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Very bright europium complexes that stain cellular mitochondria
Abstract
The synthesis, structure and photophysical properties of a series of highly emissive europium complexes is reported. Certain complexes enter mammalian cells by macropinocytosis and stain the mitochondria selectively, allowing observation of the Eu emission in cellulo by time-gated spectral imaging.
... [7,8] Potential applications include emissive optical materials and various kinds of biological probes, also including in-cell imaging. [9][10][11][12][13][14][15] Major challenges for lanthanidebased luminescence probes are the quantum yield and lifetime optimization, [12,16] and consequently, the limitation of nonradiative deactivation processes like vibrational and/or rota-tional quenching or back-energy transfer. [17][18][19] Quenching by the OÀ H overtones of water often occurs from coordinated H 2 O [20,21] but may also be due to H 2 O in the second coordination sphere, [22] and therefore, full encapsulation of the metal ions by the ligand is of great importance. ...
... [7,8] Potential applications include emissive optical materials and various kinds of biological probes, also including in-cell imaging. [9][10][11][12][13][14][15] Major challenges for lanthanidebased luminescence probes are the quantum yield and lifetime optimization, [12,16] and consequently, the limitation of nonradiative deactivation processes like vibrational and/or rota-tional quenching or back-energy transfer. [17][18][19] Quenching by the OÀ H overtones of water often occurs from coordinated H 2 O [20,21] but may also be due to H 2 O in the second coordination sphere, [22] and therefore, full encapsulation of the metal ions by the ligand is of great importance. ...
... The values for lifetimes and quantum yields are comparable to other well studied complexes with macrocylic ligands. [3,6,12,24,25,28] The efficiency of the sensitization process could be further quantified in the case of the Eu III complex (see Supporting Information for details), where the radiative decay was calculated from the emission spectra to be 3.58 ms and 4.51 ms in the solid state and aqueous solution respectively. These numbers result in an intrinsic quantum yield of 0.41 and 0.34, respectively, giving sensitization efficiencies of 0.71 and 0.97, respectively. ...
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 ([EuIIIL²]⁺, H2O, 298 K, τ=1.51 ms, ϕ=0.35; [TbIIIL²]⁺, H2O, 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.
... Apart from the dominant S 1 → T 1 → Ln 3+ energy transfer, there are other forms of energy transfer pathways in a lanthanide complex, such as direct S 1 →Ln 3+ energy transfer [37,38]. Many compounds can serve as an antenna for lanthanide complexes such as 1,10-phenanthroline [39], phenlyethynylpridine [40], azaxanthone [41], terypyridine [42], porphine [43], pyridinedicarboxylic [39][40][41][42][43][44] (c) Examples of chelators for lanthanide coordination, including cyclen, dtpa, pcta and cdta (i-iv) and chromophores themselves (v-viii) as chelating ligands [19,26,[45][46][47]52]. acid [44] (Fig. 2b). ...
... To form stable complexes, the chromophores should incorporate with chelating ligands or act as chelating antenna themselves to coordinate with lanthanide ions (Fig. 2c) [26,[45][46][47]. Chelating antennas enable the direct contact of the chromophores to lanthanide ions and shorten the orbital distance between the donors and acceptors, making the energy transfer more efficient. ...
... The chelators also break the symmetry [29] of lanthanide ions, which causes f-orbitals to mix with d-orbitals and makes the forbidden 4f-4f transition somewhat allowed. Chelators, especially with higher coordination numbers ( Fig. 2c(vii)) [45] or cage structure ( Fig. 2c(viii)) [46], also protect the lanthanide centre from solvent coordination or leakeage of lanthanide ions. Various organic ligands, such as DOTAs [26,48,49], β-diketones [47], pyridines [45,50,51], porphine [43], and carboxylic acids [52], have been successfully used to form highly luminescent lanthanide complexes with desired photophysical properties. ...
