Functional luminescent lanthanide complexes: Modulation of visible luminescence from europium complexes

The syntheses of three new ligands (L^1-3), which are based upon a DO3A core and appended with additional receptor sites for metal cations, are described, together with their corresponding Eu(III) complexes (Eu-L^1-3). The complexes are visibly luminescent in aqueous solution, following sensitization via the pyridine chromophore, showing characteristic narrow line-like emission from Eu(III). The luminescence properties show that water is effectively excluded from the inner coordination sphere of europium (q = 0). Each of the complexes showed perturbed luminescence properties upon addition of a variety of d-block metal ions. For example, emission quenching was observed for each complex following addition of Cr(III) and Cu(II). Selectivity towards Hg(II) (over Cd(II), Cu(II) and Zn(II)) was demonstrated with Eu-L³, which possesses a receptor site incorporating a softer thiophene moiety. More specifically, Hg(II) binding resulted in changes in the form of the steady state emission spectrum, together with a corresponding reduction of the luminescence lifetime in water, which can be attributed to an increase in inner sphere hydration (q = 2) and thus enhanced non-radiative deactivation of the ⁵D₀ state by proximate O-H oscillators.     

Integrated X-ray crystallography, optical and computational methods in studies of structure and luminescence of new synthesized complexes of lanthanides with ligands derived from 2,6-diformylpyridine

The reaction of 2,6-diformylpyridine-bis(benzoylhydrazone) [dfpbbh] and 2,6-diformylpyridine-bis(4-phenylsemicarbazone) [dfpbpsc] with lanthanides salts yielded the new chelates complexes [Eu(dfpbpsc-H⁺)₂]NO₃ (1), [Dy(fbhmp)₂][Dy(dfpbbh-2H⁺)₂]·2EtOH·2H₂O (fbhmp = 2-formylbenzoylhydrazone-6-methoxide-pyridine; Ph = phenyl; Py = pyridine; Et = ethyl) and [Er₂(dfpbbh-2H⁺)₂(μ-NO₃)(H₂O)₂(OH)]·H₂O.X-ray diffraction analysis was employed for the structural characterization of the three chelate complexes. In the case of complex 1, optical, synthetic and computational methods were also exploited for ground state structure determinations and triplet energy level of the ligand and HOMO-LUMO calculations, as well as for a detailed study of its luminescence properties.     

Trinuclear copper(I) acetylide complexes bearing carbonyl moiety: Synthesis, characterization, and photophysical properties

A series of trinuclear copper(I) acetylide complexes with carbonyl moiety, [Cu₃(μ-dppm)₃(μ₃-η¹-CCC(O)R)₂](ClO₄) (R = H (1), CH₃ (2), OCH₃ (3), NH₂ (4), NEt₂ (5)) (dppm = bis(diphenylphosphino)methane), have been synthesized and characterized. The crystal structures of [Cu₃(μ-dppm)₃(μ₃-η¹-CCC(O)CH₃)₂](ClO₄) (2) and [Cu₃(μ-dppm)₃(μ₃-η¹-CCC(O)NH₂)₂](ClO₄) (4) were determined by X-ray diffraction. The photophysical properties of complexes 1−5 have been studied. Complexes 1−5 show luminescence both in the solid state and in acetonitrile solution at 298 K, and their emission energies are in the order: 5 > 4 > 3 > 2 > 1. Density function theory (DFT) calculations at the hybrid Perdew, Burke, and Ernzerhof functional (PBE1PBE) level were performed on model complex 1 to elucidate the emission origin of complexes 1−5.     

Synthesis,Structures, and Luminescent Properties of Lanthanide Complexes with Triphenylphospine Oxide

Seven lanthanide complexes [Ln(OPPh₃)₃(NO₃)₃] ( 1 – 3 ) (OPPh₃ = triphenylphosphine oxide, Ln = Nd, Sm, Gd), [Dy(OPPh₃)₄(NO₃)₂](NO₃) ( 4 ), [Ln(OPPh₃)₃(NO₃)₃]₂ ( 5 – 7 ) (Ln = Pr, Eu, Gd) were synthesized by the reactions of different lanthanide salts and OPPh₃ ligand in the air. These complexes were characterized by single‐crystal X‐ray diffraction analysis, elemental analysis, IR and fluorescence spectra. Structure analysis shows that complexes 1 – 4 are mononuclear complexes formed by OPPh₃ ligands and nitrates. The asymmetric units of complexes 5 – 7 consist of two crystallographic‐separate molecules. Complex 1 is self‐assembled to construct a 2D layer‐structure of (4,4) net topology by hydrogen bond interactions. The other complexes show a 1D chain‐like structure that was assembled by OPPh₃ ligands and nitrate ions through C–H ··· O interactions. Solid emission spectra of compounds 4 and 6 are assigned to the characteristic fluorescence of Tb³⁺ (λ_em = 480, 574 nm) and Eu³⁺ (λ_em = 552, 593, 619, 668 nm).     

