Yb-to-Eu Cooperative Sensitization Upconversion in a Multifunctional Molecular Nonanuclear Lanthanide Cluster in Solution.

Lanthanide metal clusters excel in combining molecular and material chemistry properties. Here, we report an efficient cooperative sensitization UC phenomenon of a Eu3+/Yb3+ nonanuclear lanthanide cluster in CD3OD. The synthesis and characterization of the heteronuclear cluster in the solid state and solution are described together with the UC phenomenon showing Eu3+ luminescence in the visible region upon 980 nm NIR excitation of Yb3+ at concentrations as low as 100 nM. Alongside being the Eu/Yb cluster to display UC (with a quantum yield value of 4.88 × 10-8 upon 1.13 W cm-2 excitation at 980 nm), the cluster exhibits downshifted light emission of Yb3+ in the NIR region upon 578 nm visible excitation of Eu3+, which is ascribed to sensitization pathways for Yb through the 5D0 energy levels of Eu3+. Additionally, a faint emission is also observed at ca. 500 nm upon 980 nm excitation, originating from the cooperative luminescence of Yb3+. The [Eu8Yb(BA)16(OH)10]Cl cluster (BA = benzoylacetonate) is also a field-induced single-molecular magnet (SMM) under 4K with a modest Ueff/kB of 8.48 K, thereby joining the coveted list of Yb-SMMs and emerging as a prototype system for next-generation devices, combining luminescence with single-molecular magnetism in a molecular cluster.

Structural Variations in Carboxylated Bispidine Ligands: Influence of Positional Isomerism and Rigidity on the Conformation, Stability, Inertness and Relaxivity of their Mn2+ Complexes.

Mn2+ complexes of 2,4-pyridyl-disubstituted bispidine ligands have emerged as more biocompatible alternatives to Gd3+ -based MRI probes. They display relaxivities comparable to that of commercial contrast agents and high kinetic inertness, unprecedented for Mn2+ complexes. The chemical structure, in particular the substituents on the two macrocyclic nitrogens N3 and N7, are decisive for the conformation of the Mn2+ complexes, and this will in turn determine their thermodynamic, kinetic and relaxation properties. We describe the synthesis of four ligands with acetate substituents in positions N3, N7 or both. We evidence that the bispidine conformation is dependent on N3 substitution, with direct impact on the thermodynamic stability, kinetic inertness, hydration state and relaxivity of the Mn2+ complexes. These results unambiguously show that (i) solely a chair-chair conformation allows for favorable inertness and relaxivity, and (ii) in this family such chair-chair conformation is accessible only for ligands without N3-appended carboxylates.

55/52Mn2+ Complexes with a Bispidine-Phosphonate Ligand: High Kinetic Inertness for Imaging Applications.

Bispidine (3,7-diazabicyclo[3.3.1]nonane) provides a rigid and preorganized scaffold that is particularly interesting for the stable and inert complexation of metal ions, especially for their application in medical imaging. In this study, we present the synthesis of two bispidine ligands with N-methanephosphonate (H4L1) and N-methanecarboxylate (H3L2) substituents as well as the physico-chemical properties of the corresponding Mn2+ and Zn2+ complexes. The two complexes [Mn(L1)]2- and [Mn(L2)]- have relatively moderate thermodynamic stability constants according to potentiometric titration data. However, they both display an exceptional kinetic inertness, as assessed by transmetallation experiments in the presence of 50 equiv excess of Zn2+, showing only ∼40 and 20% of dissociation for [Mn(L1)]2- and [Mn(L2)]-, respectively, after 150 days at pH 6 and 37 °C. Proton relaxivities amount to r1 = 4.31 mM-1 s-1 ([Mn(L1)]2-) and 3.64 mM-1 s-1 ([Mn(L2)]-) at 20 MHz, 25 °C, and are remarkable for Mn2+ complexes with one inner-sphere water molecule (q = 1); they are comparable to that of the commercial contrast agent [Gd(DOTA)(H2O)]-. The presence of one inner-sphere water molecule and an associative water exchange mechanism was confirmed by temperature-dependent transverse 17O relaxation rate measurements, which yielded kex298 = 0.12 × 107 and 5.5 × 107 s-1 for the water exchange rate of the phosphonate and the carboxylate complex, respectively. In addition, radiolabeling experiments with 52Mn were also performed with H2(L1)2- showing excellent radiolabeling properties and quantitative complexation at pH 7 in 15 min at room temperature as well as excellent stability of the complex in various biological media over 24 h.

