Synthesis of macrocyclic salen rare-earth complexes and their framework conversion regulated by coordination sphere engineering

We now report a macrocyclic rare-earth complex whose frameworks have been regulated between a trimer and a tetramer by an external capping ligand. Coordination sphere engineering of the rare earth metals with flexible and variable coordination numbers (CN > 6) offers a unique way to control the self-assembled structures.

Highly Efficient and Thermally Durable Luminescence of 1D Eu3+ Coordination Polymers with Arsenic Bridging Ligands

In this work, bisarsine oxides were evaluated as novel bridging ligands, aiming to develop practical and efficient luminescent lanthanide coordination polymers. We have synthesized one-dimensional (1D) Eu3+ coordination polymers that incorporate bisarsine oxide bridging ligands and hexafluoroacetylacetonate anions. These polymers exhibited a denser packing of chains compared to analogous polymers bridged with bisphosphine oxides. The coordination polymers demonstrated exceptional thermal stability and substantial emission quantum yields. Additionally, the bisarsine oxides induced a pronounced polarization effect, facilitating a sensitive electric dipole transition that yields considerably narrow band red emission. Remarkably, the Eu3+ coordination polymers with bisarsine oxides maintained intense emission even at 550 K. A distinctive feature of these polymers is their heating-induced emission enhancement observed when the temperature was increased from 300 K to 400 K.

Using Chiral Auxiliaries to Mimic the Effect of Chiral Media on the Structure of Lanthanide(III) Complexes Common in Bioimaging and Diagnostic MRI

[Ln·DOTA]- complexes and systems derived therefrom are commonly used in MRI and optical bioimaging. These lanthanide(III) complexes are chiral, and, in solution, they are present in four forms, with two sets of enantiomers, with the ligand donors arranged in either a square antiprismatic, SAP, or twisted square antiprismatic geometry, TSAP. This complicated speciation is found in laboratory samples. To investigate speciation in biological media, when Ln·DOTA-like complexes interact with chiral biomolecules, six Eu·DOTA-monoamide complexes were prepared and investigated by using 1D and 2D 1H NMR. To emulate the chirality of biological media, the amide pendant arm was modified with one or two chiral centers. It is known that a chiral center on the DOTA scaffold significantly influences the properties of the system. Here, it was found that chirality much further away from the metal center changes the available conformational space and that both chiral centers and amide cis/trans isomerism may need to be considered─a fact that, for the optically enriched materials, led to the conclusion that eight chemically different forms may need to be considered, instead of the four forms necessary for DOTA. The results reported here clearly demonstrate the diverse speciation that must be considered when correlating an observation to a structure of a lanthanide(III) complex.

Mapping the distribution of electronic states within the 5D4 and 7F6 levels of Tb3+ complexes with optical spectroscopy

The Tb(III) ion has the most intense luminescence of the trivalent lanthanide(III) ions. In contrast to Eu(III), where the two levels only include a single state, the high number of electronic states in the ground (7F6) and emitting (5D4) levels makes detailed interpretations of the electronic structure-the crystal field-difficult. Here, luminescence emission and excitation spectra of Tb(III) complexes with 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA, [Tb(DOTA)(H2O)]-), ethylenediaminetetraacetic acid (EDTA, [Tb(EDTA)(H2O)3]-) and diethylenetriaminepentaacetic acid (DTPA, [Tb(DTPA)(H2O)]2-) as well as the Tb(III) aqua ion ([Tb(H2O)9]3+) were recorded at room temperature and in frozen solution. Using these data the electronic structure of the 5D4 multiplets of Tb(III) was mapped by considering the transitions to the singly degenerate 7F0 state. A detailed spectroscopic investigation was performed and it was found that the 5D4 multiplet could accurately be described as a single band for [Tb(H2O)9]3+, [Tb(DOTA)(H2O)]- and [Tb(EDTA)(H2O)3]-. In contrast, for [Tb(DTPA)(H2O)]2- two bands were needed. These results demonstrated the ability of describing the electronic structure of the emitting 5D4 multiplet using emission spectra. This offers an avenue for investigating the relationship between molecular structure and luminescent properties in detailed photophysical studies of Tb(III) ion complexes.