Lanthanide complexes of new polyaminocarboxylates with the bis-pyrazolylpyrimidine chromophore

We describe the straightforward synthesis of a new polyacid ligand derived from the bis-pyrazolylpyrimidine motif that displayed excellent properties concerning its ability to sensitize the emission of lanthanides. Emission lifetimes were in the range of ms and quantum yields were 0.15 and 0.85 for Eu and Tb, respectively, among the largest measured by us in related compounds, thus making this ligand an excellent candidate for its application in multitagging time-resolved fluoroimmunoassays.     

Synthesis,Structures, and Properties of Lanthanide Complexes with Pyrimidine‐2‐carboxylic Acid

Six lanthanide complexes [Ln(pmc)₂NO₃]_n [Hpmc = pyrimidine‐2‐carboxylic acid, Ln = La ( 1 ), Pr ( 2 )], [Ln(pmc)₂(H₂O)₃]NO₃ · H₂O [Ln = Eu ( 3 ), Tb ( 4 ) Dy ( 5 ), Er ( 6 )] were synthesized by the reactions of lanthanide nitrate and pyrimidine‐2‐carboxylic acid in water at room temperature. These complexes were characterized by single‐crystal X‐ray diffraction analysis, elemental analysis, IR, circular dichroism (CD) and fluorescence spectra. Structure analysis shows that complexes 1 and 2 are isostructural with P4₃2₁2 space group, whereas isostructural complexes 3 – 6 belong to the P2₁/c space group. In complexes 1 and 2 , the central metal atoms are coordinated by nitrates and pmc^–, which are self‐assembled to construct a 3D porous network with 6₂.6₂.6₂.6₂.6₂.6₂ (6⁶) topology. In complexes 3 – 6 , H₂O and pmc^– ligands are coordinated and the complexes exhibit a one‐dimensional zigzag chain, which is further expanded into a 3D structure by hydrogen bonding. In addition, the circular dichroism of 1 and 2 proves that the two complexes are both chiral with achiral ligand of Hpmc. Luminescent measurements of compounds 3 – 5 indicate that the characteristic fluorescence of Eu³⁺, Tb³⁺, and Dy³⁺ are observed.     

Synthesis,characterization and photoluminescence of 8-oxyquinolinato organoboron complexes derived from pyrazole

A one-pot protocol was developed for the synthesis of a series of luminescent heteroleptic diaryldiborinic complexes containing the central aryl ring bonded to two boron atoms substituted with pyrazole and complexed with 8-hydroxyquinoline. The luminescent properties of these compounds were measured. In dilute solutions they exhibited an emission at ca. 513 nm with quantum yields of 22–27% which are typical for borinic 8-hydroxyquinoline complexes. The only exception was the complex containing the bithiophene scaffold, for which no fluorescence was observed. The obtained pyrazole-based complexes show improved solubility and thermal stability with respect to their phenyl analogues. The experimental UV–vis absorption and emission data are supported by theoretical calculations of the frontier molecular orbitals, revealing the aromatic linker to quinolinato ligand excitation mechanism.     

Hydrothermal Syntheses,Crystal Structures,and Luminescent Properties of Three Organic‐Lanthanide Complexes with 3‐Hydroxycinnamic Acid

Three new homodinuclear lanthanide(III) complexes [Ln₂(L)₆(2,2′‐bipy)₂] [Ln = Tb^III ( 1 ), Sm^III ( 2 ), Eu^III ( 3 ); HL = 3‐hydroxycinnamic acid (3‐HCA); 2,2′‐bipy = 2,2′‐bipyridine] were synthesized and characterized by IR spectroscopy, elemental analyses, and X‐ray diffraction techniques. Complexes 1 – 3 crystallize in triclinic system, space group P$\bar{1}$ . In all complexes the lanthanide ions are nine‐coordinate by two nitrogen atoms from the 2,2′‐bipy ligand and seven oxygen atoms from one chelating L ligands and four bridging L ligands, forming distorted tricapped trigonal prismatic arrangements. The lanthanide(III) ions are intramolecularly bridged by eight carboxylate oxygen atoms forming dimeric complexes with Ln ··· Ln distances of 3.92747(15), 3.9664(6), and 3.9415(4) Å for complexes 1 – 3 , respectively. The luminescent properties in the solid state of HL ligand and Eu^III complex are also discussed.     

