Luminescent ruthenium(II) bipyridine-calix[4]arene complexes as receptors for lanthanide cations

The synthesis and photophysical properties of novel luminescent ruthenium(II) bipyridyl complexes containing one, two, or six lower rim acid-amide-modified calix[4]arene moieties covalently linked to the bipyridine groups are reported which are designed to coordinate and sense luminescent lanthanide ions. All the Ru-calixarene complexes synthesized in this work are able to coordinate Nd(3+), Eu(3+), and Tb(3+) ions with formation of adducts of variable stoichiometry. The absorbance changes allow the evaluation of association constants whose magnitudes depend on the nature of the complexes as well as on the nature of the lanthanide cation. Lanthanide cation complex formation affects the ruthenium luminescence which is strongly quenched by Nd(3+) ion, moderately quenched by the Eu(3+) ion, and poorly or moderately increased by the Tb(3+) ion. In the case of Nd(3+), the excitation spectra show that (i) the quenching of the Ru luminescence occurs via energy transfer and (ii) the electronic energy of the excited calixarene is not transferred to the Ru(bpy)(3) but to the neodymium cation. In the case of Tb(3+), the adduct's formation leads to an increase of the emission intensities and lifetimes. The reason for this behavior was ascribed to the electric field created around the Ru calix[4]arene complexes by the Tb(3+) ions by comparison with the Gd(3+) ion, which behaves identically and can affect ruthenium luminescence only by its charge. However, especially for compounds 1 and 3, it cannot be excluded that some contribution comes from the decrease of vibrational motions (and nonradiative processes) due to the rigidification of the structure upon Tb(3+) complexation. In the case of Eu(3+), compounds 1, 2, and 4 were quenched by the lanthanide addition but the quenching of the ruthenium luminescence is not accompanied by europium-sensitized emission which suggests that an electron-transfer mechanism is responsible for the quenching. On the contrary, compound 3 exhibits enhanced emission upon addition of Eu(3+) (as nitrate salt); it is suggested that the lack of quenching in the [3.2Eu(3+)] adduct is due to kinetic reasons because the electron-transfer quenching process is thermodynamically allowed.     

Supramolecular coordination chemistry in aqueous solution: lanthanide ion-induced triple helix formation

The self-assembly of dinuclear triple helical lanthanide ion complexes (helicates), in aqueous solution, is investigated utilizing laser-induced, lanthanide luminescence spectroscopy. A series of dinuclear lanthanide (III) helicates (Ln(III)) based on 2,6-pyridinedicarboxylic acid (dipicolinic acid, dpa) coordinating units was synthesized by linking two dpa moieties using the organic diamines (1R,2R)-diaminocyclohexane (chxn-R,R) and 4,4'-diaminodiphenylmethane (dpm). Luminescence excitation spectroscopy of the Eu3+ 7F0-->5D0 transition shows the apparent cooperative formation of neutral triple helical complexes in aqueous solution, with a [Eu2L3] stoichiometry. Eu3+ excitation peak wavelengths and excited-state lifetimes correspond to those of the [Eu(dpa)3]3- model complex. CD studies of the Nd(III) helicate Nd2(dpa-chxn-R,R)3 reveal optical activity of the f-f transitions, indicating that the chiral linking group induces a stable chirality at the metal ion center. Molecular mechanics calculations using CHARMm suggest that the delta delta configuration at the Nd3+ ion centers is induced by the chxn-R,R linker. Stability constants were determined for both ligands with Eu3+, yielding identical results: log K = 31.6 +/- 0.2 (K in units of M-4). Metal-metal distances calculated from Eu3+-->Nd3+ energy-transfer experiments show that the complexes have metal-metal distances close to those calculated by molecular modeling. The fine structure in the Tb3+ emission bands is consistent with the approximate D3 symmetry as anticipated for helicates.     