Macrocyclic Ligand Coordinating Amide-Arm Hydrolysis Reaction Activation in Aqueous Solutions: Tetravalent Uranium Does It Better

Chelation of lanthanide and actinide cations within a suitable macrocyclic ligand often results in a rigid, kinetically inert, and thermodynamically stable complex. A benchmark for such cation-ligand suitability are cyclen-derived macrocyclic ligands, frequently used as large cation hosts for various applications. Herein, a comprehensive study of the 1,4,7,10-tetrakis(carbamoylmethyl)-1,4,7,10-tetraazacyclododecane ligand (DOTAM) chelates of UIV and CeIII and their properties in aqueous solutions is presented. By employing multiple analysis techniques, including X-ray crystallography, UV-vis absorbance, 1H NMR, UPLC-MS, cyclic voltammetry, and differential pulse voltammetry, the study has revealed that the two aqueous complexes undergo a spontaneous, gradual, and stepwise hydrolysis of each of the coordinated amides toward carboxylates. The coordination of UIV in the studied reaction has been shown to significantly enhance the reaction rate, leading to an acceleration of up to 6 orders of magnitude compared to the natural process of simple aqueous amides at room temperature. An attempt to describe the unusual chelated metal cation amide-activation feature, based on the relatively lower rigidity of the complex structure, is presented. Additionally, the electrochemical properties of the complex series are discussed in detail, along with the limitations of the analytical methods employed.

The Effect of Linker-to-Metal Energy Transfer on the Photooxidation Performance of an Isostructural Series of Pyrene-Based Rare-Earth Metal-Organic Frameworks

The tetratopic linker, 1,3,6,8-tetrakis(p-benzoic acid)pyrene (H4 TBAPy) along with rare-earth (RE) ions is used for the synthesis of 9 isostructures of a metal-organic framework (MOF) with shp topology, named RE-CU-10 (RE = Y(III), Gd(III), Tb(III), Dy(III), Ho(III), Er(III), Tm(III), Yb(III), and Lu(III)). The synthesis of each RE-CU-10 analogue requires different reaction conditions to achieve phase pure products. Single crystal X-ray diffraction indicates the presence of a RE9 -cluster in Y- to Tm-CU-10, while a RE11 -cluster is observed for Yb- and Lu-CU-10. The photooxidation performance of RE-CU-10 analogues is evaluated, observing competition between linker-to-metal energy transfer versus the generation of singlet oxygen. The singlet oxygen produced is used to detoxify a mustard gas simulant 2-chloroethylethyl sulfide, with half-lives ranging from 4.0 to 5.8 min, some of the fastest reported to date using UV-irradiation and < 1 mol% catalyst, in methanol under O2 saturation.

Effect of buffers and pH in antenna sensitized Eu(III) luminescence

The photophysics of a europium(III) complex of 1,4,7,10-tetraazacycododecane-1,4,7-triacetic acid-10-(2-methylene)-1-azathioxanthone was investigated in three buffer systems and at three pH values. The buffers-phosphate buffered saline (PBS), 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES), and universal buffer (UB)-had no effect on the europium luminescence, but a lower overall emission intensity was determined in HEPES. It was found that this was due to quenching of the 1-azathioxanthone first excited singlet state by HEPES. The effect of pH on the photophysics of the complex was found to be minimal, and protonation of the pyridine nitrogen was found to be irrelevant. Even so, pH was shown to change the intensity ratio between 1-azathioxanthone fluorescence and europium luminescence. It was concluded that the full photophysics of a potential molecular probe should be investigated to achieve the best possible results in any application.

