Fluorinated EuII-based multimodal contrast agent for temperature- and redox-responsive magnetic resonance imaging

Europium(II/III) coordination chemistry toward applications

Europium is an f-block metal with two easily accessible oxidation states (+2 and +3) that have vastly different magnetic and optical properties from each other. These properties are tunable using coordination chemistry and are useful in a variety of applications, including magnetic resonance imaging, luminescence, and catalysis. This review describes important aspects of coordination chemistry of Eu from the Allen Research Group and others, how ligand design has tuned the properties of Eu ions, and how those properties are relevant to specific applications. The review begins with an introduction to the coordination chemistry of divalent and trivalent Eu followed by examples of how the coordination chemistry of Eu has made contributions to magnetic resonance imaging, luminescence, catalysis, and separations. The article concludes with a brief outlook on future opportunities in the field.

Understanding the Coordination Chemistry and Structural and Photophysical Properties of EuII- and SmII-Containing Complexes of Hexamethylhexacyclen and Noncyclic Tetradentate Amines

Ligands play a crucial role in supporting or stabilizing the divalent oxidation state of lanthanide metals. To expand the range of ligands used to chelate divalent lanthanide ions, we synthesized and studied the structural and photophysical properties of complexes of EuII and SmII with hexamethylhexacyclen, 1,1,4,7,10,10-hexamethyltriethylenetetramine, tris[2-(dimethylamino)ethyl]amine, and tris[2-(isopropylamino)ethyl]amine as supporting ligands. Coordination of hexamethylhexacyclen, an analogue of 18-crown-6, generates sterically crowded complexes of EuII and SmII that are either seven or eight coordinate and adopt a range of geometries that differ from those of their 18-crown-6 counterparts and from those of lanthanide-containing complexes with the acyclic tetradente tertiary amine ligands included in this report. The emission spectra of EuII(hexamethylhexacyclen) show a moderate sensitivity to counterion identity and are more red-shifted compared to those of complexes of EuII with 18-crown-6 and the hexamethylated aza derivative of 2.2.2-cryptand. In addition, the morphology of hexamethylhexacyclen in [LnI(hexamethylhexacyclen)]I was found to resemble that of thermally stable alkalides of the form [M(hexamethylhexacyclen)]Na- (M = K+ or Cs+), suggesting that hexamethylhexacyclen could be an interesting ligand for strongly reducing lanthanide ions.

Effect of Pyrrolidinedithiocarbamate Ligand on the Luminescence Properties of Heteroligand Samarium and Europium Complexes: Experimental and Theoretical Study

The photophysical properties of two isostructural heteroligand lanthanide complexes of general formula Ln(pdtc)3(phen) (pdtc = pyrrolidinedithiocarbamate anion, phen = 1,10-phenanthroline), Ln = Sm3+ (1), Eu3+ (2)) were studied in solid state and dichloromethane (DCM) solution. The two lanthanide complexes were investigated by experimental techniques for structural (single-crystal X-ray diffraction analysis of 1, powder XRD, TG-DTA) and spectroscopic [electron paramagnetic resonance (EPR), infrared (IR), ultraviolet-visible (UV-vis), photoluminescence (PL)] characterization. DFT/TDDFT/ωB97xD and multireference SA-CASSCF/NEVPT2 calculations with perturbative spin-orbit coupling corrections were applied to construct the Jablonski energy diagrams and to discuss the excited state energy transfer mechanism with competing excited state processes and possible sensitized mechanism of metal-centered emission. The first excited state (S1) involved in the excited state energy transfer L(antenna)-to-Ln was predicted to have interligand (pdtc-to-phen) charge transfer character in contrast to the previously predicted ligand-to-metal charge transfer character. The theoretical consideration showed similar relaxation paths and luminescence quenching channels and appropriate Donor*(phen)-Acceptor*(Ln3+) energy gap for 1 and 2. The experimental measurements in the solid state, however, showed efficient luminescence and good ability to convert UV to visible light only for the Sm(pdtc)3(phen) complex. The minor emission of 2 was explained by partial reduction of Eu3+, confirmed by EPR and calculated electron density distribution data.

Contemporary Assessment of Energy Degeneracy in Orbital Mixing with Tetravalent f-Block Compounds

The f block is a comparatively understudied group of elements that find applications in many areas. Continued development of technologies involving the lanthanides (Ln) and actinides (An) requires a better fundamental understanding of their chemistry. Specifically, characterizing the electronic structure of the f elements presents a significant challenge due to the spatially core-like but energetically valence-like nature of the f orbitals. This duality led f-block scientists to hypothesize for decades that f-block chemistry is dominated by ionic metal-ligand interactions with little covalency because canonical covalent interactions require both spatial orbital overlap and orbital energy degeneracy. Recent studies on An compounds have suggested that An ions can engage in appreciable orbital mixing between An 5f and ligand orbitals, which was attributed to "energy-degeneracy-driven covalency". This model of bonding has since been a topic of debate because different computational methods have yielded results that support and refute the energy-degeneracy-driven covalency model. In this Viewpoint, literatures concerning the metal- and ligand-edge X-ray absorption near-edge structure (XANES) of five tetravalent f-block systems─MO2 (M = Ln, An), LnF4, MCl62-, and [Ln(NP(pip)3)4]─are compiled and discussed to explore metal-ligand bonding in f-block compounds through experimental metrics. Based on spectral assignments from a variety of theoretical models, covalency is seen to decrease from CeO2 and PrO2 to TbO2 through weaker ligand-to-metal charge-transfer (LMCT) interactions, while these LMCT interactions are not observed in the trivalent Ln sesquixodes until Yb. In comparison, while XANES characterization of AnO2 compounds is scarce, computational modeling of available X-ray absorption spectra suggests that covalency among AnO2 reaches a maximum between Am and Cm. Moreover, a decrease in covalency is observed upon changing ligands while maintaining an isostructural coordination environment from CeO2 to CeF4. These results could allude to the importance of orbital energy degeneracy in f-block bonding, but there are a variety of data gaps and conflicting results from different modeling techniques that need to be addressed before broad conclusions can be drawn.