Modulating the photophysical properties of azamacrocyclic europium complexes with charge-transfer antenna chromophores

Two europium complexes with bis(bipyridine) azamacrocyclic ligands featuring pendant arms with or without π-conjugated donor groups are synthesized and fully characterized by theoretical calculations and NMR spectroscopy. Their photophysical properties, including two-photon absorption, are investigated in water and in various organic solvents. The nonfunctionalized ligand gives highly water-stable europium complexes featuring bright luminescence properties but poor two-photon absorption cross sections. On the other hand, the europium complex with an extended conjugated antenna ligand presents a two-photon absorption cross section of 45 GM at 720 nm but is poorly luminescent in water. A detailed solvent-dependent photophysical study indicates that this luminescence quenching is not due to the direct coordination of O-H vibrators to the metal center but to the increase of nonradiative processes in a protic solvent induced by an internal isomerization equilibrium.     

Lanthanide complexes of new nonadentate imino-phosphonate ligands derived from 1,4,7-triazacyclononane: synthesis, structural characterisation and NMR studies

The polyamino ligand 1,4,7-tris(2-aminoethyl)-1,4,7-triazacyclononane (1) has been used to synthesise two new ligands by Schiff-base condensation with methyl sodium acetyl phosphonate to give ligand L and methyl sodium 4-methoxybenzoyl phosphonate to give ligand L1 in the presence of lanthanide ion as templating agent to form the complexes [Ln(L)] and [Ln(L1)](Ln = Y, La, Gd, Yb). Both ligands L and L1 have nine donor atoms comprising three amine and three imine N-donors and three phosphonate O-donors and form Ln(III) complexes in which the three pendant arms of the ligands wrap around the nine-coordinate Ln(III) centres. Complexes with Y(III), La(III), Gd(III) and Yb(III) have been synthesised and the complexes [Y(L)], [Gd(L)] and [Gd(L1)] have been structurally characterised. In all the complexes the coordination polyhedron about the lanthanide centre is slightly distorted tricapped trigonal prismatic with the two triangular faces of the prism formed by the macrocyclic N-donors and the phosphonate O-donors. Interestingly, given the three chiral phosphorus centres present in [Ln(L)] and [Ln(L1)] complexes, the three crystal structures reported show the presence of only one diastereomer of the four possible. 1H, 13C and 31P NMR spectroscopic studies on diamagnetic [Y(L)] and [La(L)] and on paramagnetic [Yb(L)] complexes indicate the presence in solution of all the four different diastereomers in varying proportions. The stability of complexes [Y(L)] and [Y(L1)] in D2O in both neutral and acidic media, and the relaxivity of the Gd(III) complexes, have also been investigated.     

Two Isostructural Layered Lanthanide(III) Complexes: Syntheses,Structures, Magnetic and Luminescent Properties

Two N'-(2-hydroxybenzylidene)pyridine N-oxide-carbohydrazide (H₃L)-based coordination complexes with the formula [Ln₂(DMF)₂(OAc)₂(HL)₂]_n (Ln = Dy for 1 and Eu for 2 ) were solvothermally synthesized. Crystal structures, thermal stabilities, magnetic and luminescent properties of the two complexes were fully investigated. Both complexes are isomorphic two-dimensional layers with centrosymmetric {Ln₂} subunits extended by doubly deprotonated HL^2– connectors. Complex 1 with highly anisotropic Dy^III spin exhibits slightly frequency-dependent magnetic relaxations under zero dc field with an effective energy barrier of ca. 6.84 K. Eu^III-based complex 2 displays only one weak fluorescent emission around 532 nm with the absence of characteristic emission of Eu^III ion. These results provide helpful hints of the hydrazide Schiff-functionalized organic ligands on the function modulations of the resulting Ln complexes.     