Lanthanide complex-based medicine combining organic ligands and individual lanthanide ions is proving successful in therapeutics, especially in drug delivery, chemotherapy, and photodynamic therapy. In this review, the emerging advances of lanthanide complexes are surveyed and their key features benefiting the therapeutical monitoring and performance, such as fingerprint emissions, large pseudo-Stokes’ shifts, long lifetimes, two-photon excitations and magnetic resonance responses are discussed. Although translating the laboratory findings to clinical practices remains challenging, smart design and integration of the advances in smart point-of-care platforms, super-resolution microscopy, and time-gated techniques will boost the discovery and improvements of these versatile therapeutic agents to a higher level and promote healthcare efficiency.
... Spontaneous internalization of Ln III complexes in cells is unpredictable, and effort has been made to determine the correlation structure-cell uptake [21,42,[119][120][121][122][123][124][125][126][127]. The usual mechanism of cellular uptake of low molecular weight complexes is endocytosis [42]. ...
... In this mechanism, the complex interacts with the membrane forming vesicles that are responsible for the internalization of the Ln III complexes in the cell [42]. Thus, shape [119][120][121][122][123][124][125], chirality [128], and charge [21,42,127] are some of the factors that influence cell uptake. A thorough study conducted by Parker′s research group, using dota-and triazacyclonane-derivatized Ln III complexes, concluded that the mechanism of cell uptake involves recognition of the Ln III complex by proteins. ...
... A thorough study conducted by Parker′s research group, using dota-and triazacyclonane-derivatized Ln III complexes, concluded that the mechanism of cell uptake involves recognition of the Ln III complex by proteins. Thus, the shape and the chirality of the complex are factors that determine cell uptake [119][120][121][122][123][124][125]128]. The charge also plays an essential factor in the cell uptake of Ln III complexes. ...
The use of luminescence in biological systems allows one to diagnose diseases and understand cellular processes. Molecular systems, particularly lanthanide(III) complexes, have emerged as an attractive system for application in cellular luminescence imaging due to their long emission lifetimes, high brightness, possibility of controlling the spectroscopic properties at the molecular level, and tailoring of the ligand structure that adds sensing and therapeutic capabilities. This review aims to provide a background in luminescence imaging and lanthanide spectroscopy and discuss selected examples from the recent literature on lanthanide(III) luminescent complexes in cellular luminescence imaging, published in the period 2016–2020. Finally, the challenges and future directions that are pointing for the development of compounds that are capable of executing multiple functions and the use of light in regions where tissues and cells have low absorption will be discussed.
... The stability of sign and intensity with respect to the nature of the solvent parallels the behaviour observed in previous studies, where it was shown that the sign sequence of transitions within the ΔJ = 4 manifold correlates with the propeller chirality of each enantiomer, e. g. the Λ enantiomer has a À / + /À /À / + sequence. The complex absolute configuration has been independently determined via X-ray crystallography [22,23] The simultaneous presence within the same emitting species of transitions that are highly sensitive towards solvent polarity (ΔJ = 2) and comparatively insensitive (ΔJ = 4) allows internally referenced ratiometric measurements of medium polarity to be undertaken, with greatly enhanced sensitivity. To exemplify this point, the intensity ratio between the CPL transitions A (belonging to the ΔJ = 2 manifold) and H (belonging to the ΔJ = 4 manifold) can be considered ( Figure 5 and Figure SI 3). ...
... For instance, to terbium(III) complexes that display more intricate CPL spectra owing to the greater multiplicity of the emissive states. [4,23] For this reason, further work is necessary to verify if it is possible to exploit solvent effects to obtain mono-signate CPL emission for chiral terbium(III) systems, e. g. over a narrowly defined emission range. ...
Understanding the factors that shape the circularly polarised luminescence (CPL) emission profiles of europium(III)‐based CPL emitters to have specific sign properties, e. g. monosignate individual CPL transitions, is key to design novel complexes for applications ranging from advanced security inks to bio‐probes for live cell imaging. In order to correlate structure and spectral characteristics, a photophysical and kinetic investigation has been conducted on a series of coordinatively saturated nine‐coordinate europium(III) systems based on 1,4,7‐triazacyclononane. We highlight that lanthanide emission is sensitive to changes in the ligand field by showing the linear dependence of total emission intensity ratios as a function of solvent polarity, for europium(III) complexes displaying an internal charge transfer (ICT) excited state. This sensitivity increases by a factor of 20 when studying changes in CPL spectra, rendering these complexes accurate probes of local polarity. Solvent polarity, solvent‐specific effects, and the nature of the chromophores’ coordinating donor atoms strongly influence the kinetic stability of europium(III) complexes with respect to enantiomer interconversion. Notably, we show that the choice of donor groups to coordinating to europium(III) and the nature and polarity of the solvent affects the rate of racemisation, leading to systems with very long half‐lives at room temperature in non‐polar media.