From Mono- to Polynuclear Coordination Complexes with a 2,2'-Bipyrimidine-4,4'-dicarboxylate Ligand.

The coordination properties of the ligand 2,2'-bipyrimidine-4,4'-dicarboxylic acid (H2bpd) with lanthanide(III) ions (Ln = Eu, Tb, or Lu) were investigated. The syntheses of the H2bpd ligand and its salts, [K2(bpd)(H2O)2] (1) and [(AlkNH)Lu(bpd)2] (Alk = Et, Hex, or en), are described. In the presence of LnCl3 salts (Ln = Lu, Eu, or Tb), the formation of [Ln(bpd)2]- and [Ln(bpd)(H2O)x]+ species was assessed by 1H nuclear magnetic resonance (NMR), spectrophotometry, and spectrofluorometric titrations in aqueous solution. The solid state structure of 1, [K(H2O)2][Lu(bpd)2] (2), and [(Et3NH)Lu(bpd)2] (3) could be determined by X-ray diffraction, showing the ligand to act as a tetradentate unit with formation of three five-membered chelate rings around the central Ln(III). With the aim of building polynuclear assemblies, the coordination between [Lu(bdp)2]- and [Lu(tta)3(H2O)] units (tta = thenoyltrifluoroacetylacetonate) was also investigated. In methanol, 1H NMR titration experiments revealed the formation of complex mixtures from which two new species could be identified, [Lu2(bpd)(tta)4] (4) and H[Lu(bpd)(tta)2] (5), as confirmed by their solid state structure analysis. Using highly lipophilic cations in chloroform, the octametallic complex [enH]4[Lu8(bpd)4(tta)18] (6) could be isolated and its X-ray structure determined.

Evaluation of a Bispidine-Based Chelator for Gallium-68 and of the Porphyrin Conjugate as PET/PDT Theranostic Agent.

In this study a bispidine ligand has been applied to the complexation of gallium(III) and radiolabelled with gallium-68 for the first time. Despite its 5-coordinate nature, the resulting complex is stable in serum for over two hours, demonstrating a ligand system well matched to the imaging window of gallium-68 positron emission tomography (PET). To show the versatility of the bispidine ligand and its potential use in PET, the bifunctional chelator was conjugated to a porphyrin, producing a PET/PDT-theranostic, which showed the same level of stability to serum as the non-conjugated gallium-68 complex. The PET/PDT complex killed >90 % of HT-29 cells upon light irradiation at 50 µm. This study shows bispidines have the versatility to be used as a ligand system for gallium-68 in PET.

Ultrabright Terbium Nanoparticles for FRET Biosensing and in Situ Imaging of Epidermal Growth Factor Receptors*.

Lanthanide-doped nanoparticles (LnNPs) have become an important class of fluorophores for advanced biosensing and bioimaging. LnNPs that are photosensitized by surface-attached antenna ligands can possess exceptional brightness. However, their functional bioconjugation remains an important challenge for their translation into bioanalytical applications. To solve this problem, we designed a ligand that can be simultaneously applied as efficient light harvesting antenna for Tb surface ions and strong linker of biomolecules to the LnNPs surfaces. To demonstrate generic applicability of the photosensitized TbNP-bioconjugates, we applied them in two prototypical applications for biosensing and bioimaging. First, in-solution biorecognition was shown by time-resolved Förster resonance energy transfer (FRET) between streptavidin-functionalized TbNPs to biotinylated dyes (ATTO 610). Second, in situ detection of ligand-receptor binding on cells was accomplished with TbNP-antibody (Matuzumab) conjugates that could specifically bind to transmembrane epidermal growth factor receptors (EGFR). High specificity and sensitivity were demonstrated by time-gated imaging of EGFR on both strongly (A431) and weakly (HeLa and Cos7) EGFR-expressing cell lines, whereas non-expressing cell lines (NIH3T3) and EGFR-passivated A431 cells did not show any signals. Despite the relatively large size of TbNP-antibody conjugates, they could be internalized by A431 cells upon binding to extracellular EGFR, which showed their potential as bright and stable luminescence markers for intracellular signaling.

pH-Dependent Hydration Change in a Gd-Based MRI Contrast Agent with a Phosphonated Ligand.