Synthesis and structural properties of lanthanide complexes formed with tropolonate ligands

We have previously reported the unique luminescence properties of ML4 complexes formed between tropolonate ligands and a series of lanthanide cations, several of them emitting in the near-infrared domain. The synthesis and composition of ML4 lanthanide tropolonate complexes have been previously described in the literature, but no structural information has been available so far. In this work, the crystal structures of several lanthanide tropolonate complexes (Ln3+ = Tb3+, Dy3+, Ho3+, Er3+, Tm3+, Yb3+, Lu3+) have been isolated and systematically analyzed by X-ray diffraction and compared by using different criteria including the Kepert formalism. Such comparative work is rare in lanthanide coordination chemistry. The analysis of the structures in the solid state reveals that although the packing of the ML4 complexes depends on the nature of the metal ion, the coordination geometries around the different lanthanides is virtually similar for all the cations that have been analyzed; an indication that lanthanide-centered f orbitals play a role in controlling this coordination geometry. Analysis of the solution's behavior by stability constant determination reveals the formation of complexes with similar ML4 stoichiometries as those observed in the solid state. Nevertheless, analysis of the luminescence lifetimes indicates that the coordination environment around the lanthanide cations are different in the solid state and in solution, with the presence of one molecule of water bound to the lanthanide cation in solution. The presence of such a water molecule is a significant source of nonradiative deactivation of the excited states of the lanthanide cations, an unfavorable condition that leads to significant loss in fluorescence intensity of these lanthanide complexes. This exemplifies that such comparative analysis between the solid state and solution is important for the rationalization of the luminescence properties of the complexes. This analysis will aid us in optimizing ligand design for improved photophysical properties of the complex.     

Luminescent lanthanide ions hosted in a fluorescent polylysin dendrimer. Antenna-like sensitization of visible and near-infrared emission

We have investigated the complexation of the luminescent Nd(3+), Eu(3+), Gd(3+), Tb(3+), Er(3+), and Yb(3+) ions by a polylysin dendrimer containing 21 amide groups in the interior and, in the periphery, 24 chromophoric dansyl units which show an intense fluorescence band in the visible region. Most of the experiments were performed in 5:1 acetonitrile/dichloromethane solution at 298 K. On addition of the lanthanide ions to dendrimer solutions, the fluorescence of the dansyl units is quenched; in Nd(3+), Er(3+), and Yb(3+), a sensitized near-infrared emission of the lanthanide ion is observed. At low metal ion concentrations, each dendrimer hosts only one metal ion and when the hosted metal ion is Nd(3+) or Eu(3+), the fluorescence of all the 24 dansyl units of the dendrimer is quenched with unitary efficiency. Quantitative measurements were performed in a variety of experimental conditions, including protonation of the dansyl units and measurements in rigid matrix at 77 K where a sensitized Eu(3+) emission could also be observed. The results obtained have been interpreted on the basis of the energy levels and redox potentials of dendrimer and metal ions.     

Cationic lanthanide complexes of neutral tripodal N,O ligands: enthalpy versus entropy-driven podate formation in water

The cationic lanthanide complexes of two neutral tripodal N,O ligands, tpa (tris[(2-pyridyl)methyl]amine) and tpaam (tris[6-((2-N,N-diethylcarbamoyl)pyridyl)methyl]amine) are studied in water. The analysis of the proton lanthanide induced NMR shifts indicate that there is no abrupt structural change in the middle of the rare-earth series. Unexpectedly, the formation constant values of the lanthanide podates of tpaam and tpa in D2O at 298 K are similar, suggesting that the addition of the three amide groups to the ligand tpa does not lead to any increase in stability of the lanthanide complexes of tpaam in respect to tpa, even though the amide groups are coordinated to the metal in aqueous solution. The measurement of the enthalpy and entropy changes of the complexation reactions shows that the two similar ligands tpa and tpaam have different driving forces for lanthanide complexation. Indeed, the formation of tpa podates benefits from an exothermic enthalpy change associated with a small entropy change, whereas the complexation reaction with tpaam is clearly entropy-driven though opposed by a positive enthalpy change. The hydration states of the europium complexes were measured by luminescence and show the coordination of 4-5 water ligands in [Eu(tpa)]3+ whereas there are only 2 in [Eu(tpaam)]3+. Therefore the heptadentate ligand tpaam releases the translational entropy of more water molecules than does the tetradentate ligand tpa.     