Three Isostructural 4‐Sulfophthalate‐Based Lanthanide Complexes: Syntheses,Crystal Structures,and Luminescent Properties

Three lanthanide complexes with the ligand 4‐sulfophthalate (sp^3–), [Ln(H₂O)₂(sp)]_n [Ln = Dy ( 1 ), Tb ( 2 ), and Er ( 3 )], were solvo‐/hydrothermally synthesized by changing the rare earth cations, and were characterized structurally and photophysically. Complexes 1 – 3 are isostructural, exhibiting a two‐dimensional layered structure with centrosymmetric dinuclear subunits infinitely extended by 4‐connected sp^3– connectors. Photoluminescence spectra of 1 – 3 demonstrate that anionic sp^3– ligand can serve as a functionalized chromophore to sensitize the luminescent emission of the lanthanide ion, suggesting that the sp^3–‐involved lanthanide complexes can be used as novel optical materials.     

Lanthanide complexes with oda, ida, and nta: From discrete coordination compounds to supramolecular assemblies

Lanthanide metal–organic frameworks have been receiving special attention for the last decade. The use of polydentate organic ligands has allowed the construction of interesting network topologies with many potential applications, based on the special characteristics of the 4f electrons. In this paper, the structural and thermodynamic properties of Ln(III) complexes with oxydiacetate, iminodiacetate, and nitrilotriacetate are critically reviewed. The presence of ligands able to act as a bridge promotes the formation of Ln–M polynuclear complexes with fascinating 3D structures, whose study is also included. Equilibrium data of these systems in aqueous solution have been collected, showing the relationship between the composition of the species in solution and the nuclearity in the solid state.     

Bimetallic Pt(II)-bipyridyl-diacetylide/Ln(III) tris-diketonate adducts based on a combination of coordinate bonding and hydrogen bonding between the metal fragments: Syntheses,structures and photophysical properties

The luminescent Pt(II) complex [Pt(4,4′-^tBu₂-bipy){CC-(5-pyrimidinyl)}₂] (1) was prepared by coupling of [Pt(4,4′-^tBu₂-bipy)Cl₂] with 5-ethynyl-pyrimidine, and contains two pyrimidinyl units pendant from a Pt(II) bipyridyl diacetylide core; it shows luminescence at 520 nm which is typical of Pt(II) luminophores of this type. Reaction with [Ln(hfac)₃(H₂O)₂] (hfac = anion of hexafluoroacetylacetone) affords as crystalline solids the compounds [1 · {Ln(hfac)₃(H₂O)}{Ln(hfac)₃(H₂O)₂}] (Ln = Nd, Gd, Er, Yb), in which the {Ln(hfac)₃(H₂O)} unit is coordinated to one pyrimidine ring via an N atom, whereas the {Ln(hfac)₃(H₂O)₂} unit is associated with two N atoms, one from each pyrimidine ring of 1, via N?HOH hydrogen-bonding interactions involving the coordinated water ligands on the lanthanide centre. Solution spectroscopic studies show that the luminescence of 1 is partly quenched on addition of [Ln(hfac)₃(H₂O)₂] (Ln = Er, Nd) by formation of Pt(II)/Ln(III) adducts in which Pt(II)→Ln(III) photoinduced energy-transfer occurs to the low-lying f–f levels of the Ln(III) centre. Significant quenching occurs with both Er(III) and Nd(III) because both have several f–f states which match well the ³MLCT emission energy of 1. Time-resolved luminescence studies show that Pt(II)→Er(III) energy-transfer (7.0 × 10⁷ M⁻¹) is around three times faster than Pt(II)→Nd(III) energy-transfer (≈2 × 10⁷ M⁻¹) over the same distance because the luminescence spectrum of 1 overlaps better with the absorption spectrum of Er(III) than with Nd(III). In contrast Yb(III) causes no significant quenching of 1 because it has only a single f–f excited level which is a poor energy match for the Pt(II)-based excited state.     

Synthesis,Crystal Structures,and Photoluminescence of Lanthanide Coordination Polymers with 4‐Acetamidobenzoate

The reactions of Ln(NO₃)₃ · 6H₂O and 4‐acetamidobenzoic acid (Haba) with 4,4′‐bipyridine (4,4′‐bpy) in ethanol solution resulted in three new lanthanide coordination polymers, namely {[Ln(aba)₃(H₂O)₂] · 0.5(4,4′‐bpy) · 2H₂O}_∞ [Ln = Sm ( 1 ), Gd ( 2 ), and Er ( 3 ), aba = 4‐acetamidobenzoate]. Compounds 1 – 3 are isomorphous and have one‐dimensional chains bridged by four aba anions. 4,4′‐Bipyridine molecules don’t take part in the coordination with Ln^III ions and occur in the lattice as guest molecules. Moreover, the adjacent 1D chains in the complex are further linked through numerous N–H ··· O and O–H ··· O hydrogen bonds to form a 3D supramolecular network. In addition, complex 1 in the solid state shows characteristic emission in the visible region at room temperature.     

