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

EuIII , TbIII , GdIII and YbIII 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 LnIII cations a common highly symmetrical capped square antiprismatic (CSAPR) coordination geometry (pseudo C4v 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 ([EuIII L2 ]+ , H2 O, 298 K, τ=1.51 ms, ϕ=0.35; [TbIII L2 ]+ , H2 O, 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.

Radiative Lifetime, Non-Radiative Relaxation, and Sensitization Efficiency in Luminescent Europium and Neodymium Cryptates - The Roles of 2,2'-Bipyridine-N,N'-dioxide and Deuteration

Five luminescent tris(bipyridine)-based cryptates with the lanthanoids Eu and Nd have been prepared with a systematic increase in the number of 2,2'-bipyridine-N,N'-dioxide units and with different deuteration levels in the complexing cryptands for the europium species. Careful analysis of the radiative lifetime τ_rad in these systems reveals that an increase in N-oxide units around the metal centers uniformly lowers τ_rad by about 30-40 %. The potential involvement of nephelauxetic effects is discussed. Exchange of 30 C-D for C-H oscillators around the europium centers does not affect the radiative lifetimes but decreases non-radiative deactivation and increases the overall luminescence quantum yield in D₂ O by 45 %.

A lysosome targetable luminescent bioprobe based on a europium β-diketonate complex for cellular imaging applications

A unique bright luminescent europium coordination compound with excellent biocompatibility has been developed that serves as a selective bioprobe for particular organelles within the cells.

Host-guest complexes of mixed glycol-bipyridine cryptands: prediction of ion selectivity by quantum chemical calculations, part V

The selectivity of the cryptands [2.2.bpy] and [2.bpy.bpy] for the endohedral complexation of alkali, alkaline-earth and earth metal ions was predicted on the basis of the DFT (B3LYP/LANL2DZp) calculated structures and complex-formation energies. The cavity size in both cryptands lay between that for [2.2.2] and [bpy.bpy.bpy], such that the complexation of K(+), Sr(2+) and Tl(3+) is most favorable. While the [2.2.bpy] is moderately larger, preferring Rb(+) complexation and demonstrating equal priority for Sr(2+) and Ba(2+), the slightly smaller [2.bpy.bpy] yields more stable cryptates with Na(+) and Ca(2+). Although the CH2-units containing molecular bars fixed at the bridgehead nitrogen atoms determine the flexibility of the cryptands, the twist angles associated with the bipyridine and glycol building blocks also contribute considerably.

From antenna to assay: lessons learned in lanthanide luminescence

Ligand-sensitized, luminescent lanthanide(III) complexes are of considerable importance because their unique photophysical properties (microsecond to millisecond lifetimes, characteristic and narrow emission bands, and large Stokes shifts) make them well suited as labels in fluorescence-based bioassays. The long-lived emission of lanthanide(III) cations can be temporally resolved from scattered light and background fluorescence to vastly enhance measurement sensitivity. One challenge in this field is the design of sensitizing ligands that provide highly emissive complexes with sufficient stability and aqueous solubility for practical applications. In this Account, we give an overview of some of the general properties of the trivalent lanthanides and follow with a summary of advances made in our laboratory in the development of highly luminescent Tb(III) and Eu(III) complexes for applications in biotechnology. A focus of our research has been the optimization of these compounds as potential commercial agents for use in homogeneous time-resolved fluorescence (HTRF) technology. Our approach involves developing high-stability octadentate Tb(III) and Eu(III) complexes that rely on all-oxygen donor atoms and using multichromophore chelates to increase molar absorptivity; earlier examples utilized a single pendant chromophore (that is, a single "antenna"). Ligands based on 2-hydroxyisophthalamide (IAM) provide exceptionally emissive Tb(III) complexes with quantum yield values up to approximately 60% that are stable at the nanomolar concentrations required for commercial assays. Through synthetic modification of the IAM chromophore and time-dependent density functional theory (TD-DFT) calculations, we have developed a method to predict absorption and emission properties of these chromophores as a tool to guide ligand design. Additionally, we have investigated chiral IAM ligands that yield Tb(III) complexes possessing both high quantum yield values and strong circularly polarized luminescence (CPL) activity. To efficiently sensitize Eu(III) emission, we have used the 1-hydroxypyridin-2-one (1,2-HOPO) chelate to create remarkable ligands that combine excellent photophysical properties and exceptional aqueous stabilities. A more complete understanding of this chromophore has been achieved by combining low-temperature phosphorescence measurements with the same TD-DFT approach used with the IAM system. Eu(III) complexes with strong CPL activity have also been obtained with chiral 1,2-HOPO ligands. We have also undertaken kinetic analysis of radiative and nonradiative decay pathways for a series of Eu(III) complexes; the importance of the metal ion symmetry on the ensuing photophysical properties is clear. Lastly, we describe a Tb(III)-IAM compound--now carried through to commercial availability--that offers improved performance in the common HTRF platform and has the potential to vastly improve sensitivity.