Lanthanide Complexes (GdIII and EuIII ) Based on a DOTA-TEMPO Platform for Redox Monitoring via Relaxivity

Three lanthanide complexes (Ln=Gd, Eu) based on a DO3 A ([Ln(L1 )]) or DO2 A ([Ln(L2-3 )]+ ) platform appended by a redox active TEMPO-based arm were prepared. Complex [Ln(L2 )]+ shows an alkyne arm, offering the possibility of postfunctionalization by click reaction to yield [Ln(L3 )]+ . The complexes demonstrate a redox response whereby the hydroxylamine, nitroxide and oxoammonium forms of the arm can be obtained in turn. Luminescence measurements on the europium complexes support an octadentate (L1 , L3 ) or heptadentate (L2 ) chelation by the ligand, with one water molecule in the inner coordination sphere. The relaxivity was determined from 20 kHz to 30 MHz by fast-field cycling NMR. The three GdIII complexes under their hydroxylamine form [Gd(L1 )] and [Gd(L2-3 )]+ show r1 values of 7.0, 5.1 and 5.0 mM-1  s-1 (30 KHz), which increase to 8.8, 5.5 and 6.1 mM-1  s-1 in the nitroxide form. The radical complexes are not toxic against M21 cell lines, at least up to 40 μM. By using EPR spectroscopy we establish that they do not penetrate the cells with the exception of [Eu(L2 )]+ .

Long story short: donor set symmetry in [Eu(DOTA)(H2O)]- crystals determines the electronic structure

Lanthanide complexes of 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid DOTA have been studied in great detail due to their use as MRI contrast agents. Since the first report from Desreux in 1980, the Ln[DOTA]^- complexes of gadolinium(III) in particular have been thoroughly investigated. The forms of the nine-coordinated [Ln(DOTA)(H₂O)]^- complexes are well known, and the ligand backbone has been used extensively to create functional MRI contrast agents, luminescent probes, and as a model system for studying the properties of lanthanide(III) ions. In solution, the photophysical properties have been mapped, but as the structures are not known, direct structure-property relationships have not been created. Here, the electronic properties of two Eu[DOTA] compounds (1 and 2) and a Eu[DOTA]-like compound (3) were studied using single-crystal luminescence spectroscopy. The donor set in the three compounds is identical (4N 4O 1O), and using the symmetry deviation value σ_ideal it was shown that the coordination geometry is close to identical. Nevertheless, the electronic properties evaluated using the luminescence spectrum were found to differ significantly between the three compounds. The magnitude of the crystal field splitting was found not to scale with the symmetry of the coordination geometry. It was concluded that the donor set dictates the splitting, yet the structure-property relationships governing the electronic properties of europium(III) ions still elude us.

A high-sensitivity rapid acquisition spectrometer for lanthanide(III) luminescence

Detecting luminescence beyond 750-800 nm becomes problematic as most conventional detectors are less sensitive in this range, and as simple corrections stops being accurate. Lanthanide luminescence occurs in narrow bands across the spectrum from 350-2000 nm. The most emissive lanthanide(III) ions have bands from 450 nm to 850 nm, some with additional bands in the NIR. Investigating the NIR bands are hard, but the difficulties start already at 700 nm. In general, the photon flux from lanthanide(III) emitters is not great, and the bands beyond 700 nm are very weak, we therefore decided to build a spectrometer based on cameras for microscopy with single-photon detection capabilities. This was found to allieviate all limitations and to allow for fast and efficient recording of luminescence spectra in the range from 450 to 950 nm. The spectrometer characteristics were investigated and the performance was benchmarked against two commercial spectrometers. We conclude that this spectrometer is ideal for investigating lanthanide luminescence, an all other emitters with emission in the target range.

We are never ever getting (back to) ideal symmetry: structure and luminescence in a ten-coordinated europium(III) sulfate crystal

Our theoretical treatment of electronic structures in coordination complexes often rests on assumptions of symmetry. Experiments rarely provide fully symmetric systems to study. In solutions, fluctuations in solvation, variations in conformations, and even changes in constitution occur and complicate the picture. In crystals, lattice distortion, energy transfer, and phonon quenching play a role, but we are able to identify distinct symmetries. Yet the question remains: How is the real symmetry in a crystal compared to ideal symmetries?