Determining the Effects of Zero-Field Splitting and Magnetic Exchange in Dimeric Europium(II) Complexes

Related BAP [BAP = bis(acyl)phosphide] and Acac (Acac = β-diketonate) molecules perform as robust supports for both lanthanide and actinide metals. Here, a molecular bimetallic Eu2+ complex was successfully targeted and isolated by employing sodium bis(mesitoyl)phosphide [Na(mesBAP)] in a salt metathesis with EuI2, producing [Eu(mesBAP)2(et2o)]2 (et2o = metal-coordinated diethyl ether). The corresponding Acac-Eu2+ complex was targeted using mesAcac- (1,3-dimesityl-1,3-propanedione), generating [Eu(mesAcac)2(et2o)]2. Both complexes were characterized by single-crystal X-ray diffraction, UV-vis, IR, and NMR spectroscopies, and variable-temperature magnetic susceptibility. [Eu(mesBAP)2(et2o)]2 was persistent under anaerobic, anhydrous conditions, whereas the analogous [Eu(mesAcac)2(et2o)]2 showed evidence of decomposition under identical conditions. Variable-temperature magnetic susceptibility and magnetization studies of [Eu(mesBAP)2(et2o)]2 and [Eu(mesAcac)2(et2o)]2 were performed, resulting in similar magnetic exchange coupling values of Jex = -0.018 and -0.023 cm-1 and axial zero-field-splitting D values of -0.38 and -0.51 cm-1, respectively.

Diastereoselective Synthesis of α-Aryl-Substituted LnDOTA Chelates from Achiral Starting Materials by Deracemization Under Mild Conditions

Substituted derivatives of the DOTA framework are of general interest to alter chelate properties and facilitate the conjugation of chelates to other molecular structures. However, the scope of substituents that can be introduced into the α-position has traditionally been limited by the availability of a suitable enantiopure starting materials to facilitate a stereoselective synthesis. Tetra-substituted DOTA derivatives with phenyl and benzoate substituents in the α-position have been prepared. Initial syntheses used enantiopure starting materials but did not afford enantiopure products. This indicates that the integrity of the stereocenters was not preserved during synthesis, despite the homo-chiral diastereoisomer being the major reaction product. The homochiral diastereoisomer could be produced as the major or sole reaction product when starting from racemic or even achiral materials. Deracemization was found to occur during chelation through the formation of an enolate stabilized by the aryl substituent. This general ability of aryl groups to enable deracemization greatly increases the range of substituents that can be introduced into DOTA-type ligands with diastereochemical selectivity.

Donor Radii in Rare-Earth Complexes

We present a set of donor radii for the rare-earth cations obtained from the analysis of structural data available in the Cambridge Structural Database (CSD). Theoretical calculations using density functional theory (DFT) and wave function approaches (NEVPT2) demonstrate that the Ln-donor distances can be broken down into contributions of the cation and the donor atom, with the minimum in electron density (ρ) that defines the position of (3,-1) critical points corresponding well with Shannon's crystal radii (CR). Subsequent linear fits of the experimental bond distances for all rare earth cations (except Pm3+) afforded donor radii (rD) that allow for the prediction of Ln-donor distances regardless of the nature of the rare-earth cation and its oxidation state. This set of donor radii can be used to rationalize structural data and identify particularly weak or strong interactions, which has important implications in the understanding of the stability and reactivity of complexes of these metal ions. A few cases of incorrect atom assignments in X-ray structures were also identified using the derived rD values.

Fluorinated Mn(III)/(II)-Porphyrin with Redox-Responsive 1 H and 19 F Relaxation Properties

A new fluorinated manganese porphyrin, (Mn-TPP-p-CF3 ) is reported capable of providing, based on the Mn(III)/Mn(II) equilibrium, dual 1 H relaxivity and 19 F NMR response to redox changes. The physical-chemical characterization of both redox states in DMSO-d6 /H2 O evidenced that the 1 H relaxometric and 19 F NMR properties are appropriate for differential redox MRI detection. The Mn(III)-F distance (dMn-F =9.7-10 Å), as assessed by DFT calculations, is well tailored to allow for adequate paramagnetic effect of Mn(III) on 19 F T1 and T2 relaxation times. Mn-TPP-p-CF3 has a reversible Mn(II)/Mn(III) redox potential of 0.574 V vs. NHE in deoxygenated aqueous HEPES/ THF solution. The reduction of Mn(III)-TPP-p-CF3 in the presence of ascorbic acid is slowly, but fully reversed in the presence of air oxygen, as monitored by UV-Vis spectrometry and 19 F NMR. The broad 1 H and 19 F NMR signals of Mn(III)-TPP-p-CF3 disappear in the presence of 1 equivalent ascorbate replaced by a shifted and broadened 19 F NMR signal from Mn(II)-TPP-p-CF3 . Phantom 19 F MR images in DMSO show a MRI signal intensity decrease upon reduction of Mn(III)-TPP-p-CF3 , retrieved upon complete reoxidation in air within ~24 h. 1 H NMRD curves of the Mn(III)/(II)-TPP-p-CF3 chelates in mixed DMSO/water solvent have the typical shape of Mn(II)/Mn(III) porphyrins.