A disymmetric terpyridine based ligand for the formation of luminescent di-aquo lanthanide complexes

The synthesis of ligand H3 based on a disymmetrically substituted terpyridine core functionalised by a carboxylic acid in the 6-position and a bis(carboxymethyl)aminomethyl function in the 6'-position is described. The coordination behaviour of this heptadentate (4N/3O) ligand with lanthanide cations (Ln=Eu, Gd and Tb) was studied in solution showing the formation of complexes with [Ln] stoichiometry. Complexes with general formula [Ln(H2O)2] were isolated from neutral water solutions containing equimolar amounts of cations and ligands, and the complexes were characterized in the solid state (elemental analysis, IR) and in solution (mass spectrometry). The photo-physical properties of the luminescent complexes of Eu and Tb were studied in water solution by means of absorption, steady state and time-resolved emission spectroscopies. Evolution of the luminescence lifetimes of the Eu and Tb complexes in H2O and D2O reveals the presence of two water molecules coordinated in the first coordination sphere of the cations. Despite this important hydration number, the overall luminescence quantum yields of the complexes remained elevated, especially in the case of Tb (Phi=22.0 and 6.5% respectively for Tb and Eu). Upon crystallisation the Gd complex formed dimeric species in which two gadolinium atoms are each heptacoordinated by one ligand, the coordination sphere being completed by a single water molecule and a bridging carboxylate function, pointing to different behaviours in the solid and liquid states.     

Luminescent lanthanide complexes with stereocontrolled tris(2-pyridylmethyl)amine ligands: chirality effects on lanthanide complexation and luminescence properties

A series of tris(2-pyridylmethyl)amines including one and two asymmetric centers were synthesized in a stereo-controlled fashion as potential ligands of lanthanide cations. The reaction of chiral pyridylethyl methanesulfonates and bis(pyridylmethyl)amines occurred via an S(N)2 mechanism with complete inversion of asymmetric centers and gave the stereocontrolled tris(2-pyridylmethyl)amines, the stereochemical purity of which was ascertained by GPC, NMR, X-ray, and polarimetry experiments. They formed stable Tb(3+) and Eu(3+) complexes having 1:1, 1:2, and 1:3 stoichiometry (metal:ligand) in CH(3)CN solutions. NMR and UV titration experiments revealed that their complexation behaviors were rarely influenced by ligand chirality but significantly affected by the nature of the counteranion and the concentration ratio of metal to ligand. The Tb(3+) and Eu(3+) complexes with these tripodal ligands exhibited characteristic luminescence spectra upon excitation for pyridine chromophores (260 nm), the intensities of which were largely dependent on the ligand chirality. The meso isomer of the disubstituted tripods particularly exhibited the enhanced terbium luminescence ca. three times more than its diastereomer and un- and monosubstituted tripods. Direct excitation at the lanthanide center had similar chirality effects on the luminescence profiles, indicating that the stereochemistry of the employed ligand largely influenced the lanthanide emitting processes. Since the ligand chirality finely modified the local coordination environments around the lanthanide center, the use of stereocontrolled ligands is applicable in design of the luminescent lanthanide complexes.     

A water-soluble Pybox derivative and its highly luminescent lanthanide ion complexes

A new water-soluble Pybox ligand, 1, has been synthesized and found to crystallize in the monoclinic P2(1)/n space group with unit cell parameters a = 6.0936(1) ?, b = 20.5265(4) ?, c = 12.0548(2) ?, and β = 90.614(1)°. In the crystal, a water molecule is bound through hydrogen-bonding interactions to the nitrogen atoms of the oxazoline rings. This ligand was used to complex a variety of lanthanide ions, opening up new avenues for luminescence and catalysis in aqueous environment. These complexes are highly luminescent in aqueous solutions, in acetonitrile, and in the solid state. Aqueous quantum yields are high at 30.4% for Eu(III), 26.4% for Tb(III), 0.32% for Yb(III), and 0.11% for Nd(III). Er(III) did not luminesce in water, but an emission efficiency of 0.20% could be measured in D(2)O. Aqueous emission lifetimes were also determined for the visible emitting lanthanide ions and are 1.61 ms for Eu(III) and 1.78 ms for Tb(III). Comparing emission lifetimes in deuterated and nondeuterated water indicates that no water molecules are coordinated to the metal ion. Speciation studies show that three species form successively in solution and the log β values are 5.3, 9.6, and 13.8 for Eu(III) and 5.3, 9.2, and 12.7 for Tb(III) for 1:1, 2:1, and 3:1 ligand to metal ratios, respectively.     