Invisible strings. The first single crystal of the cTSAP form of [Eu(DOTA)(H2O)]− has an electronic structure similar to one of the reported cSAP forms

Counter ions can be used to tune the solid state structure of Eu·DOTA between the cSAP and cTSAP form, but the electronic properties does not match the observations seen in solution.

Delicate, a study of the structural changes in ten-coordinated La(iii), Ce(iii), Pr(iii), Nd(iii), Sm(iii) and Eu(iii) sulfates

A study of doped ten-coordinated structures of the lanthanide(iii) crystals series, K6[Ln2(SO4)6] (Ln(iii) = La, Ce, Pr) and K5Na[Ln2(SO4)6] (Ln(iii) = Nd, Sm, Eu) to determine luminescence from Eu(iii) in distorted host lattices.

Rigid, Highly Reactive and Stable DOTA-like Tags Containing a Thiol-Specific Phenylsulfonyl Pyridine Moiety for Protein Modification and NMR Analysis*

Site specific installation of a paramagnetic ion with magnetic anisotropy in a biomolecule generates valuable structural restraints, such as pseudocontact shifts (PCSs) and residual dipolar couplings (RDCs). These paramagnetic effects can be used to characterize the structures, interactions and dynamics of biological macromolecules and their complexes. Two single-armed DOTA-like tags, BrPSPy-DO3M(S)A-Ln and BrPSPy-6M-DO3M(S)A-Ln, each containing a thiol-specific reacting group, that is, a phenylsulfonyl pyridine moiety, are demonstrated as rigid, reactive and stable paramagnetic tags for protein modification by formation of a reducing resistant thioether bond between the protein and the tag. The two tags present high reactivity with the solvent exposed thiol group in aqueous solution at room temperature. The introduction of Br at the meta-position in pyridine enhances the reactivity of 4-phenylsulfonyl pyridine towards the solvent exposed thiol group in a protein, whereas the ortho-methyl group in pyridine increases the rigidity of the tag in the protein conjugates. The high performance of these two tags has been demonstrated in different cysteine mutants of ubiquitin and GB1. The high reactivity and rigidity of these two tags can be added in the toolbox of paramagnetic tags suitable for the high-resolution NMR measurements of biological macromolecules and their complexes.

Second-Sphere Interaction Promoted Turn-On Fluorescence for Selective Sensing of Organic Amines in a TbIII -based Macrocyclic Framework

Guided by a second-sphere interaction strategy, we fabricated a Tb(III)-based metal-organic framework (MMCF-4) for turn-on sensing of methyl amine with ultra-low detection limit and high turn-on efficiency. MMCF-4 features lanthanide nodes shielded in a nonacoordinate geometry along with secondary coordination spheres that are densely populated with H-bond interacting sites. Nonradiative routes were inhibited by binding-induced rigidification of the ligand on the second coordination sphere, resulting in luminescence amplification. Such remote interacting mechanism involved in the turn-on sensing event was confirmed by single-crystal X-ray diffraction and molecular dynamic simulation studies. The design of both primary and secondary coordination spheres of Tb(III) enabled the first turn-on sensing of organic amines in aqueous conditions. Our work suggests a promising strategy for high-performance turn-on sensing for Ln-MOFs and luminous materials driven by other metal chromophores.

A Walk Across the Lanthanide Series: Trend in Affinity for Phosphate and Stability of Lanthanide Receptors from La(III) to Lu(III)