Light- and Chemical-Doping-Induced Magnetic Behavior of Eu Molecular Systems

Variable temperature electron paramagnetic resonance (VT-EPR) was used to investigate the role of the environment and oxidation states of several coordinated Eu compounds. We find that while Eu(III) chelating complexes are diamagnetic, simple chemical reduction results in the formation of paramagnetic species. In agreement with the distorted D3h symmetry of Eu molecular complexes investigated in this study, the EPR spectrum of reduced complexes showed axially symmetric signals (g⊥ = 2.001 and g∥ = 1.994) that were successfully simulated with two Eu isotopes with nuclear spin 5/2 (151Eu and 153Eu with 48% and 52% natural abundance, respectively) and nuclear g-factors 151Eu/153Eu = 2.27. Illumination of water-soluble complex Eu(dipic)3 at 4 K led to the ligand-to-metal charge transfer (LMCT) that resulted in the formation of Eu(II) in a rhombic environment (gx = 2.006, gy = 1.995, gz = 1.988). The existence of LMCT affects the luminescence of Eu(dipic)3, and pre-reduction of the complex to Eu(II)(dipic)3 reversibly reduces red luminescence with the appearance of a weak CT blue luminescence. Furthermore, encapsulation of a large portion of the dipic ligand with Cucurbit[7]uril, a pumpkin-shaped macrocycle, inhibited ligand-to-metal charge transfer, preventing the formation of Eu(II) upon illumination.

Outersphere Approach to Increasing the Persistance of Oxygen-Sensitive Europium(II)-Containing Contrast Agents for Magnetic Resonance Imaging with Perfluorocarbon Nanoemulsions toward Imaging of Hypoxia

Radiographic mapping of hypoxia is needed to study a wide range of diseases. Complexes of Eu(II) are a promising class of molecules to fit this need, but they are generally limited by their rapid oxidation rates in vivo. Here, a perfluorocarbon-nanoemulsion perfused with N2 , forms an interface with aqueous layers to hinder oxidation of a new perfluorocarbon-soluble complex of Eu(II). Conversion of the perfluorocarbon solution of Eu(II) into nanoemulsions results in observable differences between reduced and oxidized forms by magnetic resonance imaging both in vitro and in vivo. Oxidation in vivo occurrs over a period of ≈30 min compared to <5 min for a comparable Eu(II)-containing complex without nanoparticle interfaces. These results represent a critical step toward delivery of Eu(II)-containing complexes in vivo for the study of hypoxia.

An Fe complex for 19F magnetic resonance-based reversible redox sensing and multicolor imaging

We report a first-in-class responsive, pentafluorosulfanyl (-SF₅)-tagged ¹⁹F MRI agent capable of reversibly detecting reducing environments via an Fe^II/III redox couple. In the Fe^III form, the agent displays no ¹⁹F MR signal due to paramagnetic relaxation enhancement-induced signal broadening; however, upon rapid reduction to Fe^II with one equivalent of cysteine, the agent displays a robust ¹⁹F signal. Successive oxidation and reduction studies validate the reversibility of the agent. The -SF₅ tag in this agent enables 'multicolor imaging' in conjunction with sensors containing alternative fluorinated tags and this was demonstrated via simultaneous monitoring of the ¹⁹F MR signal of this -SF₅ agent and a hypoxia-responsive agent containing a -CF₃ group.

Toward quantification of hypoxia using fluorinated EuII/III-containing ratiometric probes

Hypoxia is a prognostic biomarker of rapidly growing cancers, where the extent of hypoxia is an indication of tumor progression and prognosis; therefore, hypoxia is also used for staging while performing chemo- and radiotherapeutics for cancer. Contrast-enhanced MRI using EuII-based contrast agents is a noninvasive method that can be used to map hypoxic tumors, but quantification of hypoxia using these agents is challenging due to the dependence of signal on the concentration of both oxygen and EuII. Here, we report a ratiometric method to eliminate concentration dependence of contrast enhancement of hypoxia using fluorinated EuII/III-containing probes. We studied three different EuII/III couples of complexes containing 4, 12, or 24 fluorine atoms to balance fluorine signal-to-noise ratio with aqueous solubility. The ratio between the longitudinal relaxation time (T1) and 19F signal of solutions containing different ratios of EuII- and EuIII-containing complexes was plotted against the percentage of EuII-containing complexes in solution. We denote the slope of the resulting curves as hypoxia indices because they can be used to quantify signal enhancement from Eu, that is related to oxygen concentration, without knowledge of the absolute concentration of Eu. This mapping of hypoxia was demonstrated in vivo in an orthotopic syngeneic tumor model. Our studies significantly contribute toward improving the ability to radiographically map and quantify hypoxia in real time, which is critical to the study of cancer and a wide range of diseases.

Tuning the pH of Activation of Fluorinated Hydrazone-Based Switches─A Pathway to Versatile 19F Magnetic Resonance Imaging Contrast Agents

Molecular switches have become an area of great interest in recent years. They are explored as high-density data storage and organic diodes in molecular electronics as well as chemosensors due to their ability to undergo a transition between well-defined structures under the action of external stimuli. One of the types of such switches is hydrazones. They work by changing the configuration from E to Z under the influence of pH or light. The change in configuration is accompanied by a change in the absorption band and changes in the nuclear magnetic resonance (NMR) spectrum. In this publication, the structure-property relationship of fluorinated hydrazone switches was established. A linear relationship between the Hammett substituent constants and the pH where the switching occurs was found. Introduction of strong electron-donating groups allowed obtaining a hydrazone switch of pK_a = 6 suitable for application in ¹⁹F MRI as contrast agents.