... The europium complex, as shown in Fig. 7, has been designed by Parker and his colleagues and is used as a luminescent probe for live-cell imaging applications [45]. The design of the probe is based on the very effective shielding of the Eu(III) ion using nonadentate ligands based on triazacyclononane with strongly absorbing p-substituted aryl-alkynyl groups, [EuL2]. ...
... (b) Bidirectional confocal microscopy images showing staining of cellular mitochondria in NIH 3T3 cells for [Eu-L 2 ]. Reproduced from ref.[45]. Copyright 2013 Royal Society of Chemistry. ...
Mitochondria are vital organelles in eukaryotic cells, and can perform critical roles in supplying energy and maintaining cell functions. As a consequence, the design of mitochondria-targeting luminescent probes and the realisation of real-time mitochondrial activity acquisition are valuable tools in the study and therapy of mitochondrial dysfunction in diseases. Recently, lanthanide molecular compounds have emerged as potential mitochondria-targeting probes due to their low photobleaching propensity, sharp emission lines, and long excited-state lifetimes, which allow for time-resolved detection for increased sensitivity. A wide range of mitochondria-targeting luminescent lanthanide probes have been developed and employed for the detection of various reactive oxygen and bioactive species at the subcellular level in recent years, and the current review summarises all these advances. The design strategies, sensing mechanisms, and applications in bio-imaging of various reactive oxygen species in mitochondria are detailed here.
... A selection of the chromophores that have been explored as sensitisers for lanthanide emission, with their triplet energies, (E T ), and absorption maxima (l exc ), highlights the predominance of heterocyclic and aromatic systems (Scheme 2). [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26] The Ln ion needs to be as close as possible in space to the chromophore, in order to optimise the efficiency of the ligand to metal energy transfer step, and so is best coordinated to the lanthanide ion by an integral donor, typically a pyridine nitrogen or an anionic oxygen atom. By minimizing the donor-acceptor distance, efficient wave function overlap is created that allows optimal energy transfer to occur by the Dexter electron exchange mechanism. ...
... In particular, such a situation arises for sensitisers with strongly electron donating groups on the aryl ring, e.g. in arylalkynylpyridines and certain acridones. 11,[21][22][23][24][25] In the former case with relatively weak donor groups on the aryl ring, the ICT state lies at higher energy and consequently a classical triplet-mediated sensitisation process occurs, with energy transfer via the Dexter exchange mechanism. ...
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.
... The few examples described showed moderate yields, even when using adapted copper-free Sonogashira procedures or performing the coupling reaction on metalated macrocycles [20,39]. Therefore, the picolinate moieties is mostly functionalized via C-C cross coupling reactions prior to the alkylation of the macrocyclic platform [34,[39][40][41][42][43]. In our case, we were confident that the presence of the protecting phosphoryl group on the cyclam moiety 11 should avoid any parasite interactions between the metal catalyst and the macrocyclic coordinating entity, therefore allowing the Buchwald-Hartwig reaction to perform gently. ...
Positron emission tomography (PET) imaging of Aβ plaques, is recognized as a tool for the diagnosis of Alzheimer’s dis- ease. As a contribution to the development of new strategies for early diagnosis of the disease, using PET medical imaging technique, a new copper complex, the [Cu(TE1PA-ONO)]+ was synthesized in ten steps. The key step of our strategy is the coupling of a monopicolinate-N-alkylated cyclam-based ligand with a moiety capable of recognizing Aβ plaques via a successful and challenging Buchwald-Hartwig coupling reaction. To our knowledge, it is the first time that such a strategy is used to functionalize polyazamacrocyclic derivatives. The thermodynamic stability constants determined in MeOH/ H2O solvent indicate that the attachment of this moiety does not weaken the chelating properties of TE1PA-ONO ligand in relation to parent HTE1PA. The novel complex described here is able to recognize amyloid plaques in brain sections from Alzheimer's disease patients and shows low toxicity to human neuronal cells.