The heptadentate ligand L was shown to form an extremely stable Gd complex at neutral pH with a pGd value of 18.4 at pH 7.4. The X-ray crystal structures of the complexes formed with Gd and Tb displayed two very different coordination behaviors being, respectively, octa- and nonacoordinated. The relaxometric properties of the Gd complex were studied by field-dependent relaxivity measurements at various temperatures and by 17 O NMR spectroscopy. The pH-dependence of the longitudinal relaxivity profile indicated large changes around neutral pH leading to a very large value of 10.1 mm-1 ⋅s-1 (60 MHz, 298 K) at pH 4.7. The changes were attributed to an increase of the hydration number from one water molecule in basic conditions to two at acidic pH. A similar trend was observed for the luminescence of the Eu complex, confirming the change in hydration state. DOSY experiments were performed on the Lu analogue, pointing to the absence of dimers in solution in the considered pH range. A breathing mode of the complex was postulated, which was further supported by 1 H and 31 P NMR spectroscopy of the Yb complex at varying pH and was finally modeled by DFT calculations.

Formation of Heteropolynuclear Lanthanide Complexes Using Macrocyclic Phosphonated Cyclam-Based Ligands.

Ligands L1 and L2, respectively based on a cyclam and a cross-bridged cyclam scaffold functionalized at N1 and N8 by 6-phosphonic-2-methylene pyridyl groups, are described. While complexation of lanthanide (Ln) cations with L2 was not possible, a family of complexes has been prepared with L1, of the general formulae [LnL1H2]Cl (Ln3+ = Lu, Tb, Yb) or [LnL1H] (Ln3+ = Eu). The solution, structural, potentiometric, and photophysical data for these novel ligands and their complexes have been investigated, including a solid-state study by X-ray diffraction (L1, L2, and [EuL1H]), 1H NMR complexation investigations (Lu3+), as well as UV-vis absorption and luminescence spectroscopy in water and D2O (pH ≈ 7). L1 forms 1:1 metal-ligand stoichiometric octadentate complexes in solution. Importantly, the pyridyl phosphonate functions are capable of simultaneous chelation to the metal center and of interaction with a second metal center. 1H NMR (Lu3+) and spectrophotometric titrations of the isolated [TbL1]- complex by EuCl3 salts demonstrated the formation of high-order (hetero)polymetallic species in aqueous solution (H2O, pH = 7). Global analysis of the luminescence titration experiment points to the formation of 4:1, 3:1, and 3:2 [TbL1]/Eu heteropolynuclear assemblies, exhibiting a strong preference to forming [TbL1]3Eu2 at increased europium concentrations, with energy transfer occurring between the kinetically inert terbium complex and added europium cations.

Unprecedented Kinetic Inertness for a Mn2+ -Bispidine Chelate: A Novel Structural Entry for Mn2+ -Based Imaging Agents.

The search for more biocompatible alternatives to Gd3+ -based MRI agents, and the interest in 52 Mn for PET imaging call for ligands that form inert Mn2+ chelates. Given the labile nature of Mn2+ , high inertness is challenging to achieve. The strongly preorganized structure of the 2,4-pyridyl-disubstituted bispidol ligand L1 endows its Mn2+ complex with exceptional kinetic inertness. Indeed, MnL1 did not show any dissociation for 140 days in the presence of 50 equiv. of Zn2+ (37 °C, pH 6), while recently reported potential MRI agents MnPyC3A and MnPC2A-EA have dissociation half-lives of 0.285 h and 54.4 h under similar conditions. In addition, the relaxivity of MnL1 (4.28 mm-1 s-1 at 25 °C, 20 MHz) is remarkable for a monohydrated, small Mn2+ chelate. In vivo MRI experiments in mice and determination of the tissue Mn content evidence rapid renal clearance of MnL1 . Additionally, L1 could be radiolabeled with 52 Mn and the complex revealed good stability in biological media.