Modulation of luminescence intensity of lanthanide complexes by photoinduced electron transfer and its application to a long-lived protease probe

Luminescent lanthanide complexes (Tb(3+), Eu(3+), etc.) have excellent properties for biological applications, including extraordinarily long lifetimes and large Stokes shifts. However, there have been few reports of lanthanide-based functional probes, because of the difficulty in designing suitable complexes with a luminescent on/off switch. Here, we have synthesized a series of complexes which consist of three moieties: a lanthanide chelate, an antenna, and a luminescence off/on switch. The antenna is an aromatic ring which absorbs light and transmits its energy to the metal, and the switch is a benzene derivative with a different HOMO level. If the HOMO level is higher than a certain threshold, the complex emits no luminescence at all, which indicates that the lanthanide luminescence can be modulated by photoinduced electron transfer (PeT) from the switch to the sensitizer. This approach to control lanthanide luminescence makes possible the rational design of functional lanthanide complexes, in which the luminescence property is altered by a biological reaction. To exemplify the utility of our approach to the design of lanthanide complexes with a switch, we have developed a novel protease probe, which undergoes a significant change in luminescence intensity upon enzymatic cleavage of the substrate peptide. This probe, combined with time-resolved measurements, was confirmed in model experiments to be useful for the screening of inhibitors, as well as for clinical diagnosis.     

Highly luminescent, triple- and quadruple-stranded, dinuclear Eu, Nd, and Sm(III) lanthanide complexes based on bis-diketonate ligands

The bis(beta-diketone) ligands 1,3-bis(3-phenyl-3-oxopropanoyl)benzene, H(2)L(1) and 1,3-bis(3-phenyl-3-oxopropanoyl) 5-ethoxy-benzene, H(2)L(2), have been prepared for the examination of dinuclear lanthanide complex formation and investigation of their properties as sensitizers for lanthanide luminescence. The ligands bear two conjugated diketonate binding sites linked by a 1,3-phenylene spacer. The ligands bind to lanthanide(III) or yttrium(III) ions to form neutral homodimetallic triple stranded complexes [M(2)L(1)(3)] where M = Eu, Nd, Sm, Y, Gd and [M(2)L(2)(3)], where M = Eu, Nd or anionic quadruple-stranded dinuclear lanthanide units, [Eu(2)L(1)(4)](2-). The crystal structure of the free ligand H(2)L(1) has been determined and shows a twisted arrangement of the two binding sites around the 1,3-phenylene spacer. The dinuclear complexes have been isolated and fully characterized. Detailed NMR investigations of the complexes confirm the formation of a single complex species, with high symmetry; the complexes show clear proton patterns with chemical shifts of a wide range due to the lanthanide paramagnetism. Addition of Pirkle's reagent to solutions of the complexes leads to splitting of the peaks, confirming the chiral nature of the complexes. Electrospray and MALDI mass spectrometry have been used to identify complex formulation and characteristic isotope patterns for the different lanthanide complexes have been obtained. The complexes have high molar absorption coefficients (around 13 x 10(4) M(-1)cm(-1)) and display strong visible (red or pink) or NIR luminescence upon irradiation at the ligand band around 350 nm, depending on the choice of the lanthanide. Emission quantum yield experiments have been performed and the luminescence signals of the dinuclear complexes have been found to be up to 11 times more intense than the luminescence signals of the mononuclear analogues. The emission quantum yields and the luminescence lifetimes are determined to be 5% and 220 micros for [Eu(2)L(1)(3)], 0.16% and 13 micros for [Sm(2)L(1)(3)], and 0.6% and 1.5 micros for [Nd(2)L(1)(3)]. The energy level of the ligand triplet state was determined from the 77 K spectrum of [Gd(2)L(1)(3)]. The bis-diketonate ligand is shown to be an efficient sensitizer, particularly for Sm and Nd. Photophysical studies of the europium complexes at room temperature and 77 K show the presence of a thermally activated deactivation pathway, which we attribute to ligand-to-metal charge transfer (LMCT). Quenching of the luminescence from this level seems to be operational for the Eu(III) complex but not for complexes of Sm(III) and Nd(III), which exhibit long lifetimes. The quadruple-stranded europium complex has been isolated and characterized as the piperidinium salt of [Eu(2)L(1)(4)](2-). Compared with the triple-stranded Eu(III) complex in the solid state, the quadruple-stranded complex displays a more intense emission signal with a distinct emission pattern indicating the higher symmetry of the quadruple-stranded complex.     