... However, this system can be adapted for use with other long-lifetime luminescent probes such as lanthanide chelates and lanthanide-doped nanocrystals with lifetimes ranging from μs to ms 39 or porous silicon nanoparticles with a lifetime of 5-13 μs. 34 Lanthanide doped nanoparticles can be used for probing human cervical carcinoma (HeLa) cells 39 or cellular mitochondria, 54 and porous silicon nanoparticles can be used for screening ovarian cancer. 34 Currently, the closest application of time-gated photoluminescence to lensfree imaging uses single photon avalanche diode (SPAD) arrays for applications such as neural imaging 55 or fluorescence lifetime imaging. ...
Fluorescence and, more generally, photoluminescence enable high contrast imaging of targeted regions of interest through the use of photoluminescent probes with high specificity for different targets. Fluorescence can be used for rare cell imaging; however, this often requires a high space-bandwidth product: simultaneous high resolution and large field of view. With bulky traditional microscopes, high space-bandwidth product images require time-consuming mechanical scanning and stitching. Lensfree imaging can compactly and cost-effectively achieve a high space-bandwidth product in a single image through computational reconstruction of images from diffraction patterns recorded over the full field of view of standard image sensors. Many methods of lensfree photoluminescent imaging exist, where the excitation light is filtered before the image sensor, often by placing spectral filters between the sample and sensor. However, the sample-to-sensor distance is one of the limiting factors on resolution in lensfree systems and so more competitive performance can be obtained if this distance is reduced. Here, we show a time-gated lensfree photoluminescent imaging system that can achieve a resolution of 8.77 µm. We use europium chelate fluorophores because of their long lifetime (642 µs) and trigger camera exposure ∼50 µs after excitation. Because the excitation light is filtered temporally, there is no need for physical filters, enabling reduced sample-to-sensor distances and higher resolutions.
... 72,73 [Eu(L2)]displays an absorption band at 335 nm attributed to an ILCT transition. 54,74 Excitation into this band induces Eu 3+ emission. The Eu 3+ emission lifetime is long (1.09 ms, mono-exponential decay) and complementary measurements in D2O confirm that no water molecule is bound to the Eu 3+ ion. ...
Lanthanide(III) (Ln3+) complexes feature desirable luminescence properties for cell microscopy imaging, but cytosolic delivery of Ln3+ complexes and their use for 2P imaging of live cells are challenging. In this article, we describe the synthesis and spectroscopic characterizations of a series of Ln3+ complexes based on two ligands, L1 and L2, featuring extended picolinate push-pull antennas for longer wavelength absorption and 2P absorption properties as well as a free carboxylate function for conjugation to peptides. Several cell penetrating peptide/Ln3+ complex conjugates were then prepared with the most interesting luminescent complexes, Tb(L1) and Eu(L2), and with two cell penetrating peptides (CPPs), ZF5.3 and TP2. A spectroscopic analysis demonstrates that the luminescence properties of the complexes are not affected by conjugation to the peptide. The conjugates were evaluated for one-photon (1P) time-gated microscopy imaging, which suppresses biological background fluorescence, and 2P confocal microscopy. Whereas TP2-based conjugates were unable to enter cells, successful 1P and 2P imaging was performed with ZF5.3[Tb(L1)]. 2P confocal imaging suggests proper internalization and cytosolic delivery as expected for this CPP. Noteworthy, 2P confocal microscopy also allowed characterization of the luminescence properties of the complex (spectrum, lifetime) within the cell, opening the way to functional luminescent probes for 2P confocal imaging of live cells.