Molecular Upconversion in Water in Heteropolynuclear Supramolecular Tb/Yb Assemblies.

Piling up excited states to reach upconversion (UC) is severely restricted by vibrational quenching mechanisms, especially when one looks at discrete molecular entities in solution. By carefully controlling the supramolecular assembly processes resulting from the strong electrostatic interactions between negatively charged Yb complexes and Tb3+ cations in aqueous solutions, we engineered the formation of heteropolynuclear complexes of [(YbL)2Tb x] compositions ( x = 1 and 2). These edifices display a phenomenon of cooperative photosensitization UC with green emission of the Tb cations upon NIR excitation at 980 nm in the Yb absorption band. The photophysical properties of the complexes were carefully investigated by steady-state and time-resolved luminescence experiments in D2O, allowing one to quantify the impact of the composition and pD of the solution on the emission intensity as well as clarifying the exact cooperative photosensitization upconversion mechanism. Using optimized conditions, the energy transfer UC process could be observed for the first time in nondeuterated water with discrete molecular compounds.

Energy Transfer in Supramolecular Heteronuclear Lanthanide Dimers and Application to Fluoride Sensing in Water.

In the presence of fluoride anions, [LnL(H2 O)]+ complexes, based on the coordination of a lanthanide (Ln) cation into the cavity of a C2v symmetrical cyclen-based ligand (L), self-assemble in water to form [(LnL)2 F]+ dimers. The crystal structures of the Yb hydrated monomer and of the fluorinated dimer are reported and analyzed to unravel the impact of the cumulative effect of weak hydrogen bonding and aromatic stacking interactions in the supramolecular assembly. The assembly is stable over a broad range of pH 3-8. A combination of equimolar amounts of Eu and Tb complexes led to a quasistatistical mixture of homo- and heterodimers, as observed by using electrospray mass spectrometry. In the heterodimers, selective excitation into the 7 F6 →5 D4 absorption band of the Tb center at λ=488 nm allowed the observation of a Tb-to-Eu downshifting energy transfer, not observed in the absence of fluoride ions. Analysis of the excited-state lifetimes of the dimers within the frame of the Förster theory of energy transfer showed the transfer to have an efficiency of 34 %, with a corresponding Förster radius of 4.1 Å; thereby, unraveling the short Ln-Ln distance as a crucial parameter of the energy-transfer process. By using equimolar mixtures of the Tb and Eu complexes, the energy-transfer phenomenon was used for a ratiometric sensing of fluoride anions in water with a detection limit of 17.7 nm.

Formation of Mono- and Polynuclear Luminescent Lanthanide Complexes based on the Coordination of Preorganized Phosphonated Pyridines.

A series of polynuclear assemblies based on ligand L (1,4,7-tris[hydrogen (6-methylpyridin-2-yl)phosphonate]-1,4,7-triazacyclononane) has been developed. The coordination properties of ligand L with LnIII (Ln = La, Eu, Tb, Yb, Lu) have been studied in water (pH = 7.0) and in D2O (pD = 7.0) by UV-absorption spectrometry, spectrofluorimetry, 1H and 31P NMR, DOSY, ESI-mass spectrometry, and X-ray diffraction. This nonadentate ligand forms highly stable mononuclear complexes in water and provides a very efficient shielding of the Ln cations, as emphasized by the very good luminescence properties of the Yb complex in D2O, especially regarding its lifetime (τD2O = 10.2 µs) and quantum yield (ϕD2O = 0.42%). In the presence of excess LnIII cation, polynuclar complexes of [(LnL)2Ln x] stoichiometry (x = 1 and x = 2) are observed in solution. In the solid state, a dinuclear complex of La could be isolated and structurally characterized by X-ray diffraction, unraveling the presence of strong hydrogen bonding interactions between a La(H2O)93+ cation and the [LaL]3- complex.

Sensing Uranyl(VI) Ions by Coordination and Energy Transfer to a Luminescent Europium(III) Complex.