The trend in affinity of two 1,2-hydroxypyridinonate lanthanide(III) receptors-Ln^III-2,2-Li-HOPO and Ln^III-3,3-Gly-HOPO (Ln^III = La^III, Pr^III, Nd^III, Sm^III, Eu^III, Gd^III, Tb^III, Dy^III, Ho^III, Er^III, Tm^III, Yb^III, and Lu^III)-for phosphate across the series was investigated by luminescence spectroscopy via competition against the central europium(III) analog. Regardless of the ligand, the rare earth receptors display a steep and continuous increase in affinity for their phosphate guest across the series, with the later lanthanides displaying the highest affinity for the oxyanion. This trend mirrors that of the stability of the lanthanide receptors, which also increases significantly and continuously from La^III to Lu^III. For these two ligands, the ionic radius of a rare earth, a parameter directly linked to its Lewis acidity, correlates strongly with its affinity for anions, regardless of whether that anion is the one coordinating it (in this case the 1,2-hydroxypyridinonate ligand) or the guest targeted by the lanthanide receptor (in this case phosphate). These observations are indicative of a lack of steric hindrance for coordination of phosphate. Advantageously, increased efficacy of the lanthanide receptor comes with increased stability. The remarkably high stability of Lu^III-2,2-Li-HOPO, combined with its high affinity for phosphate, makes it a particularly promising candidate for translational application to medical or environmental sequestration of phosphate since the higher stability will further reduce the risk of the rare earth leaching during anion separation. The unusually large difference in stability between lanthanide complexes (the Lu^III complex of 2,2-Li-HOPO is at least 7 orders of magnitude more stable than the La^III one) bodes well for potential applications in rare earth separation.

Temperature Dependence of Fundamental Photophysical Properties of [Eu(MeOH-d4)9]3+ Solvates and [Eu·DOTA(MeOH-d4)]- Complexes

The trivalent lanthanide ions show optical transitions between energy levels within the 4f shell. All these transitions are formally forbidden according to the quantum mechanical selection rules used in molecular photophysics. Nevertheless, highly luminescent complexes can be achieved, and terbium(III) and europium(III) ions are particularly efficient emitters. This report started when an apparent lack of data in the literature led us to revisit the fundamental photophysics of europium(III). The photophysical properties of two complexes-[Eu·DOTA(MeOH-d₄)]^- and [Eu(MeOH-d₄)₉]³⁺-were investigated in deuterated methanol at five different temperatures. Absorption spectra showed decreased absorbance as the temperature was increased. Luminescence spectra and time-resolved emission decay profiles showed a decrease in intensity and lifetime as the temperature was increased. Having corrected the emission spectra for the actual number of absorbed photons and differences in the non-radiative pathways, the relative emission probability was revealed. These were found to increase with increasing temperature. The transition probability for luminescence was shown to increase with temperature, while the transition probability for light absorption decreased. The changes in transition probabilities were correlated with a change in the symmetry of the absorber or emitter, with an average increase in symmetry lowering absorbance and access to more asymmetric structures increasing the emission rate constant. Determining luminescence quantum yields and the Einstein coefficient for spontaneous emission allowed us to conclude that lowering symmetry increases both. Furthermore, it was found that collisional self-quenching is an issue for lanthanide luminescence, when high concentrations are used. Finally, detailed analysis revealed results that show the so-called "Werts' method" for calculating radiative lifetimes and intrinsic quantum yields is based on assumptions that do not hold for the two systems investigated here. We conclude that we are lacking a good theoretical description of the intraconfigurational f-f transitions, and that there are still aspects of fundamental lanthanide photophysics to be explored.

Structural dynamism of chiral sodium peraza-macrocycle complexes derived from cyclic peptoids

A variety of cyclen and hexacyclen derivatives decorated with (S)-1-phenylethyl side chains or (S)-pyrrolidine units have been prepared via a reductive approach from the corresponding cyclic peptoids containing N-(S)-(1-phenylethyl)glycine and l-proline residues. Spectroscopic and DFT studies on their Na+ complexes show that point chirality and ring size play a crucial role in controlling the structural dynamism of 1,2-diaminoethylene units and pendant arms. The detection of highly symmetric C4- and C3-symmetric metalated species demonstrates that a full understanding of the relationship between the structure and conformational properties of peraza-macrocyclic metal complexes is possible.