Magnetic and Luminescence Properties of Two Dinuclear Lanthanide Complexes with Butterfly‐like Arrangement

Two dinuclear lanthanide complexes with pentadentate ligand 3‐[bis(pyridine‐2‐ylmethyl)amino]propane‐1,2‐diol (H₂L), formulated as [Ln₂(HL)₂(NO₃)₂(H₂O)₂] · 1.5NO₃ · 0.5I [Ln = Tb ( ZTU‐1 ) and Eu ( ZTU‐2 ); ZTU = Zhaotong University] were synthesized and structurally characterized. ZTU‐1 and ZTU‐2 are isomorphous and feature a butterfly‐like arrangement. The fluorescence properties of ZTU‐1 and ZTU‐2 are investigated and slow magnetic relaxation behavior is observed in ZTU‐1 .     

Porphyrin-appended europium(III) bis(phthalocyaninato) complexes: synthesis, characterization, and photophysical properties

Mixed cyclization of 3-mono-, 4-mono-, or 4,5-di(porphyrinated) phthalonitrile compounds 2, 3, or 6 and unsubstituted phthalonitrile with the half-sandwich complex [EuIII(acac)(Pc)] (Pc=phthalocyaninate, acac=acetylacetonate) as the template in the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) in n-pentanol afforded novel porphyrin-appended europium(III) bis(phthalocyaninato) complexes 7-9 in 30-40% yield. These mixed tetrapyrrole triads and tetrad were spectroscopically and electrochemically characterized and their photophysical properties were also investigated with steady-state and transient spectroscopic methods. It has been found that the fluorescence of the porphyrin moiety is quenched effectively by the double-decker unit through an intramolecular photoinduced electron-transfer process, which takes place in several hundred femtoseconds, while the recombination of the charge-separated state occurs in several picoseconds. By using different phthalocyanines containing different numbers of porphyrin substituents at the peripheral or nonperipheral position(s) of the ligand, while the other unsubstituted phthalocyanine remains unchanged in these double-deckers, the effects of the number and the position of the porphyrin substituents on these photophysical processes were also examined.     

Excited State Properties of Lanthanide(III) Complexes with a Nonadentate Bispidine Ligand

Eu^III, Tb^III, Gd^III and Yb^III complexes of the nonadentate bispidine derivative L² (bispidine=3,7-diazabicyclo[3.3.1]nonane) were successfully synthesized and their emission properties studied. The X-ray crystallography reveals full encapsulation by the nonadentate ligand L² that enforces to all Ln^III cations a common highly symmetrical capped square antiprismatic (CSAPR) coordination geometry (pseudo C_4v symmetry). The well-resolved identical emission spectra in solid state and in solution confirm equal structures in both media. As therefore expected, this results in long-lived excited states and high emission quantum yields ([Eu^IIIL²]⁺, H₂O, 298 K, τ=1.51 ms, ϕ=0.35; [Tb^IIIL²]⁺, H₂O, 298 K, τ=1.95 ms, ϕ=0.68). Together with the very high kinetic and thermodynamic stabilities, these complexes are a possible basis for interesting biological probes.     

Triphenylarsane Oxide Complexes of Lanthanide Nitrates: Polymorphs and Photophysics

trans‐[Ln(NO₃)₂(Ph₃AsO)₄](NO₃)₂ ( 1 ) and mer‐[Ln(NO₃)₃(Ph₃AsO)₃] ( 2 ) complexes were prepared from Ln(NO₃)₃ · xH₂O and Ph₃AsO in chloroform (Ln = Y, Sm, Eu, Tb, and Dy). Production of complexes 1 vs. 2 and solvent content was found to be highly dependent on crystallization solvent choice. Tb and Eu produced only 1 , while the other Ln metals produced both 1 and 2 . Solvent‐free, acetone‐, and methanol‐containing polymorph series were identified for complexes 1 . Acetone/ether‐ and CH₂Cl₂‐containing polymorph series were identified for complexes 2 . Luminescence measurements were performed on solvent‐free 1 (Ln = Y, Eu, Tb, and Dy) and 2 (Ln = Sm) at 78 K. Sensitized lanthanide emission bands via resonance energy transfer were observed in all cases, except the control (Ln = Y). The efficiency of this energy transfer process varies amongst the lanthanide metals studied and was rationalized using Latva's empirical rule and Density Functional Theory calculations.