The release of uranyl(VI) is a hazardous environmental issue, with limited ways to monitor accumulation in situ. Here, we present a method for the detection of uranyl(VI) ions through the utilization of a unique fluorescence energy transfer process to europium(III). Our system displays the first example of a "turn-on" europium(III) emission process with a small, water-soluble lanthanide complex triggered by uranyl(VI) ions.

Upconverted Photosensitization of Tb Visible Emission by NIR Yb Excitation in Discrete Supramolecular Heteropolynuclear Complexes.

Addition of Tb3+ salts to a solution of a (YbLD) complex in D2O resulted in the formation of [(YbLD)2Tbx] (x = 1 to 3) complexes that, upon NIR excitation at 980 nm, showed an unprecedented Yb to Tb upconversion sensitization phenomenon resulting in the observation of the typical green emission of Tb.

A Bispidol Chelator with a Phosphonate Pendant Arm: Synthesis, Cu(II) Complexation, and 64Cu Labeling.

Here we present the synthesis and characterization of a new bispidine (3,7-diazabicyclo[3.3.1]nonane) ligand with N-methanephosphonate substituents (L2). Its physicochemical properties in water, as well as those of the corresponding Cu(II) and Zn(II) complexes, have been evaluated by using UV-visible absorption spectroscopy, potentiometry, 1H and 31P NMR, and cyclic voltammetry. Radiolabeling experiments with 64CuII have been carried out, showing excellent radiolabeling properties. Quantitative complexation was achieved within 60 min under stoichiometric conditions, at room temperature and in the nanomolar concentration range. It was also demonstrated that the complexation occurred below pH 2. Properties have been compared to those of the analogue bispidol bearing a N-methanecarboxylate substituent (L1). Although both systems meet the required criteria to be used as new chelator for 64/67Cu in terms of the kinetics of formation, thermodynamic stability, selectivity for Cu(II), and kinetic inertness regarding redox- or acid-assisted decomplexation processes, substitution of the carboxylic acid function by the phosphonic moiety is responsible for a significant increase in the thermodynamic stability of the Cu(II) complex (+2 log units for pCu) and also leads to an increase in the radiochemical yields with 64CuII which is quantitative for L2.

Bifunctional bispidine derivatives for copper-64 labelling and positron emission tomography.

The first radiolabelling studies of a bispidine (3,7-diazabicyclo[3.3.1]nonane) derivative substituted by a glycinate pendant arm (L1) with 64Cu are reported. Labelling was fast and easily performed at room temperature and in a wide range of pH values. Under these conditions, radiochemical yields over 90% were achieved within 5 minutes at micromolar concentration of the ligand. A bifunctional analogue of L1 (L2) has been obtained by introducing an l-lysine amino acid on the bispidine skeleton. Ligand L2 demonstrates good radiolabelling capacities at room temperature and in water (pH 4 to pH 6). This new bispidine is a versatile platform which can easily react with NHS esters and can be subsequently coupled to a recognition unit in order to perform targeted Positron Emission Tomography (PET) imaging. As a proof of concept, two new bifunctional chelators (BFCs) with a biotin (L3) or a maleimide functional group (L4) have been synthesized. The biotinylated BFC is very valuable for pretargeting strategies using streptavidin-conjugated antibodies. The reactivity of the maleimide derivative L4 has been studied with the model peptide GP120. Quantitative coupling has been achieved under physiological conditions, showing a good regioselectivity towards cysteine residues versus lysine amino acids.

Importance of outer-sphere and aggregation phenomena in the relaxation properties of phosphonated gadolinium complexes with potential applications as MRI contrast agents.

