sm) and in which the Ln3+ is considered as a 6-connecting node and the L ligand is a 3-connecting node to afford a (3,6)-connected 2D layer with kgd topology, which further stacks into 3D supramolecular networks through C-H…O weak interactions. Luminescent properties of these lanthanide MOFs have also been assessed at ambient temperature, in which the Pr3+-1 and the La3+-5 are slightly blue-shifted with respect to the ligand. Series of sharp peaks characteristic of the Eu3+-4, the Tb3+-3 and Sm3+−6 metal-centered luminescence appear. The Nd3+-2 has no emission.

Ligand Design for Luminescent Lanthanide-Containing Metallopolymers.

The isolation of emissive materials with lanthanide ions is a topic of interest for imaging, display, and sensing applications, and it requires tuning of the electronics of the systems, such that, during the process of sensitization of the metal-centered luminescence, energy transfer from the sensitizer to the lanthanide ion is efficient and the resulting materials have the characteristics required for a given application. We discuss here our group's work in controlling the singlet- and triplet-state energies of sensitizing ligands through the choice of functional groups on the pyridine rings chelating to the lanthanide ions and show how we achieved progressively higher emission efficiencies. We describe targeted functionalization that led to highly emissive systems in solution and in the solid state and to new metallopolymers with norbornene or dicyclopentadiene-based backbones and pyridinebis(oxazoline) pendants, which display red EuIII-based, green TbIII-based, and, for the first time, blue TmIII-based emission.

F-Element coordination and extraction selectivity of a carbamoylmethylphosphine oxide ligand based on a tripodal phosphine oxide scaffold

We report here the synthesis and study of a multipodal ligand bearing three CMPO units attached to a trialkylphosphine oxide cap along with the resultant Ln3+, Th4+ and UO22+ metal complexes. This tripodal ligand demonstrates extraction efficiencies of 47% and 44% for the actinide species Th(NO3)4 and UO2(NO3)2 out of 1M nitric acid into a solution of dichloromethane, while all lanthanide nitrates were extracted with relatively low efficiencies. Further extraction studies utilizing 5M nitric acid solutions resulted in nearly quantitative actinide extraction. The ability of this ligand to sensitize the metal-centered luminescence of Eu3+ and Tb3+ in DMSO solutions was also investigated. Finally, possible conformations for the structure of the [La(NO3)-ligand]2+ complex were calculated using density functional theory.

Ln3I5(S2N2)(S2)(THF)10 - a new type of molecular compounds.

Unprecedented complexes of the composition Ln3I5(S2N2)(S2)(THF)10 were obtained in the reactions of neodymium and dysprosium iodide-nitrides with sulfur. The inorganic core of the molecules contains the cyclic fragments Ln(μ-S2)Ln, LnSNSN and LnSN. Ten of the fourteen atoms of the core are coplanar, the remaining four S2 and I2 atoms lie in the other two orthogonal planes. The dysprosium complex upon excitation with UV light exhibits the metal-centered luminescence characteristic of the Dy(3+) ion. Geometric parameters of the molecules, computational data, electron spectroscopy and fluorescence suggest the existence of some conjugation in the mentioned heterocycles.

Analysis of the Role of Peripheral Ligands Coordinated to Zn(II) in Enhancing the Energy Barrier in Luminescent Linear Trinuclear Zn-Dy-Zn Single-Molecule Magnets.

Three new Dy complexes have been prepared according to a complex-as-ligand strategy. Structural determinations indicate that the central Dy ion is surrounded by two LZn units (L(2-) is the di-deprotonated form of the N2 O2 compartmental N,N'-2,2-dimethylpropylenedi(3-methoxysalicylideneiminato) Schiff base. The Dy ions are nonacoordinate to eight oxygen atoms from the two L ligands and to a water molecule. The Zn ions are pentacoordinate in all cases, linked to the N2 O2 atoms from L, and the apical position of the Zn coordination sphere is occupied by a water molecule or bromide or chloride ions. These resulting complexes, formulated (LZnX)-Dy-(LZnX), are tricationic with X=H2 O and monocationic with X=Br or Cl. They behave as field-free single-molecule magnets (SMMs) with effective energy barriers (Ueff ) for the reversal of the magnetization of 96.9(6) K with τ0 =2.4×10(-7) s, 146.8(5) K with τ0 =9.2×10(-8) s, and 146.1(10) K with τ0 =9.9×10(-8) s for compounds with ZnOH2 , ZnBr, and ZnCl motifs, respectively. The Cole-Cole plots exhibit semicircular shapes with α parameters in the range of 0.19 to 0.29, which suggests multiple relaxation processes. Under a dc applied magnetic field of 1000 Oe, the quantum tunneling of magnetization (QTM) is partly or fully suppressed and the energy barriers increase to Ueff =128.6(5) K and τ0 =1.8×10(-8) s for 1, Ueff =214.7 K and τ0 =9.8×10(-9) s for 2, and Ueff =202.4 K and τ0 =1.5×10(-8) s for 3. The two pairs of largely negatively charged phenoxido oxygen atoms with short DyO bonds are positioned at opposite sides of the Dy(3+) ion, which thus creates a strong crystal field that stabilizes the axial MJ =±15/2 doublet as the ground Kramers doublet. Although the compound with the ZnOH2 motifs possesses the larger negative charges on the phenolate oxygen atoms, as confirmed by using DFT calculations, it exhibits the larger distortions of the DyO9 coordination polyhedron from ideal geometries and a smaller Ueff value. Ab initio calculations support the easy-axis anisotropy of the ground Kramers doublet and predict zero-field SMM behavior through Orbach and TA-QTM relaxations via the first excited Kramers doublet, which leads to large energy barriers. In accordance with the experimental results, ab initio calculations have also shown that, compared with water, the peripheral halide ligands coordinated to the Zn(2+) ions increase the barrier height when the distortions of the DyO9 have a negative effect. All the complexes exhibit metal-centered luminescence after excitation into the UV π-π* absorption band of ligand L(2-) at λ=335 nm, which results in the appearance of the characteristic Dy(III) ((4) F9/2 →(6) HJ/2 ; J=15/2, 13/2) emission bands in the visible region.