Study of lanthanide complexes with salicylic acid by photoacoustic and fluorescence spectroscopy

Solid complexes Ln(Sal)3.H2O (Sal: salicylic acid; Ln: La3+, Nd3+, Eu3+, Tb3+) are synthesized, and their photoacoustic (PA) spectra in the UV-Vis region have been recorded. PA intensities of central lanthanide ions are interpreted in terms of the probability of nonradiative transitions. It is found that PA intensity of the ligand increases in the order of Tb(Sal)3.H2O < La(Sal3).H2O < Eu(Sal)3.H2O < Nd(Sal)3.H2O. Different PA intensities of the ligand are interpreted by comparison with the fluorescence spectra. Ternary complexes Eu(Sal)3Phen and Tb(Sal)3Phen (Phen: 1,10-phenanthroline) are synthesized. Compared with their binary complexes, PA intensity of the ligand Sal decreases for Eu(Sal)3Phen, while the reverse is true for that of Tb(Sal)3Phen. The luminescence of Eu3+ increases remarkably when Phen is introduced, and luminescence of Tb3+ decreases greatly when Phen is added. The intramolecular energy transfer and relaxation processes in the complexes are discussed from two aspects: radiative and nonradiative relaxations.     

Remarkable luminescence properties of lanthanide complexes with asymmetric dodecahedron structures

The distorted coordination structures and luminescence properties of novel lanthanide complexes with oxo‐linked bidentate phosphane oxide ligands—4,5‐bis(diphenylphosphoryl)‐9,9‐dimethylxanthene (xantpo), 4,5‐bis(di‐tert‐butylphosphoryl)‐9,9‐dimethylxanthene (tBu‐xantpo), and bis[(2‐diphenylphosphoryl)phenyl] ether (dpepo)—and low‐vibrational frequency hexafluoroacetylacetonato (hfa) ligands are reported. The lanthanide complexes exhibit characteristic square antiprism and trigonal dodecahedron structures with eight‐coordinated oxygen atoms. The luminescence properties of these complexes are characterized by their emission quantum yields, emission lifetimes, and their radiative and nonradiative rate constants. Lanthanide complexes with dodecahedron structures offer markedly high emission quantum yields (Eu: 55–72 %, Sm: 2.4–5.0 % in [D₆]acetone) due to enhancement of the electric dipole transition and suppression of vibrational relaxation. These remarkable luminescence properties are elucidated in terms of their distorted coordination structures.     

Tuning the coordination sphere around highly luminescent lanthanide complexes

A series of three ligands designed for the formation of water-soluble luminescent lanthanide complexes is described. All ligands are based on a 6'-carboxy-2,2':6',2'-terpyridine framework linked via a methylene bridge to n-butylamine. The second negatively charged arm consists of a 6-carboxy-2-methylenepyridine for L1, a 6'-carboxy-6-methylene-2,2'-bipyridine for L2, and a 6'-carboxy-6-methylene-2,2':6',2'-terpyridine for L3. The photophysical properties of the Eu and Tb complexes were studied in aqueous solutions by means of absorption spectroscopy and steady-state and time-resolved luminescence spectroscopy. Luminescence excited-state lifetimes were recorded and led to the determination of two water molecules in the first coordination sphere. The europium complexes were characterized by means of (1)H NMR spectroscopy in D 2O and DFT calculations performed at the B3LYP level both in vacuo and in aqueous solution. Finally, the influence of different phosphorylated anions such as HPO 4 (2-), ATP (4-), ADP (3-), and AMP (2-) on the luminescence properties of the [Eu L X (H 2O) 2] (+) complexes ( X = 1-3) was investigated in buffered aqueous solutions (0.01 M TRIS, pH 7.0), showing a significant interaction of ATP (4-) with [Eu( L2)(H 2O) 2] (+). The coordination of anions was understood in terms of partial decomplexation of one arm of the ligands and water displacement, with formation of ternary species, and it was rationalized on the basis of the structural models of the complexes obtained from DFT calculations.     

Cyclam-based dendrimers as ligands for lanthanide ions

We have investigated the complexation of lanthanide ions (Nd3+, Eu3+, Gd3+, Tb3+, Dy3+) with three cyclam-based ligands (cyclam = 1,4,8,11-tetraazacyclotetradecane), namely 1,4,8,11-tetrakis(naphthylmethyl)cyclam (1), and two dendrimers consisting of a cyclam core appended with four dimethoxybenzene and eight naphthyl units (2) and twelve dimethoxybenzene and sixteen naphthyl units (3). In the free ligands the fluorescence of the naphthyl units is strongly quenched by exciplex formation with the cyclam nitrogens. Complexation with the metal ions prevents exciplex formation and revives the intense naphthyl fluorescence. Fluorescence and NMR titration experiments have revealed the formation of complexes with different metal/ligand stoichiometries in the case of 1, 2 and 3. Surprisingly, the large dendrimer 3 gives rise to a stable [M(3)3]3+ species. Energy transfer from the lowest singlet and triplet excited states of the peripheral naphthyl units to the lower lying excited states of Nd3+, Eu3+, Tb3+, Dy3+ coordinated to the cyclam core does not take place.     