A series composed of a tetra-, a tris- and a bisphosphonated ligand based on a pyridine scaffold (L(4) , L(3) and L(2) , respectively) was studied within the frame of lanthanide (Ln) coordination. The stability constants of the complexes formed with lanthanide cations (Ln=La, Nd, Eu, Gd, Tb, Er and Lu) were determined by potentiometry in aqueous solutions (25.0 °C, 0.1 M NaClO4 ), showing that the tetraphosphonated complexes are among the most stable Ln(III) complexes reported in the literature. The complexation of L(4) was further studied by different titration experiments using mass spectrometry and various spectroscopic techniques including UV/Vis absorption, and steady state and time-resolved luminescence (Ln=Eu and Tb). Titration experiments confirmed the formation of highly stable [LnL(4) ] complexes. (31) P NMR experiments of the LuL(4) complex revealed an intramolecular interconversion process which was studied at different temperatures and was rationalized by DFT modelling. The relaxivity properties of the Gd(III) complexes were studied by recording their (1) H NMRD profiles at various temperatures, by temperature dependent (17) O NMR experiments (GdL(4) ) and by pH dependent relaxivity measurements at 0.47 T (GdL(3) and GdL(2) ). In addition to the high relaxivity values observed for all complexes, the results showed an important second-sphere contribution to relaxivity and pH dependent variations associated with the formation of aggregates for GdL(2) and GdL(3) . Finally, intravenous injection of GdL(4) to a mouse was followed by dynamic MRI imaging at 1.5 T, which showed that the complex can be immediately found in the blood stream and rapidly eliminated through the liver and in large part through the kidneys.

Kinetically Inert Bispidol-Based Cu(II) Chelate for Potential Application to (64/67)Cu Nuclear Medicine and Diagnosis.

A family of 2,4-pyridyl-disubstituted bispidol derivatives bearing methylene carboxylic acid ethyl esters (L1-L3), methylene carboxylic acids (L4 and L5), or methylenethiophene (L6) groups were synthesized. In water, all ligands form rigid 1:1 complexes in the presence of Zn(II) in which the bicycle adopts a chair-chair conformation (cis isomer), as observed by (1)H NMR and, in the case of ligand L1, by an X-ray diffraction crystal structure. Interestingly, addition of Zn(II) ions on ligand L1 induces a metal-mediated selective hydrolysis of the ethyl esters. This selective hydrolysis was not observed upon addition of other cations such as Na(+), Mg(+), and Ca(2+). Reduction of the central ketone was achieved to prevent ring opening via retro Diels-Alder reactions and to afford highly stable and water-soluble ligands (L4, L5, L6). The complexation properties of L4 and L6 were studied in solution, with a particular interest for ligand L4. Fast complexation occurs in strongly acidic media (pH = 1), with a high affinity toward Cu(II) (log KCuL4 = 19.2(3), pCu = 17.0 at pH 7.4, pCu = -log[Cufree], [Cu] = 1 × 10(-6) M, [L] = 1 × 10(-5) M) and high selectivity versus Co(II), Ni(II), and Zn(II), as shown by the values of the binding constants obtained from potentiometric and spectrophotometric titrations. Reversible redox potential with E1/2 = -430 mV (vs normal hydrogen electrode) was measured. The complex was found to be fairly inert from acid-assisted dissociation experiments in 5 M HClO4 (t1/2 = 110 d at 25 °C).

Supramolecular luminescent lanthanide dimers for fluoride sequestering and sensing.

Lanthanide complexes (Ln=Eu, Tb, and Yb) that are based on a C2 -symmetric cyclen scaffold were prepared and characterized. The addition of fluoride anions to aqueous solutions of the complexes resulted in the formation of dinuclear supramolecular compounds in which the anion is confined into the cavity that is formed by the two complexes. The supramolecular assembly process was monitored by UV/Vis absorption, luminescence, and NMR spectroscopy and high-resolution mass spectrometry. The X-ray crystal structure of the europium dimer revealed that the architecture of the scaffold is stabilized by synergistic effects of the EuFEu bridging motive, π stacking interactions, and a four-component hydrogen-bonding network, which control the assembly of the two [EuL] entities around the fluoride ion. The strong association in water allowed for the luminescence sensing of fluoride down to a detection limit of 24 nM.

Upconverting photons at the molecular scale with lanthanide complexes.

In this perspective, we summarise the major milestones to date in the field of molecular upconversion (UC) with lanthanide based coordination complexes. This begins from the leap firstly from solid-state to nanoparticular regimes, and further down the scale to the molecular domain. We explain the mechanistic intricacies of each differing way of generating upconverted photons, critiquing them and outlining our views on the benefits and limitations of each process, also offering our perspective and opinion on where these new molecular UC edifices will take us. This nascent area is already rapidly expanding and improving, having increased in luminance efficiency by more than four orders of magnitude in the last decade: we conclude that the future is bright for molecular UC.