Two-Photon Circularly Polarized Luminescence of Chiral Eu Complexes

We report the synthesis of chiral lanthanide complexes with extended π conjugation for efficient circularly polarized luminescence (CPL) via two-photon excitation (2PE). The pyridine bis-oxazoline (PyBox) core provides the chiral Ln3+ environment, while the extension of the conjugated backbone through the pyridine 4-position with a phenylacetylene unit increases the two-photon absorption cross section. This work presents an important step toward the development of chiral systems displaying enhanced nonlinear optical properties, with potential applications in imaging and sensing, as well as in photodynamic therapy due to the selective excitation of molecules within a specific focal volume.

Carbazole-Functionalized Dipicolinato LnIII Complexes Show Two-Photon Excitation and Viscosity-Sensitive Metal-Centered Emission

Eu^III and Yb^III complexes with the carbazole-dipicolinato ligand dpaCbz^2-, namely K₃[Eu(dpaCbz)₃] and K₃[Yb(dpaCbz)₃], were isolated. The Eu^III complex displayed metal-centred emission upon one-photon excitation with a sensitized emission efficiency Φ L Ln of 1.8±0.3 %, corresponding to an intrinsic emission efficiency Φ Ln Ln of 46% and a sensitization efficiency of η_sens 3.9%, with an emission lifetime of the emissive state τ of 1.087±0.005 ms. The Yb^III complex displayed Φ L Ln of 0.010±0.001 %, and a τ of 2.32±0.06 μs. The Eu^III-centred emission was sensitized as well upon two-photon excitation and a two-photon absorption cross-section σ_2PA of 63 GM at 750 nm was determined for the complex. The one- or two-photon sensitized emission intensity of the Eu^III complex changes by more than two-fold when the solvent viscosity is varied in the range 0.5 - 200 cP and the emission is independent of dissolved oxygen. The Yb^III complex displays a change in emission intensity as well. However, in this case, a dependence of the emission intensity on dissolved oxygen content was observed.

Coordination-Directed Self-Assembly of Functional Polynuclear Lanthanide Supramolecular Architectures

Lanthanide supramolecular chemistry is a fast growing and intriguing research field due to the unique photophysical, magnetic, and coordination properties of lanthanide ions (Ln^III). Compared with the intensively investigated mononuclear Ln-complexes, polymetallic lanthanide supramolecular assemblies offer more structural superiority and functional advantages. In recent decades, significant progress has been made in polynuclear lanthanide supramolecules, varying from structural evolution to luminescent and magnetic functional materials. This review summarizes the design principles in ligand-induced coordination-driven self-assembly of polynuclear Ln-structures and intends to offer guidance for the construction of more elegant Ln-based architectures and optimization of their functional performances. Design principles concerning the water solubility and chirality of the lanthanide-organic assemblies that are vital in extending their applications are emphasized. The strategies for improving the luminescent properties and the applications in up-conversion, host-guest chemistry, luminescent sensing, and catalysis have been summarized. Magnetic materials based on supramolecular assembled lanthanide architectures are given in an individual section and are classified based on their structural features. Challenges remaining and perspective directions in this field are also briefly discussed.

The design of responsive luminescent lanthanide probes and sensors

The principles of the design of responsive luminescent probes and sensors based on lanthanide emission are summarised, based on a mechanistic understanding of their mode of action. Competing kinetic pathways for deactivation of the excited states that occur are described, highlighting the need to consider each of the salient quenching processes. Such an analysis dictates the choice of both the ligand and its integral sensitising moiety for the particular application. The key aspects of quenching involving electron transfer and vibrational and electronic energy transfer are highlighted and exemplified. Responsive systems for pH, pM, pX and pO₂ and selected biochemical analytes are distinguished, according to the nature of the optical signal observed. Signal changes include both simple and ratiometric intensity measurements, emission lifetime variations and the unique features associated with the observation of circularly polarised luminescence (CPL) for chiral systems. A classification of responsive lanthanide probes is introduced. Examples of the operation of probes for reactive oxygen species, citrate, bicarbonate, α₁-AGP and pH are used to illustrate reversible and irreversible transformations of the ligand constitution, as well as the reversible changes to the metal primary and secondary coordination sphere that sensitively perturb the ligand field. Finally, systems that function by modulation of dynamic quenching of the ligand or metal excited states are described, including real time observation of endosomal acidification in living cells, rapid urate analysis in serum, accurate temperature assessment in confined compartments and high throughput screening of drug binding to G-protein coupled receptors.