Unique emission from polymer based lanthanide alloys

The incorporation of metal-ligand interactions into macromolecules imparts them with unique and potentially useful properties. We report novel macromolecules prepared via ATRP that contain activated esters for subsequent incorporation of terpyridine. The addition of lanthanide ions (Eu3+ and Tb3+) gave metal functionalized polymers that exhibited excellent emission of either pink (Eu3+) or green (Tb3+) light. A unique yellow luminescence was generated when these two different metal ions were incorporated into the same molecular backbone at a 1:1 ratio, producing an alloy. Upon heating above 50 degrees C, selective thermochromism, from yellow to orange/pink, was observed.     

Highly luminescent lanthanide complexes of 1-hydroxy-2-pyridinones

The synthesis, X-ray structure, stability, and photophysical properties of several trivalent lanthanide complexes formed from two differing bis-bidentate ligands incorporating either alkyl or alkyl ether linkages and featuring the 1-hydroxy-2-pyridinone (1,2-HOPO) chelate group in complex with Eu(III), Sm(III), and Gd(III) are reported. The Eu(III) complexes are among some of the best examples, pairing highly efficient emission (Phi tot (Eu) approximately 21.5%) with high stability (pEu approximately 18.6) in aqueous solution, and are excellent candidates for use in biological assays. A comparison of the observed behavior of the complexes with differing backbone linkages shows remarkable similarities, both in stability and photophysical properties. Low temperature photophysical measurements for a Gd(III) complex were also used to gain insight into the electronic structure and were found to agree with corresponding time-dependent density functional theory (TD-DFT) calculations for a model complex. A comparison of the high resolution Eu(III) emission spectra in solution and from single crystals also revealed a more symmetric coordination geometry about the metal ion in solution due to dynamic rotation of the observed solid state structure.     

Structural and near-IR photophysical studies on ternary lanthanide complexes containing poly(pyrazolyl)borate and 1,3-diketonate ligands

The ligands tris[3-(2-pyridyl)pyrazol-1-yl]hydroborate (L1, potentially hexadentate) and bis[3-(2-pyridyl)pyrazol-1-yl]dihydroborate (L2, potentially tetradentate) have been used to prepare ternary lanthanide complexes in which the remaining ligands are dibenzoylmethane anions (dbm). [Eu(L1)(dbm)2] is eight-coordinate, with L1 acting only as a tetradentate chelate (with one potentially bidentate arm pendant) and two bidentate dbm ligands. [Nd(L1)(dbm)2] was also prepared but on recrystallization some of it rearranged to [Nd(L1)2][Nd(dbm)4], which contains a twelve-coordinate [Nd(L1)2]+ cation (two interleaved hexadentate podand ligands) and the eight-coordinate anion [Nd(dbm)4]- which, uniquely amongst eight-coordinate complexes having four diketonate ligands, has a square prismatic structure with near-perfect O8 cubic coordination. Formation of this sterically unfavourable geometry is assumed to arise from favourable packing with the pseudo-spherical cation. The isostructural series of complexes [Ln(L2)(dbm)2](Ln = Pr, Nd, Eu, Gd, Tb, Er, Yb) was also prepared and all members structurally characterised; again the metal ions are eight-coordinate, from one tetradentate ligand L2 and two bidentate dbm ligands. Photophysical studies on the complexes with Ln = Pr, Nd, Er, and Yb were carried out; all show the near-IR luminescence characteristic of these metal ions, with longer lifetimes in CD3OD than in CH3OH. For [Yb(L2)(dbm)2], two species with different luminescence lifetimes were observed in CH3OH solution, corresponding to species with zero or one coordinated solvent molecules, in slow exchange on the luminescence timescale. For [Nd(L2)(dbm)2] a single average solvation number of 0.7 was observed in MeOH. For [Pr(L2)(dbm)2] a range of emission lines in the visible and NIR regions was detected; time-resolved measurements show a particularly high susceptibility to quenching by solvent CH and OH oscillators.     