Influence of symmetry on the luminescence and radiative lifetime of nine-coordinate europium complexes.

Homoleptic mononuclear nine-coordinate lanthanum(III) and europium(III) tris-complexes [Ln(N(∧)N(∧)O)3]·nH2O with two tridentate N-benzylbenzimidazole pyridine-2-carboxylates exhibit a rare C3-symmetry of the lanthanide coordination polyhedron in the solid state, as confirmed by luminescence spectroscopy and by X-ray crystallography (the three N(∧)N(∧)O ligands are arranged "up-up-up" around the lanthanide ion). The symmetry, however, is changed to the more common C1 upon dissolution of the complexes in dichloromethane, as revealed by luminescence spectroscopy (the three ligands are likely to be arranged "up-up-down"). The new europium complexes emit efficient ligand-sensitized metal-centered luminescence with excited-state lifetimes of 1.56-2.18 ms and quantum yields of 25-41% in the solid and in solution. The change of the symmetry from (a higher) C3 to (a lower) C1 alters the luminescence spectrum, shortens the radiative lifetime, and increases the luminescence efficiency of the europium complexes.

Erratum to: Influence of Uncoordinated Anion-Radical TCNQ∸ on the Photo- and Electroluminescence Properties of Eu3+ and Tb3+ Tris(pyrazolyl)borate Complexes

Complexes of Eu3+ and Tb3+ with tris(3-(2-pyridyl)pyrazolyl)borate (TpPy–) of composition [Tp 2 Ln]Tp Py and [Tp 2 Ln]TCNQ · show metal-centered luminescence even in the presence of efficient luminescence quencher such as TCNQ∸ as the counter-ion. The electroluminescence spectrum of an ITO/PVK/[Tp 2 Tb]TpPy/Al (PVK = poly(9-vinyl)carbazole) device showed Tb3+ emission. When [Tp 2 Ln]TCNQ · is used as the emitting layers presumably the emission of separate carbazole fragments of PVK was observed, which may be due to a breakdown in packing of the carbazole fragments upon the inclusion into stacks of the TCNQ∸ anion-radicals. Very weak emission of the Tb3+ ion was detected in the LED with the [Tp 2 Ln]TCNQ · complex.

A 2-D dysprosium-organic complex constructed from 6,7-dihydropyrido(2,3-d)pyridazine-5,8-dione and oxalic acid: synthesis, characterization and photoluminescence

A pyridine derivative 6,7-dihydropyrido(2,3-d)pyridazine-5,8-dione (H2PDH) and oxalic acid (H2ox) are reacted with dysprosium oxide to develop a new 2-D dysprosium–organic complex [Dy(PDH)(ox)0.5(H2O)2]. The single-crystal structure reveals that acylamino oxygens and coordinated waters develop a 3-D supramolecular network, which is composed of 2-D layers, and each 2-D layer is formed by 1-D Dy–H2O–ox chains linked by tridentate PDH2− ligands. The complex exhibits metal-centered luminescence with yellowish blue emission. The energy transfer mechanism and photoluminescence are also investigated.

Lanthanide(III) compounds with the N2O4-donor Schiff base – Synthesis, spectral, thermal, magnetic and luminescence properties

New Schiff base complexes [Ln(NO3)3H2L] (where: Ln=Gd (1), Tb (2), Dy (3), Ho (4) and Er (5), H2L=N,N′-bis(5-bromo-3-methoxysalicylidene)propylene-1,3-diamine have been synthesized and characterized using elemental analysis, FTIR spectroscopy, thermogravimetric methods (TG–DSC), magnetic measurements, UV–Vis and luminescence studies. The compounds 1–5 follow the Curie–Weiss law all through the investigated temperature range 1.8–300K. The 1 shows behavior characteristic for the well-isolated mononuclear system whereas the magnetic properties of 2–5 are dominated by the crystal field effect on the Ln(III) site, masking the magnetic interaction between the paramagnetic centers. The Tb(III) complex 2 emits the characteristic metal centered luminescence.