Systemic Delivery of Divalent Europium from Ligand Screening with Implications to Direct Imaging of Hypoxia

Hypoxia is a hallmark of many diseases, including cancer, arthritis, heart and kidney diseases, and diabetes, and it is often associated with disease aggressiveness and poor prognosis. Consequently, there is a critical need for imaging hypoxia in a noninvasive and direct way to diagnose, stage, and monitor the treatment and development of new therapies for these diseases. Eu-containing contrast agents for magnetic resonance imaging have demonstrated potential for in vivo imaging of hypoxia via changes in metal oxidation state from +2 to +3, but rapid oxidation in blood limits EuII-containing complexes to studies compatible with direct injection to sites. Here, we report a new EuII-containing complex that persists in oxygenated environments and is capable of persisting in blood long enough for imaging by magnetic resonance imaging. We describe the screening of a library of ligands that led to the discovery of the complex as well as a pH-dependent mechanism that hinders oxidation to enable usefulness in vivo. These studies of the first divalent lanthanide complex that persists in oxygenated solutions open the door to the use of EuII-based contrast agents for imaging hypoxia in a wide range of diseases.

Symmetric CEST-active lanthanide complexes for redox monitoring

Two symmetric ligands harbouring two TEMPO radicals and two functionalized acetamide arms (R = OMe (L1), CF₃ (L2)) were prepared and chelated to lanthanide ions (Eu^III, Yb^III for both L1 and L2, Dy^III for L1). Luminescence measurements on the europium complexes support the coordination of a single water molecule. The TEMPO arms are magnetically interacting in L1 (and its complexes) but not in L2. The TEMPO moieties can be reversibly oxidized into an oxoammonium (0.33-0.36 V vs. Fc⁺/Fc) or reduced into a hydroxylamine (ill-defined redox wave, reduction by ascorbate), which are both diamagnetic. The europium complexes [Eu(L1)]3+ and [Eu(L2)]3+ in their hydroxylamine form exhibit a temperature dependent CEST effect, which is maximal at 25 °C (30%) and 37 °C (12%), respectively. The CEST activity is dramatically reduced in the corresponding nitroxide forms due to the paramagnetism of the ligand. The europium complexes show no cytotoxicity against M21 cell lines over long incubation times (72 h) at high concentration (40 μM).

Dual-Mode Tumor Imaging Using Probes That Are Responsive to Hypoxia-Induced Pathological Conditions

Hypoxia in solid tumors is associated with poor prognosis, increased aggressiveness, and strong resistance to therapeutics, making accurate monitoring of hypoxia important. Several imaging modalities have been used to study hypoxia, but each modality has inherent limitations. The use of a second modality can compensate for the limitations and validate the results of any single imaging modality. In this review, we describe dual-mode imaging systems for the detection of hypoxia that have been reported since the start of the 21st century. First, we provide a brief overview of the hallmarks of hypoxia used for imaging and the imaging modalities used to detect hypoxia, including optical imaging, ultrasound imaging, photoacoustic imaging, single-photon emission tomography, X-ray computed tomography, positron emission tomography, Cerenkov radiation energy transfer imaging, magnetic resonance imaging, electron paramagnetic resonance imaging, magnetic particle imaging, and surface-enhanced Raman spectroscopy, and mass spectrometric imaging. These overviews are followed by examples of hypoxia-relevant imaging using a mixture of probes for complementary single-mode imaging techniques. Then, we describe dual-mode molecular switches that are responsive in multiple imaging modalities to at least one hypoxia-induced pathological change. Finally, we offer future perspectives toward dual-mode imaging of hypoxia and hypoxia-induced pathophysiological changes in tumor microenvironments.

19 F MRI Probes for Multimodal Imaging*

Magnetic resonance imaging (MRI) is one of the most powerful imaging tools today, capable of displaying superior soft-tissue contrast. This review discusses developments in the field of ¹⁹ F MRI multimodal probes in combination with optical fluorescence imaging (OFI), ¹ H MRI, chemical exchange saturation transfer (CEST) MRI, ultrasonography (USG), X-ray computed tomography (CT), single photon emission tomography (SPECT), positron emission tomography (PET), and photoacoustic imaging (PAI). In each case, multimodal ¹⁹ F MRI probes compensate for the deficiency of individual techniques and offer improved sensitivity or accuracy of detection over unimodal counterparts. Strategies for designing ¹⁹ F MRI multimodal probes are described with respect to their structure, physicochemical properties, biocompatibility, and the quality of images.

Understanding the Stabilization and Tunability of Divalent Europium 2.2.2B Cryptates

Lanthanides such as europium with more accessible divalent states are useful for studying redox stability afforded by macrocyclic organic ligands. Substituted cryptands, such as 2.2.2B cryptand, that increase the oxidative stability of divalent europium also provide coordination environments that support synthetic alterations of Eu(II) cryptate complexes. Two single crystal structures were obtained containing nine-coordinate Eu(II) 2.2.2B cryptate complexes that differ by a single coordination site, the occupation of which is dictated by changes in reaction conditions. A crystal structure containing a [Eu(2.2.2B)Cl]⁺ complex is obtained from a methanol-THF solvent mixture, while a methanol-acetonitrile solvent mixture affords a [Eu(2.2.2B)(CH₃OH)]²⁺ complex. While both crystals exhibit the typical blue emission observed in most Eu(II) containing compounds as a result of 4f⁶5d¹ to 4f⁷ transitions, computational results show that the substitution of a Cl^- anion in the place of a methanol molecule causes mixing of the 5d excited states in the Eu(II) 2.2.2B cryptate complex. Additionally, magnetism studies reveal the identity of the capping ligand in the Eu(II) 2.2.2B cryptate complex may also lead to exchange between Eu(II) metal centers facilitated by π-stacking interactions within the structure, slightly altering the anticipated magnetic moment. The synthetic control present in these systems makes them interesting candidates for studying less stable divalent lanthanides and the effects of precise modifications of the electronic structures of low valent lanthanide elements.