Experimental and theoretical approaches toward anion-responsive tripod-lanthanide complexes: mixed-donor ligand effects on lanthanide complexation and luminescence sensing profiles

A new series of tripods were designed to form anion-responsive, luminescent lanthanide complexes. These tripods contain pyridine, thiazole, pyrazine, or quinoline chromophores combined with amide carbonyl oxygen and tertiary nitrogen atoms. Crystallographic and EXAFS studies of the 10-coordinated tripod-La(NO(3))(3) complexes revealed that each La(3+) cation was cooperatively coordinated by one tetradentate tripod and three bidentate NO(3)(-) anions in the crystal and in CH(3)CN. Quantum chemical calculations indicated that the aromatic nitrogen plays a significant role in lanthanide complexation. The experimentally determined stability constants of complexes of the tripod with La(NO(3))(3), Eu(NO(3))(3), and Tb(NO(3))(3) were in good agreement with the theoretically calculated interaction energies. Complexation of each tripod with lanthanide triflate gave a mixture of several lanthanide complex species. Interestingly, the addition of a coordinative NO(3)(-) or Cl(-) anion to the mixture significantly influenced the lanthanide complexation profiles. The particular combination of tripod and a luminescent Eu(3+) center gave anion-selective luminescence enhancements. Pyridine-containing tripods exhibited the highest NO(3)(-) anion-selective luminescence and thus permit naked-eye detection of the NO(3)(-) anion.     

Lanthanide metal polymer complexes. Synthesis,characterization and application

Our recent results on the investigation of lanthanide metal polymer complexes were presented. Luminescence properties of Tb³⁺ or Eu³⁺ -polycarboxylate complexes in aqueous solution were investigated. The excitation band near 300 nm for Tb or Eu(polyacrylate) solutions were drastically enhanced by the addition of hydroxy radical generating reagents as well as ultrasonic irradiation. These spectral changes were attributed to the energy transfer from chromophore molecules formed by generated hydroxy radicals in both systems. Since the increase in the luminescence intensity was proportional to the hydroxy radical concentration, the Eu³⁺ or Tb³⁺ (PAA) system can provide a convenient method for the determination of hydroxy radical concentration in aqueous solution. We have also utilized lanthanide metal ion complexes as a luminescent emitter in electroluminescence (EL) devices. The configuration of the EL cell and experimental results were discussed.     

Sensitized emission of luminescent lanthanide complexes based on 4-naphthalen-1-yl-benzoic acid derivatives by a charge-transfer process.

The photophysical properties of 4-naphthalen-1-yl-benzoic acid ligands and their Eu(III)-cored complexes were systematically investigated to elucidate the effective energy-transfer pathway in luminescent lanthanide complexes. A series of 4-naphthalen-1-yl-benzoic acid ligands, such as 4-naphthalen-1-yl-benzoic acid (NA-1), 4-[4-(4-methoxyphenyl)-naphthalen-1-yl]-benzoic acid (NA-2), and 4-{4-[4-(4-methoxyphenyl)-naphthalen-1-yl]-benzyloxy}-benzoic acid (NA-3), were synthesized and utilized for the synthesis of their Eu(III)-cored complexes, corresponding to NAC-1, NAC-2, and NAC-3. The fluorescence spectra of NA-1 and NA-2 show large Stokes shifts with increasing solvent polarity. These large Stokes shifts might be dominantly due to the formation of an intramolecular charge transfer (ICT) complex in the excited state. Also, the intensive luminescence of the Eu(III) ions by the photoexcitation of the ligand in NAC-1 and NAC-2 in polar solvents supports that the energy transfer from the ligand to the Eu(III) ion takes place efficiently. In the case of NA-3, which has a -CH2OPh- group that acts as a blocking group, there is no dependence of the fluorescence spectrum on the solvent nature and no luminescence of the Eu(III) ions by the photoexcitation of the ligand, indicating no formation of the ICT state. This can be due to the fact that the formation of the ICT state in NA-3 was prevented because the -OCH2- group acts as a blocking group by interrupting the pi-conjugation between the benzoic acid and the naphthalene unit. From these photophysical studies, we suggest that the ICT state plays a very important role in the energy-transfer pathway from the ligand to the Eu(III) ion. To our best knowledge, this is the first demonstration of sensitized emission of luminescent lanthanide complexes based on 4-naphthalen-1-yl-benzoic acid derivatives by the charge-transfer process.