... Coordination complexes with various metals have been studied. However, surprisingly, although lanthanide (Ln) complexes have been intensively investigated for their luminescent and magnetic properties [27,28], as well as for biological applications [29][30][31][32][33], there are only a few reports on their peculiar NLO properties [34][35][36][37][38][39][40][41][42][43]. ...
Substitution of the diglyme ligand of [Eu(hfa)3(diglyme)] (where hfa is hexafluoroacetylacetonate) with a simple 1,10-phenanthroline leads to a six-fold increase of the product μβEFISH, as measured by the Electric-Field-Induced Second Harmonic generation (EFISH) technique. Similarly, [Eu(tta)3(1,10-phenanthroline)] (where Htta is 2-thenoyltrifluoroacetone) is characterized by a large second-order NLO response. Both 1,10-phenanthroline europium complexes have great potential as multifunctional materials for photonics.
... The PYCLEN core is also known to be an attractive platform for the design of stable and inert lanthanide complexes, as a result of the rigidifying effect of the pyridyl ring [118][119][120][121][122]. The smaller TACN platform has been also exploited for lanthanide complexation, in particular to design luminescent probes and MRI probes relying on the PARASHIFT principle [73,[123][124][125]. However, the number of complexes based on TACN that have been characterized through thermodynamic or kinetic studies is very scarce [126,127]. ...
In this chapter, we analyze ligand structural features necessary for the design of stable and inert lanthanide complexes for use in biomedical applications. In the first sections of the chapter we provide a brief introduction to the coordination chemistry of the lanthanides and some general complex design principles. We then discuss the main synthetic strategies available for the preparation of macrocycles for lanthanide complexation, paying particular attention to the regioselective functionalization of selected macrocycles. Subsequently, we review the thermodynamic stability of lanthanide complexes and the different stability trends observed across the lanthanide series. In the last part of the review, we present the main mechanisms responsible for the dissociation of lanthanide complexes and ligand characteristics that result in particularly inert complexes.
... This quest strongly stimulates the researchers' creativity as illustrated by the exceptional variety of polydentate or macrocyclic ligands published each year, where large brightness has been achieved using very different structures. As selected examples, record brightness has been obtained for (i) Eu(III) using macrocyclic triazacyclononane platform functionalized by a -conjugated chromophore, 18 (ii) Tb(III) thanks to a cryptate ligand [19][20] and (iii) Yb(III) using a sandwich complex between a porphyrin and a Kläui's tripodal ligand. 21 A large variety of chromophores have been tested as antenna among which polyaromatic dyes (anthracene, pyrene), [22][23] xanthene 24 and coumarin derivatives, [25][26] -conjugated chromophores (Michler ketone, 27 tetra-thiafulavalene, 28 dipolar dyes [29][30] ), coordination or organometallic complexes, 31-32 porphyrins, 21 dithienylethene, 33 murexide, [34][35] Herein, we report the use of [2.2]-paracyclophane (pCp) chromophore for the design of tunable antennas. ...
The multi-step synthesis of original antennas incorporating substituted [2.2]paracyclophane (pCp) moieties in the -conjugated skeleton is described. These antennas, functionalized with electron an donor alkoxy fragment (A1) or with a fused coumarin derivative (A2) are incorporated in a triazacyclonane macrocyclic ligand L1 or L2, respectively for the design of Eu(III), Yb(III) and Gd(III) complexes. A combined photophysical/theoretical study reveals that A1 presents a charge transfer character via the through-space paracyclophane conjugation, whereas A2 presents only local excited states centered on the coumarin-paracyclophane moiety, strongly favoring triplet state population via intersystem crossing. The resulting complexes EuL1 and YbL2 are fully emissive in the red and near infrared, respectively whereas the GdL2 complex acts as photosensitizer for the generation of singlet oxygen.
... There are more than 9,000 papers on europium luminescence on Web of Science, a number that has been growing exponentially for more than a decade. 14 The appeal of europium (III) luminescence lies in that it is highly sensitive to various environmental effects, [15][16][17] and can thus be used in optical sensors, to document self-assembly, [18][19] and for bioimaging 2,[20][21][22] . More recently, europium(III) luminescence has evolved to be a highly effective tool for determining the structure, speciation and electronic structure of europium(III) complexes in solution. ...