Copper(II) Pyridyl Aminophenolates: Hypoxia-Selective, Nucleus-Targeting Cytotoxins, and Magnetic Resonance Probes

Targeting the low-oxygen (hypoxic) environments found in many tumours by using redox-active metal complexes is a strategy that can enhance efficacy and reduce the side effects of chemotherapies. We have developed a series of Cu^II complexes with tridentate pyridine aminophenolate-based ligands for preferential activation in the reduction window provided by hypoxic tissues. Furthermore, ligand functionalization with a pendant CF₃ group provides a ¹⁹ F spectroscopic handle for magnetic-resonance studies of redox processes at the metal centre and behaviour in cellular environments. The phenol group in the ligand backbone was substituted at the para position with H, Cl, and NO₂ to modulate the reduction potential of the Cu^II centre, giving a range of values below the window expected for hypoxic tissues. The NO₂ -substituted complex, which has the highest reduction potential, showed enhanced cytotoxic selectivity towards HeLa cells grown under hypoxic conditions. Cell death occurs by apoptosis, as determined by analysis of the cell morphology. A combination of ¹⁹ F NMR and ICP-OES indicates localization of the NO₂ complex in HeLa cell nuclei and increased cellular accumulation under hypoxia. This correlates with DNA nuclease activity being the likely origin of cytotoxic activity, as demonstrated by cleavage of DNA plasmids in the presence of the Cu^II nitro complex and a reducing agent. Selective detection of the paramagnetic Cu^II complexes and their diamagnetic ligands by ¹⁹ F MRI suggests hypoxia-targeting theranostic applications by redox activation.

Magnetic resonance thermometry using a GdIII-based contrast agent

The complexes described here serve as contrast agents for magnetic resonance imaging thermometry. The complexes differentially enhance contrast between 275 and 325 K. The basis of the temperature response of the fluorinated contrast complex is the modulation of water exchange caused by trifluoromethyl groups that can be chemically controlled.

Rational Design of High-Relaxivity EuII -Based Contrast Agents for Magnetic Resonance Imaging of Low-Oxygen Environments

Metal-based contrast agents for magnetic resonance imaging present a promising avenue to image hypoxia. EuII -based contrast agents have a unique biologically relevant redox couple, EuII/III , that distinguishes this metal for use in hypoxia imaging. To that end, we investigated a strategy to enhance the contrast-enhancing capabilities of EuII -based cryptates in magnetic resonance imaging by controlling the rotational dynamics. Two dimetallic, EuII -containing cryptates were synthesized to test the efficacy of rigid versus flexible coupling strategies. A flexible strategy to dimerization led to a modest (114 %) increase in contrast enhancement per Eu ion (60 MHz, 298 K), but a rigid linking strategy led to an excellent (186 %) increase in contrast enhancement despite this compound's having the smaller molecular mass of the two dimetallic complexes. We envision the rigid linking strategy to be useful in the future design of potent EuII -based contrast agents for magnetic resonance imaging.

Rational Ligand Design Enables pH Control over Aqueous Iron Magnetostructural Dynamics and Relaxometric Properties

Complexes of Fe³⁺ engage in rich aqueous solution speciation chemistry in which discrete molecules can react with solvent water to form multinuclear μ-oxo and μ-hydroxide bridged species. Here we demonstrate how pH- and concentration-dependent equilibration between monomeric and μ-oxo-bridged dimeric Fe³⁺ complexes can be controlled through judicious ligand design. We purposed this chemistry to develop a first-in-class Fe³⁺-based MR imaging probe, Fe-PyCy2AI, that undergoes relaxivity change via pH-mediated control of monomer vs dimer speciation. The monomeric complex exists in a S = 5/2 configuration capable of inducing efficient T₁-relaxation, whereas the antiferromagnetically coupled dimeric complex is a much weaker relaxation agent. The mechanisms underpinning the pH dependence on relaxivity were interrogated by using a combination of pH potentiometry, ¹H and ¹⁷O relaxometry, electronic absorption spectroscopy, bulk magnetic susceptibility, electron paramagnetic resonance spectroscopy, and X-ray crystallography measurements. Taken together, the data demonstrate that PyCy2AI forms a ternary complex with high-spin Fe³⁺ and a rapidly exchanging water coligand, [Fe(PyCy2AI)(H₂O)]⁺ (ML), which can deprotonate to form the high-spin complex [Fe(PyCy2AI)(OH)] (ML(OH)). Under titration conditions of 7 mM Fe complex, water coligand deprotonation occurs with an apparent pK_a 6.46. Complex ML(OH) dimerizes to form the antiferromagnetically coupled dimeric complex [(Fe(PyCy2AI))₂O] ((ML)₂O) with an association constant (K_a) of 5.3 ± 2.2 mM^-1. The relaxivity of the monomeric complexes are between 7- and 18-fold greater than the antiferromagnetically coupled dimer at applied field strengths ranging between 1.4 and 11.7 T. ML(OH) and (ML)₂O interconvert rapidly within the pH 6.0-7.4 range that is relevant to human pathophysiology, resulting in substantial observed relaxivity change. Controlling Fe³⁺ μ-oxo bridging interactions through rational ligand design and in response to local chemical environment offers a robust mechanism for biochemically responsive MR signal modulation.