Design of polyazamacrocyclic Gd3+ theranostic agents combining magnetic resonance imaging and two-photon photodynamic therapy

New “all-in-one” theranostic systems, combining a magnetic resonance imaging contrast agent with a biphotonic photodynamic therapy photosensitiser generating cytotoxic singlet oxygen, were successfully developed and characterized.

High-Throughput Platform for Real-Time Monitoring of ATP-Generating Enzymes in Living Cells Based on a Lanthanide Probe

Remarkable variation between cell-free and cellular measurements of enzyme activity triggered the unmet need to develop tools for monitoring enzyme activity in living cells. Such tools will advance our understanding of the biological functions of enzymes and their potential impact on drug discovery. We report in this study a universal assay for monitoring ATP-generating enzymes in living cells using a self-assembled Tb³⁺ complex probe. Modulation of the rheological properties of cell culture media enabled shifting the lifetime of the Tb³⁺ complex in the presence of ATP from micro-to-millisecond range. Based on the response of the Tb³⁺ complex to ATP, cellular assays for 5 ATP-generating enzymes were developed. Remarkably, assessment of the activity of these enzymes in living cells is made possible for the first time. The pyruvate kinase M2 (PKM2) assay has been optimized for high-throughput screening (HTS) and further implemented in the identification of novel scaffolds as PKM2 inhibitors.

Luminescent Carbazole-Based EuIII and YbIII Complexes with a High Two-Photon Absorption Cross-Section Enable Viscosity Sensing in the Visible and Near IR with One- and Two-Photon Excitation

The newly synthesized Eu^III and Yb^III complexes with the new carbazole-based ligands CPAD^2- and CPAP^4- display the characteristic long-lived metal-centered emission upon one- and two-photon excitation. The Eu^III complexes show the expected narrow emission bands in the red region, with emission lifetimes between 0.382 and 1.464 ms and quantum yields between 2.7% and 35.8%, while the Yb^III complexes show the expected emission in the NIR region, with emission lifetimes between 0.52 and 37.86 μs and quantum yields between 0.028% and 1.12%. Two-photon absorption cross sections (σ_2PA) as high as 857 GM were measured for the two ligands. The complexes showed a strong dependence of the one- and two-photon sensitized emission intensity on solvent viscosity in the range of 0.5-200 cP in the visible and NIR region.

Tripyridinophane Platform Containing Three Acetate Pendant Arms: An Attractive Structural Entry for the Development of Neutral Eu(III) and Tb(III) Complexes in Aqueous Solution

We report a detailed characterization of Eu³⁺ and Tb³⁺ complexes derived from a tripyridinophane macrocycle bearing three acetate side arms (H₃tpptac). Tpptac^3- displays an overall basicity (∑ log K_i^H) of 24.5, provides the formation of mononuclear ML species, and shows a good binding affinity for Ln³⁺ (log K_LnL = 17.5-18.7). These complexes are also thermodynamically stable at physiological pH (pEu = 18.6, pTb = 18.0). It should be noted that the pGd value of Gd-tpptac (18.4) is only slightly lower than that of commercially available MRI contrast agents such as Gd-dota (pGd = 19.2). Moreover, a very good selectivity for these ions over the endogenous cations (log K_CuL = 14.4, log K_ZnL = 12.9, and log K_CaL = 9.3) is observed. The X-ray structure of the terbium complex shows the metal coordinated by the nine N₆O₃ donor set of the ligand and one inner-sphere water molecule. DFT calculations result in two Eu-tpptac structures with similar bond energies (ΔE = 0.145 eV): one structure in which the water is coordinated to the metal ion and one structure in which the water molecule is farther away from the ion, bound to the ligand with an OH-π bond. By detailed luminescence experiments, we demonstrate that the europium complex in aqueous solution presents a hydration equilibrium between nine-coordinate, dehydrated [Eu-tpptac]⁰ and ten-coordinate, monohydrated [Eu-tpptac(H₂O)]⁰ species. A similar trend is observed for the terbium complex. Despite the presence of this hydration equilibrium, the H₃tpptac ligand sensitizes Eu³⁺ and Tb³⁺ luminescence efficiently in buffered water at physiological pH. Particularly, the terbium complex displays a long excited-state lifetime of 2.24 ms and an overall quantum yield of 33% with a brightness of 3600 M^-1 cm^-1. Such features of Ln³⁺ complexes of H₃tpptac indicate that this platform appears to be particularly appealing for the further development of luminescent lanthanide labels.