Noncovalent Interactions at Lanthanide Complexes

Lanthanide complexes have attracted a widespread attention due to their structural diversity, as well as multifunctional and tunable properties. The development of lanthanide based functional materials has often relied on the design of the secondary coordination sphere of the corresponding lanthanide complexes. For instance, usually simple lanthanide salts (solvento complexes) do not catalyze effectively organic reactions or provide low yield of the expected product, whereas the presence of a suitable organic ligand with a noncovalent bond donor or acceptor centre (secondary coordination sphere) modifies the symmetry around the metal centre in lanthanide complexes which then successfully can act as catalysts in both homogenous and heterogenous catalysis. In this minireview, we discuss several relevant examples, based on X-ray crystal structure analyses, in which the hydrogen, halogen, chalcogen, pnictogen, tetrel and rare-earth bonds, as well as cation-π, anion-π, lone pair-π, π-π and pancake interactions, are used as a synthon in the decoration of the secondary coordination sphere of lanthanide complexes.

Helicate-to-tetrahedron transformation of chiral lanthanide supramolecular complexes induced by ionic radii effect and linker length

Controlled formation of desired lanthanide supramolecular complexes is challenging because of the difficulties in predicting coordination geometry, as well as a labile coordination number. Herein, we explore the effect of ionic radii and linker length on supramolecular species formation. A helicate-to-tetrahedron transformation occurred between [Ln₂L1₃] and [Ln₄L1₆] (Ln = La, Sm, Eu, Gd, Tb and Lu). For six lanthanide ions, the unfavored tetrahedron [La₄L1₆] can only be observed in a concentrated mixture with the helicate [La₂L1₃] where no pure [La₄L1₆] species was isolated via crystallization. For Sm, Eu, Gd, Tb, the [Ln₄L1₆] supramolecular tetrahedron can be isolated via crystallization from diisopropyl ether. A similar result was also observed for Lu, but the tetrahedral structure was found to be relatively stable and transformed back to [Lu₂L1₃] much slower upon dissolution.  No tetrahedron formation was observed with L3 giving rise to only [Ln₂L3₃] species, in which L3 contains a longer and more flexible linker compared with that of L1. Results show that the supramolecular transformation in these systems is governed by both the ionic radii as well as the ligand design. Special focus is on both [Eu₂L1₃] and [Eu₄L1₆] which form chiral entities and exhibit interesting circular polarized luminescence.

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.

Accessing lanthanide-based, in situ illuminated optical turn-on probes by modulation of the antenna triplet state energy

Luminescent lanthanides possess ideal properties for biological imaging, including long luminescent lifetimes and emission within the optical window. Here, we report a novel approach to responsive luminescent Tb(iii) probes that involves direct modulation of the antenna excited triplet state energy. If the triplet energy lies too close to the 5D4 Tb(iii) excited state (20 500 cm-1), energy transfer to 5D4 competes with back energy transfer processes and limits lanthanide-based emission. To validate this approach, a series of pyridyl-functionalized, macrocyclic lanthanide complexes were designed, and the corresponding lowest energy triplet states were calculated using density functional theory (DFT). Subsequently, three novel constructs L3 (nitro-pyridyl), L4 (amino-pyridyl) and L5 (fluoro-pyridyl) were synthesized. Photophysical characterization of the corresponding Gd(iii) complexes revealed antenna triplet energies between 25 800 and 30 400 cm-1 and a 500-fold increase in quantum yield upon conversion of Tb(L3) to Tb(L4) using the biologically relevant analyte H2S. The corresponding turn-on reaction can be monitored using conventional, small-animal optical imaging equipment in presence of a Cherenkov radiation emitting isotope as an in situ excitation source, demonstrating that antenna triplet state energy modulation represents a viable approach to biocompatible, Tb-based optical turn-on probes.

Computational Characterization of AcIII-DOTA Complexes in Aqueous Solution

The 1,4,7,10-tetrazacyclodecane-1,4,7,10-tetraacetic acid (DOTA) aqueous complexes of Ac^III with H₂O, dimethyl sulfoxide (DMSO), OH^-, and F^- as axial ligands were studied using density functional theory. Formation of the [Ac^III(DOTA)(OH)]^2- and [Ac^III(DOTA)(F)]^2- complexes is predicted to be significantly more favorable than that of [Ac^III(DOTA)(H₂O)]^- and [Ac^III(DOTA)(DMSO)]^- because of the enhanced relative Gibbs free energies. Further electronic structure analyses demonstrate that the type and nature of the bond between Ac and the ligand donor atom is the main driving force that determines the thermodynamic stability of the complexes. Specifically, the [Ac^III(DOTA)]^- complex strongly binds to OH^- and F^- via covalent bonds, while the bonding to H₂O and DMSO is ionic and relatively weaker.