Versatile Nickel(II) Scaffolds as Coordination-Induced Spin-State Switches for 19 F Magnetic Resonance-Based Detection

¹⁹ F magnetic resonance (MR) based detection coupled with well-designed inorganic systems shows promise in biological investigations. Two proof-of-concept inorganic probes that exploit a novel mechanism for ¹⁹ F MR sensing based on converting from low-spin (S=0) to high-spin (S=1) Ni²⁺ are reported. Activation of diamagnetic NiL₁ and NiL₂ by light or β-galactosidase, respectively, converts them into paramagnetic NiL₀ , which displays a single ¹⁹ F NMR peak shifted by >35 ppm with accelerated relaxation rates. This spin-state switch is effective for sensing light or enzyme expression in live cells using ¹⁹ F MR spectroscopy and imaging that differentiate signals based on chemical shift and relaxation times. This general inorganic scaffold has potential for developing agents that can sense analytes ranging from ions to enzymes, opening up diverse possibilities for ¹⁹ F MR based biosensing.

Macrocyclic Pyclen-Based Gd3+ Complex with High Relaxivity and pH Response

We report the synthesis and characterization of the macrocyclic ligand 2,2'-((2-(3,9-bis(carboxymethyl)-3,6,9-triaza-1(2,6)-pyridinacyclodecaphane-6-yl)ethyl)azanediyl)diacetic acid (H₄L) and several of its complexes with lanthanide ions. The structure of the free ligand was determined using X-ray diffraction measurements. Two N atoms of the pyclen moiety in the trans position are protonated in the solid state, together with the exocyclic N atom and one of the carboxylate groups of the ligand. The relaxivity of the Gd³⁺ complex was found to increase from 6.7 mM^-1 s^-1 at pH 8.6 to 8.5 mM^-1 s^-1 below pH ≈ 6.0. Luminescence lifetime measurements recorded from H₂O and D₂O solutions of the Eu³⁺ complex evidence the presence of a single complex species in solution at low pH (∼5.0) that contains two inner-sphere water molecules. DFT calculations suggest that the coordination environment of the Ln³⁺ ion is fulfilled by the four N atoms of the pyclen unit, two oxygen atoms of the macrocyclic acetate groups, and an oxygen atom of an exocyclic carboxylate group. The two inner-sphere water molecules complete coordination number nine around the metal ion. At high pH (∼9.3), the lifetime of the excited ⁵D₀ level of Eu³⁺ displays a biexponential behavior that can be attributed to the presence of two species in solution with hydration numbers of q = 0 and q = 1. The ¹H NMR and DOSY spectra recorded from solutions of the Eu³⁺ and Y³⁺ complexes reveal a structural change triggered by pH and the formation of small aggregates at high pH values.

Spectroscopic and Electrochemical Trends in Divalent Lanthanides through Modulation of Coordination Environment

Due to the importance of both visible-light luminescence and lanthanides in modern society, the influence of the ligand environment on complexes of Yb^II were studied and compared with analogous complexes of Eu^II. Four ligands with systematically varied electronic and steric characteristics were used to probe the coordination environment and electronic and redox properties of the corresponding Yb^II-containing complexes. Strong-field nitrogenous donors gave rise to bathochromic shifts, leading to visible-light absorption by Yb^II. Trends in properties across the series of Yb^II-containing complexes were compared to trends reported for the analogous Eu^II-containing complexes, revealing the translatability of coordination environment effects across the divalent lanthanide series. These studies provide valuable information regarding the behavior of small and medium-sized divalent lanthanides outside of the solid state.

The solid-state structures and ligand cavity evaluation of lanthanide(iii) complexes of a DOTA analogue with a (dibenzylamino)methylphosphinate pendant arm

A series of lanthanide(iii) complexes of a monophosphinate analogue of H4dota, 1,4,7,10-tetraazacyclododecane-1,4,7-triacetic-10-methyl[(N,N-dibenzylamino)methyl]phosphinic acid (H4do3apDBAm = H4L1), were prepared and their solid-state structures were studied using single-crystal X-ray diffraction. In all structures, the ligand anion was octadentately coordinated to the Ln(iii) or Sc(iii) ions similarly to other DOTA-like ligands, i.e. forming parallel N4- and O4-planes. The lighter lanthanide(iii) complexes (till dysprosium) were nonacoordinated in the twisted square-antiprismatic (TSA) configuration with the apical coordination of water molecules or oxygen atoms from the neighbouring complex unit. The heavier lanthanide(iii) complexes (from terbium) were found as the "anhydrous" octacoordinated twisted square-antiprismatic (TSA') isomer. For the terbium(iii) ion, both forms were structurally characterized. The structural data of the Ln(iii)-H4L1 complexes and complexes of several related DOTA-like ligands were analysed. It clearly showed that the structural parameters for the square-antiprismatic (SA) isomers were clustered in a small range while those for the TSA/TSA' isomers were significantly more spread. The analysis also gave useful information about the influence of various pendant arms on the structure of the complexes of the DOTA-like ligands. The twist angle (torsion) of the chelate ring containing a larger phosphorus atom was similar to those of the remaining three acetate pendants. It led to a larger separation of the N4O4 planes and to smaller trans-O-Ln-O angles than the parameters found in the complexes of H4dota and its tetraamide derivatives dotam(R). It resulted in a relatively long bond between the metal ion and the coordinated water molecule. It led, together with the negative charge of the oxygen atoms forming the O4-plane, to an extremely fast water exchange rate reported for the Gd(iii)-H4L1 complex and, generally, to a fast water exchange of Gd(iii) complexes with the monophosphorus acid analogues of H4dota, H5do3ap/H4do3apR.