Luminescent Mesoporous Silica Nanohybrid Based on Drug Derivative Terbium Complex

Mesoporous silica nanoparticles prepared by organic template-driven synthesis have been successfully explored as carriers of the drug-derivate green luminescent complex of terbium (III) with the nonsteroidal anti-inflammatory drug ketoprofen. The terbium (III) complex was synthesized by reacting ketoprofen sodium salt with terbium (III) chloride, which was further adsorbed onto the surface of mesoporous nanoparticles with a mean particle size of 47 ± 4 nm and pore size of 11 nm. The incorporation of the complex into mesoporous silica nanoparticles was tracked by the decrease in the surface area and pore size of the nanoparticles, and successfully demonstrated by substantial changes in the adsorption isotherms and thermal and vibrational spectroscopy results. The cytotoxicity assay and confocal microscopy have shown that the novel luminescent nanohybrid presents high cell viability and the characteristic terbium (III) emission can be assessed through two-photon excitation, which paves the way for bioimaging applications in nanomedicine.

Lanthanide-based tools for the investigation of cellular environments

Biological probes constructed from lanthanides can provide a variety of readout signals, such as the luminescence of Eu(iii), Tb(iii), Yb(iii), Sm(iii) and Dy(iii), and the proton relaxation enhancement of Gd(iii) and Eu(ii). For numerous applications the intracellular delivery of the lanthanide probe is essential. Here, we review the methods for the intracellular delivery of non-targeted complexes (i.e. where the overall complex structure enhances cellular uptake), as well as complexes attached to a targeting unit (i.e. to a peptide or a small molecule) that facilitates delivery. The cellular applications of lanthanide-based supramolecules (dendrimers, metal organic frameworks) are covered briefly. Throughout, we emphasize the techniques that can confirm the intracellular localization of the lanthanides and those that enable the determination of the fate of the probes once inside the cell. Finally, we highlight methods that have not yet been applied in the context of lanthanide-based probes, but have been successful in the intracellular delivery of other metal-based probes.

Near infrared two photon imaging using a bright cationic Yb(iii) bioprobe spontaneously internalized into live cells

An Yb(iii) complex based on a dimethyl cyclen macrocyclic ligand functionalized by charge transfer antennae was prepared. This cationic [YbL3]⁺ complex is stable and soluble in water and presents interesting photophysical nonlinear properties. It is spontaneously internalized and accumulates in live cells. High quality images have been obtained both in a classical NIR-to-vis configuration and in the more challenging NIR-to-NIR one.

Responsive, Water-Soluble Europium(III) Luminescent Probes

The design principles, mechanism of action and performance of europium(III) complexes that serve as strongly emissive and responsive molecular probes in water are critically discussed. Examples of systems designed to assess pH, selected metal ions and anions, including chiral species, as well as selected small molecules and biopolymers are considered, and prospects evaluated for improved performance in more complex biological media such as in bio-fluids and within living cells. Modulation of the emission spectral form, lifetime and degree of circular polarisation can be used to quantify the spectral response and permit calibration.

Solvent polarity and oxygen sensitivity, rather than viscosity, determine lifetimes of biaryl-sensitised terbium luminescence

The lifetime of terbium luminescence sensitised by a biaryl group is influenced by solvent polarity and dissolved oxygen quenching the sensitiser.

Lanthanide-based luminescence biolabelling

Multiplexing, time-resolution, FRET…lanthanide-based biolabels reveal exceptional spectroscopic properties for bioanalytical applications.

Influence of the oxazole ring connection on the fluorescence of oxazoyl-triphenylamine biphotonic DNA probes

On the basis of our previous work on DNA fluorophores derived from vinylpyridinium-triphenylamine, we explored the structure space around the electron-rich triphenylamine (TP) core by changing the vinyl bond to an oxazole ring. As 2,5-diaryloxazoles are known to be highly fluorescent and efficient two photon absorbers, we synthesized analogues with two different connections of the oxazole to the triphenylamine core: TP-Ox2Py and TP-Ox5Py sets. Since the benzimidazolium group was proven to be more effective in the TP series than the pyridinium, we also synthesized a TP-Ox5Bzim set. The TP-Ox5Py series retains the TP-Py properties: on/off behavior on DNA, good two-photon cross-section and bright staining of nuclear DNA by microscopy under both one or two-photon excitation. On the other hand, the TP-Ox2Py series does not display fluorescence upon binding to DNA. The TP-Ox5Bzim set is fluorescent even in the absence of DNA and displays lower affinity than the corresponding TP-Ox5Py. CD experiments and docking were performed to understand these different behaviors.

A pair of highly biotolerated diamagnetic and paramagnetic iron(ii) complexes displaying electroneutrality

A pair of structurally analogous macrocyclic iron(ii) complexes with a magnetic off-on relationship is reported that exhibit electroneutrality at neutral pH and high stability in physiological media.