Oligomers Intermediates in Between Two New Distinct Homonuclear Uranium(IV) DOTP Complexes*

Two new aqueous U^IV complexes were synthesized by the interaction between the tetravalent uranium cation and the (1,4,7,10-Tetraazacyclododecane-1,4,7,10-tetra(methylene phosphonic acid) (DOTP) macrocyclic ligand. Two distinct homonuclear complexes were identified; the first was characterized by X-ray crystallography as a unique "out-of-cage", [U(DOTPH₆ )₂ ] complex, in which the U^IV cation is octa-coordinated to 4 phosphonic arms from each ligand in a square anti-prism geometry, with a C₄ symmetry. The second is the "in-cage" [U(DOTPH₄ )] complex, in which the tetravalent cation is located between the macrocycle O₄ and N₄ planes. With the help of UV-Vis absorption, ¹ H/³¹ P NMR, ATR-IR, and MALDI-TOFMS analytical techniques, the chemical interchange between both species is presented. It is shown that the one-way transition is governed by the formation of a multiple number of soluble oligomeric species consisting of varied stoichiometric ratios of both characterized homonuclear complexes.

Electronic Structure of Ytterbium(III) Solvates-a Combined Spectroscopic and Theoretical Study

The wide range of optical and magnetic properties of lanthanide(III) ions is associated with their intricate electronic structures which, in contrast to lighter elements, is characterized by strong relativistic effects and spin-orbit coupling. Nevertheless, computational methods are now capable of describing the ladder of electronic energy levels of the simpler trivalent lanthanide ions, as well as the lowest energy term of most of the series. The electronic energy levels result from electron configurations that are first split by spin-orbit coupling into groups of energy levels denoted by the corresponding Russell-Saunders terms. Each of these groups are then split by the ligand field into the actual electronic energy levels known as microstates or sometimes m_J levels. The ligand-field splitting directly informs on the coordination geometry and is a valuable tool for determining the structure and thus correlating the structure and properties of metal complexes in solution. The issue with lanthanide complexes is that the determination of complex structures from ligand-field splitting remains a very challenging task. In this paper, the optical spectra-absorption, luminescence excitation, and luminescence emission-of ytterbium(III) solvates were recorded in water, methanol, dimethyl sulfoxide (DMSO), and N,N-dimethylformamide (DMF). The electronic energy levels, that is, the microstates, were resolved experimentally. Subsequently, density functional theory calculations were used to model the structures of the solvates, and ab initio relativistic complete active space self-consistent field calculations (CASSCF) were employed to obtain the microstates of the possible structures of each solvate. By comparing the experimental and theoretical data, it was possible to determine both the coordination number and solution structure of each solvate. In water, methanol, and N,N-dimethylformamide, the solvates were found to be eight-coordinated and have a square antiprismatic coordination geometry. In DMSO, the speciation was found to be more complicated. The robust methodology developed for comparing experimental spectra and computational results allows the solution structures of homoleptic lanthanide complexes to be determined.

Combining the Best of Two Chelating Titans: A Hydroxypyridinone-Decorated Macrocyclic Ligand for Efficient and Concomitant Complexation and Sensitized Luminescence of f-Elements

An ideal chelator for f-elements features rapid kinetics of complexation, high thermodynamic stability, and slow kinetics of dissociation. Here we present the facile synthesis of a macrocyclic ligand bearing four 1-hydroxy-2-pyridinone units linked to a cyclen scaffold that rapidly forms thermodynamically stable complexes with lanthanides (Sm³⁺ , Eu³⁺ , Tb³⁺ , Dy³⁺ ) and a representative late actinide (Cm³⁺ ) in aqueous media and concurrently sensitizes them. Extended X-ray absorption fine structure (EXAFS) spectroscopy revealed an increase in the Ln/An-O bond lengths following the trend Cm>Eu>Tb and EXAFS data were compatible with time-resolved luminescence studies, which indicated one to two water molecules in the inner metal coordination sphere of Eu(III) and two water molecules for the Cm(III) complex. Spectrofluorimetric ligand competition titrations against DTPA confirmed the high thermodynamic stability of DOTHOPO complexes, with pM values between 19.9(1) and 21.9(2).