19F Magnetic Resonance Activity-Based Sensing Using Paramagnetic Metals

Fluorine magnetic resonance imaging (¹⁹F MRI) is a promising bioimaging technique due to the favorable magnetic resonance properties of the ¹⁹F nucleus and the lack of detectable biological background signal. A range of imaging agents have been developed for this imaging modality including small molecule perfluorocarbons, fluorine-rich macromolecules and nanoparticles, and paramagnetic metal-containing agents. Incorporation of paramagnetic metals into fluorinated agents provides a unique opportunity to manipulate relaxation and chemical shift properties of ¹⁹F nuclei. Paramagnetic centers will enhance relaxation rates of nearby ¹⁹F nuclei through paramagnetic relaxation enhancement (PRE). Further, metals with anisotropic unpaired electrons can induce changes in ¹⁹F chemical shift through pseudocontact shift (PCS) effects. PRE and PCS are dependent on the nature of the metal center itself, the molecular scaffold surrounding it, and the position of the ¹⁹F nucleus relative to the metal center. One intriguing prospect in ¹⁹F magnetic resonance molecular imaging is to design responsive agents that can serve to provide a read out biological activity, including the activity of enzymes, redox activity, the activity of ions, etc. Paramagnetic agents are well suited for this activity-based sensing as metal complexes can be designed to respond to specific biological activities and give a corresponding ¹⁹F response that results from changes in the metal complex structure and subsequently PRE/PCS. Broadly speaking, when designing paramagnetic ¹⁹F MR biosensors, one can envision that in response to changes in analyte activity, the number of unpaired electrons of the metal changes or the ligand conformation/chemical composition changes. This Account highlights activity-based probes from the Que lab that harness paramagnetic metals to modulate ¹⁹F signal. We discuss probes that use conversion from Cu²⁺ to Cu⁺ in response to reducing environments to dequench the ¹⁹F MR signal. Probes in which oxidants convert Co²⁺ to Co³⁺, resulting in chemical shift responses, are also described. Finally, we explore our foray into using Ni²⁺ coordination switching to furnish probes with different ¹⁹F signals when they are converted between 4-coordinate square planar and higher coordination numbers. A major barrier for ¹⁹F MR molecular imaging is in vivo application, as signal sensitivity is relatively low, requiring long imaging times to detect imaging agents. Nanoparticle and macromolecular agents show promise due to their higher fluorine density and longer circulation times; however, their analyte scope is limited to analytes that induce cleavage events. A grand challenge for researchers in this area is adapting lessons learned from small molecule paramagnetic probes with promising in vitro activities for the development of probes with enhanced in vivo utility for basic biological and clinical applications.

Measurement of the Dissociation of EuII-Containing Cryptates Using Murexide

The dissociation rates of five Eu^II-containing cryptates in water were measured using UV-visible spectroscopy and murexide at pH 6.5, 7, 7.5, 8, and 9. Murexide was used as a coordinating dye for Eu^II. The results for a known cryptate were within experimental error of the value obtained using other methods and enabled the measurement of other cryptates. This validation of the use of murexide to study the dissociation of Eu^II-containing cryptates enables its use with other complexes of Eu^II.

A 19F-MRI probe for the detection of Fe(ii) ions in an aqueous system

An activity-based ¹⁹F-MRI probe that showed a chemical shift change in response to Fe(ii) was developed.

Reversible redox-responsive 1H/19F MRI molecular probes

The redox-responsive manganese(ii)/(iii) complexes serve as a pair of reversible probes for ¹H MRI and ¹⁹F MRI of biological redox species.

Molecular Magnetic Resonance Imaging with Gd(III)-Based Contrast Agents: Challenges and Key Advances

In an era of personalized medicine, the clinical community has become increasingly focused on understanding diseases at the cellular and molecular levels. Magnetic resonance imaging (MRI) is a powerful imaging modality for acquiring anatomical and functional information. However, it has limited applications in the field of molecular imaging due to its low sensitivity. To expand the capability of MRI to encompass molecular imaging applications, we introduced bioresponsive Gd(III)-based magnetic resonance contrast agents (GBCAs) in 1997. Since that time, many research groups across the globe have developed new examples of bioresponsive GBCAs. These contrast agents have shown great promise for visualizing several biochemical processes, such as gene expression, neuronal signaling, and hormone secretion. They are designed to be conditionally retained, or activated, in vivo in response to specific biochemical events of interest. As a result, an observed MR signal change can serve as a read-out for molecular events. A significant challenge for these probes is how to utilize them for noninvasive diagnostic and theranostic applications. This Perspective focuses on the design strategies that underlie bioresponsive probes, and describes the key advances made in recent years that are facilitating their application in vivo and ultimately in clinical translation. While the field of bioresponsive agents is embryonic, it is clear that many solutions to the experimental and clinical radiologic problems of today will be overcome by the probes of tomorrow.