Luminescent Eu(III) and Gd(III) Trisbipyridine Cryptates: Experimental and Theoretical Study of the Substituent Effects

Synthesis, absorption spectra and luminescebce properties of a series of lanthanide trisbipyridine cryptates Ln within R-Bpy x R-Bpy x R-Bpy, where Ln = Eu, Gd and R = H, COOH, COOCH3, CONH(CH2)2NH2 are described. Comparison of the unsubstituted parent compound with the substituted compounds shows that bipyridine substitution doesn't alter significantly the photophysical properties of the lanthanide cryptate. The absorption maximum is slightly red-shifted when three bipyridines are substituted, whereas substituting one bipyridines has a negligible effect on the absorption spectra. The experimental triplet state energy is between 21600 and 22 100 cm(-1) for the series of compounds and the luminescence lifetimes at 77 K are between 0.5 and 0.8 ms in HO2 and equal to 1.7 ms in D2O. The experimental characterizations are completed by DFT and TD-DFT calculations to assess the ability of these approaches to predict absorption maxima, triplet state energies and structural parameters of lanthanide cryptates and to characterize the electronic structure of the excited states. The calculations on the unsubstituted parent and substituted compounds show that absorption maxima and lowest 3pipi* triplet state energies can be accurately determined from density functional theory (DFT) and time-dependent (TD) DFT calculations.

Host - Guest Complexes of Bicyclic Hexaamine Cryptands - Prediction of Ion Selectivity by Quantum Chemical Calculations. III

Density functional theory (B3LYP/LANL2DZp) calculations on the structures and complex formation energies were performed to predict the alkali and alkaline earth ion selectivity of the cryptands 1,4,7,10,13,16,21,24-octaazabicyclo(8.8.8)hexacosane (N2N2N2) 1, 3,6,10,13,16,19-hexaazabicyclo(6.6.6)icosane (sarcophagine) 2 and 1,3,6,8,10,13,16,19-octaazabicyclo(6.6.6)icosane (sepulchrate) 3. Compounds 2 and 3 favour binding of Li+ and Mg2+ and exhibit a cavity size similar to [2.1.1], whereas cryptand 1 has a cavity size similar to [2.2.2] and prefers the selective binding of K+ and Ba2+. The cryptand flexibility of 1 is attributed mainly to the groups adjacent to the bridgehead nitrogens, whereas the main guiding feature for the flexibility of 2 and 3 can be traced back to the coordination of the ethylenediamine spacer.

Coordination features of a terpyridine-containing polyamine receptor. Effect of protonation on the photophysical properties of the complexes

The synthesis of the new terpyridine-containing macrocycle 2,6,10,14-tetraaza[15](6,6'')cyclo(2,2':6',2'')terpyridinophane (L) is reported. The ligand contains a tetraamine chain linking the 6,6'' positions of a terpyridine unit. A potentiometric, (1)H NMR, UV-vis spectrophotometric and fluorescence emission study on the basicity properties of in aqueous solutions shows that the first four protonation steps occur on the polyamine chain, while the terpyridine nitrogens are involved in proton binding only in the last protonation step at strongly acidic pH values. Cu(II), Zn(II), Cd(II) and Pb(II) complexation was studied in aqueous solution by means of potentiometric, spectrophotometric and spectrofluorimetric measurements. Cu(II) and Zn(II) can form both mono- and dinuclear complexes in solution, while the larger Cd(II) and Pb(II) give only mononuclear complexes. In the [ML](2+) complexes (M = Zn(II) or Cd(II)) the metal is unequivocally bound to the terpyridine unit. Some amine groups are not coordinated and can quench the fluorescence emission of the terpyridine unit thanks to an electron transfer process. Protonation of the unbound amine groups inhibits the eT process, affording fluorescent [MLH(x)]((2+x)+) complexes.

Experimental and Theoretical Study of the Photophysics and Structures of Europium Cryptates Incorporating 3,3?-Bi-isoquinoline-2,2?-dioxide

A detailed photophysical study of [Eu within (biqO2.2.2)(CF3SO3)](CF3SO3)2. CH3CN.H2O (Eu within 1) and two other types of cryptates incorporating three 3,3'-biisoquinoline-2,2'-dioxide units has been performed. Structural crystallographic data of Eu within 1, electronic structure calculations and theoretical models were used to obtain the intramolecular energy transfer rates and the appropriate set of rate equations, which was solved numerically. Quantum yields and decay lifetimes were obtained from these results and compared to the experimental data. The role of the ligand-to-metal charge transfer (LMCT) states was ascertained. A theoretical ligand field and intensity analysis was carried out and the results agree very well with the emission spectra. The molecular structures of the lanthanide cryptates were successfully modelled by the YIII ion using the restricted Hartree-Fock (RHF) method, with the advantage of dealing with closed-shell systems. These molecular structures were used to explain the drastic differences in the photophysics of the three EuIII cryptates.

Photophysics and crystal structure of a europium(III) cryptate incorporating 3,3'-biisoquinoline-2,2'-dioxide

Increased interest in the emission properties of lanthanide(III) (Eu and Tb) complexes containing ultraviolet and visible sensitizers is being driven by the desire to produce efficient and selective luminescent probes of biological structure. Of special interest are cryptates and other macrocyclic chelating ligands that efficiently encapsulate the lanthanide ions. These species also form relatively stable systems and in some cases are well protected from penetration of the first coordination sphere by solvent molecules and counterions. This work describes the X-ray structure and various spectroscopic measurements on a europium cryptate containing 3,3'-biisoquinoline-2,2'-dioxide (biqO2). This cryptate has been previously recognized for special stability and luminescence efficiency. The compound, (Eu:biqO2.2.2)(CF3SO3)3.CH3CN.H2O, forms rhombic crystals with the space group Pbca. Absorption, emission, and excitation spectra at 293, 77, and 4 K as well as luminescence decay time measurements are used to characterize the solid state and solutions. The ligand-to-metal energy-transfer mechanism and thermally activated back-energy-transfer processes are analyzed and compared to previously published results on related Eu(III) cryptate systems. Preliminary results on the use of high liquid pressure to perturb ligand singlet and triplet states and, as a consequence, probe the ligand-metal orbital interactions are also presented.