Structural variation of self-assembled lanthanide supramolecular complexes induced by reaction conditions

The structural variation of self-assembled lanthanide supramolecular complexes which can be induced by different factors such as concentration, anion and solvent, cationic radii, stoichiometric ratio and light.

HOCl Responsive Lanthanide Complexes Using Hydroquinone Caging Units

Redox biology is still looking for tools to monitor redox potential in cellular biology and, despite a large and sustained effort, reliable molecular probes have yet to emerge. In contrast, molecular probes for reactive oxygen and nitrogen have been widely explored. In this manuscript, three kinetically inert lanthanide complexes that selectively react with hypochlorous acid are prepared and characterized. The design is based on 1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid (DO3A) and 1,4,7,10-tetraazacyclododecane-1,7-diacetic acid (DO2A) ligands appended with one or two redox active hydroquinone derived arms, thereby forming octadentate ligands ideally suited to complex trivalent lanthanide ions. The three complexes are found to react selectively with hypochlorous acid to form highly symmetric lanthanide(III) 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacedic acid (DOTA) complexes. The conversion of the probe to [Ln.DOTA]- is followed by luminescence, absorption, and NMR spectroscopy in a model system comprised of a Triton-X modified HEPES buffer. It was concluded that the design principle works, and that simple caging units like hydroquinones can work well in conjugation with lanthanide(III) complexes.

π-Expanded Thioxanthones - Engineering the Triplet Level of Thioxanthone Sensitizers for Lanthanide-Based Luminescent Probes with Visible Excitation

Bright lanthanide based probes for optical bioimaging must rely on the antenna principle, where the lanthanide-centred excited state is formed by a complex sensitization process. Efficient sensitization of lanthanide-centred emission occurs via triplet states centred on the sensitizing chromophore. Here, the triplet state of thioxanthone chromophores is modulated by extending the π-system. Three thioxanthone chromophores-thioxanthone, benzo[c]thioxanthone, and naphtho[2,3-c]thioxanthone were synthesised and characterised. The triplet state energies and lifetimes is found to change as expected, and two dyes are found to be suitable sensitizers for europium(iii) luminescence. Reactive derivatives of thioxanthone and benzo[c]thioxanthone were prepared and coupled to a 1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid (DO3A) lanthanide binding pocket. The photophysics and the performance in optical bioimaging of the resulting europium(iii) complexes were investigated. It is concluded that while the energetics favour efficient sensitization, the solution structure does not. While it was found that the complexes are too lipophilic to be efficient luminescent probes for optical bioimaging, we successfully demonstrated bioimaging using europium(iii) luminescence following 405 nm excitation.

The effect of weighted averages when determining the speciation and structure–property relationships of europium(iii) dipicolinate complexes

Europium(iii) dipicolinate complexes have been a model system in lanthanide solution chemistry for decades, here it is investigated in unprecedented detail.

Luminescent europium(iii) and terbium(iii) complexes of β-diketonate and substituted terpyridine ligands: synthesis, crystal structures and elucidation of energy transfer pathways

A series of coordinatively saturated Ln^III complexes: [Ln(R-TPY)(TTA)₃] (1–6) were designed and structurally characterized and plausible energy transfer (ET) pathways determined using a theoretical method.

Shining light on the excited state energy cascade in kinetically inert Ln(iii) complexes of a coumarin-appended DO3A ligand

Lanthanide based molecular probes for bioimaging relies on the antenna effect, here we are unravelling the excited state energy cascade that results in sensitized lanthanide luminescence.