Fluorinated MRI contrast agents and their versatile applications in the biomedical field

MRI has been recognized as one of the most applied medical imaging techniques in clinical practice. However, the presence of background signal coming from water protons in surrounding tissues makes sometimes the visualization of local contrast agents difficult. To remedy this, fluorine has been introduced as a reliable perspective, thanks to its magnetic properties being relatively close to those of protons. In this review, we aim to give an overall description of fluorine incorporation in contrast agents for MRI. The different kinds of fluorinated probes such as perfluorocarbons, fluorinated dendrimers, polymers and paramagnetic probes will be described, as will their imaging applications such as chemical exchange saturation transfer (CEST) imaging, physico-chemical changes detection, drug delivery, cell tracking and inflammation or tumors detection.

Molecular Magnetic Resonance Imaging Using a Redox-Active Iron Complex

We introduce a redox-active iron complex, Fe-PyC3A, as a biochemically responsive MRI contrast agent. Switching between Fe3+-PyC3A and Fe2+-PyC3A yields a full order of magnitude relaxivity change that is field-independent between 1.4 and 11.7 T. The oxidation of Fe2+-PyC3A to Fe3+-PyC3A by hydrogen peroxide is very rapid, and we capitalized on this behavior for the molecular imaging of acute inflammation, which is characterized by elevated levels of reactive oxygen species.  Injection of Fe2+-PyC3A generates strong, selective contrast enhancement of inflamed pancreatic tissue in a mouse model (caerulein/LPS model). No significant signal enhancement is observed in normal pancreatic tissue (saline-treated mice). Importantly, signal enhancement of the inflamed pancreas correlates strongly and significantly with ex vivo quantitation of the pro-inflammatory biomarker myeloperoxidase. This is the first example of using metal ion redox for the MR imaging of pathologic change in vivo. Redox-active Fe3+/2+ complexes represent a new design paradigm for biochemically responsive MRI contrast agents.

A dual-modal nanoprobe based on Eu(iii) complex-MnO2 nanosheet nanocomposites for time-gated luminescence-magnetic resonance imaging of glutathione in vitro and in vivo

Dual-modal fluorescence-magnetic resonance imaging (MRI) techniques have gained great interest in biomedical research and clinical practice, since they integrate the advantages of both imaging techniques and provide a useful approach to simultaneously investigate both molecular and anatomical information at the same biological structures. Herein, we report the construction of a dual-modal time-gated luminescence (TGL)/MRI nanoprobe, BHHBB-Eu3+@MnO2, for glutathione (GSH) by anchoring luminescent β-diketone-Eu3+ complexes on layered MnO2 nanosheets. The fabricated nanoprobe exhibited very week luminescence and MR signals since the luminescence of the Eu3+ complex was quenched by MnO2 nanosheets and Mn atoms were isolated from water. Upon exposure to GSH, the MnO2 nanosheets were rapidly and selectively reduced to Mn2+ ions, resulting in remarkable enhancements of TGL and MR signals simultaneously. The combination of TGL and MR detection modes enables the nanoprobe to be used for detecting GSH in a wide concentration range (1-1000 μM) and imaging GSH at different resolutions and depths ranging from the subcellular level to the whole body. Furthermore, the as-prepared nanoprobe exhibited a low cytotoxicity and good biocompatibility, rapid response rate, long-lived luminescence, and high sensitivity and selectivity for responding to GSH. These features allowed it to be successfully used for the TGL detection of GSH in human sera, TGL imaging of GSH in living cells and zebrafish, as well as dual-modal TGL/MR imaging of GSH in tumor-bearing mice. All of these results highlighted the applicability and advantages of the nanoprobe for detecting GSH in vitro and in vivo.

Summary of Imaging in 2020: Visualizing the Future of Healthcare with MR Imaging

The Imaging in 2020 meeting convenes biannually to discuss innovations in medical imaging. The 2018 meeting, titled "Visualizing the Future of Healthcare with MR Imaging," sought to encourage discussions of the future goals of MRI research, feature important discoveries, and foster scientific discourse between scientists from a variety of fields of expertise. Here, we highlight presented research and resulting discussions of the meeting.

A fluorinated bihydrazide conjugate for activatable sensing and imaging of hypochlorous acid by 19F NMR/MRI

A fluorinated bihydrazide conjugate as a ¹⁹F NMR/MRI probe with a “turn-on” character for activatable sensing and imaging of HClO was reported.

Copper(ii) complexes for cysteine detection using 19F magnetic resonance

Cysteine plays an essential role in maintaining cellular redox homeostasis and perturbations in cysteine concentration are associated with cardiovascular disease, liver disease, and cancer. 19F MRI is a promising modality for detecting cysteine in biology due to its high tissue penetration and negligible biological background signal. Herein we report fluorinated macrocyclic copper complexes that display a 19F NMR/MRI turn-on response following reduction of the Cu(ii) complexes by cysteine. The reactivity with cysteine was studied by monitoring the appearance of a robust diamagnetic 19F signal following addition of cysteine in conjunction with UV-vis and EPR spectroscopies. Importantly, complexes with -CH2CF3 tags display good water solubility. Studies with this complex in HeLa cells demonstrate the applicability of these probes to detect cysteine in complex biological environments.

Screening of ligands for redox-active europium using magnetic resonance imaging

We report a screening procedure to predict ligand coordination to Eu^II and Eu^III using magnetic resonance imaging in which bright images indicate complexation and dark images indicate no complexation. Here, paramagnetic Gd^III is used as a surrogate for Eu^III in the screening procedure to enable detection with magnetic resonance imaging. The screening procedure was tested using a set of eight ligands with known coordination to Eu^II and Eu^III, and results were found to be consistent with expected binding. Validation of the screening procedure with known coordination chemistry enables use with new ligands in the future.