Lanthanide luminescence has been treated separate from molecular photophysics, although the underlying phenomena are the same. As the optical transitions observed in the trivalent lanthanide ions are forbidden, they do belong to the group that molecular photophysics have yet to conquer, yet the experimental descriptors remains valid. Determining these have proven challenging as full control/knowledge of sample composition is a prerequisite. This has been achieved, and here the luminescence quantum yields (ϕlum), luminescence lifetimes (τobs), oscillator strengths (f ), and the rates of non-radiative (knr) and radiative (kr ≡ A) deactivation of [Eu(H2O)9]3+ was determined for the trigonal tricapped prismatic (TTP) coordination geometry. Further, it was shown that instead of a full photophysical characterization, it is possible to relate changes in transition probabilities to the relative parameter Arel, which does not require reference data. While Arel does not afford comparisons between experiments, it resolves emission intensity changes due to emitter properties—changes in A—from intensity changes due to environmental effects—changes in knr, and differences in the number of photons absorbed. When working with fluorescence this may seem trivial, when working with lanthanide luminescence it is not.
... The tri(phenylphosphinate) C 3 symmetric complexes, [EuL 11 ] and [TbL 11 ], 36 and their methylphosphinate analogues, [LnL 12 ], form the core structures in the EuroTracker™ series of emissive lanthanide probes. 2,14,15,23,24 In each case, the Tb 3+ species was found to be more susceptible to quenching by Mn 2+ . Such behaviour is not consistent with a quenching mechanism involving electron transfer to the Ln* state, as the reduction of Tb 3+ is highly energetically unfavourable. ...
The relative sensitivities of structurally related Eu(III) complexes to quenching by electron and energy transfer processes have been compared. In two sets of 9-coordinate complexes based on 1,4,7-triazacyclononane, the Eu emission lifetime decreased as the number of conjugated sensitising groups and the number of unbound ligand N atoms increased, consistent with photoinduced electron transfer to the excited Eu(III) ion that is suppressed by N-protonation. Quenching of the Eu 5D0 excited state may also occur by electronic energy transfer, and the quenching of a variety of 9-coordinate complexes by a cyanine dye with optimal spectral overlap occurs by an efficient FRET process, defined by a Förster radius (R0) value of 68 Å and characterised by second rate constants in the order of 109 M-1 s-1; these values were insensitive to changes in the ligand structure and to the overall complex hydrophilicity. Quenching of the Eu and Tb excited states by energy transfer to Mn(II) and Cu(II) aqua ions occurred over much shorter distances, with rate constants of around 106 M-1 s-1, owing to the much lower spectral overlap integral. The calculated R0 values were estimated to be between 2.5 to 4 Å in the former case, suggesting the presence of a Dexter energy transfer mechanism that requires much closer contact, consistent with the enhanced sensitivity of the rate of quenching to the degree of steric shielding of the lanthanide ion provided by the ligand.
... This quest strongly stimulates the researchers' creativity as illustrated by the exceptional variety of polydentate or macrocyclic ligands published each year, where ample brightness has been achieved using very different structures. As selected examples, record brightness has been obtained for (i) Eu(III) using a macrocyclic triazacyclononane platform functionalized by a π-conjugated chromophore, 18 (ii) Tb(III) using a cryptate ligand, 19,20 and (iii) Yb(III) using a sandwich complex between a porphyrin and a Klaüi's tripodal ligand. 21 A large variety of chromophores have been tested as antennae, such as polyaromatic dyes (anthracene or pyrene), 22 Herein, we report the use of [pCp chromophores for the design of tunable antennas. ...
... 53,56,57 It should be noted that the observed QY is the product of the efficiency of ligand conversion to the donor state (e.g., the efficiency of intersystem crossing), the efficiency of energy transfer from the ligand to the lanthanide ion, and the actual emission QY of the lanthanide ion. Molar absorption coefficients (for complexes) are on the order of 10 4 M −1 cm −1 and most often centered in the UV, 54,55,58,59 resulting in large spectral separation between excitation and emission. 51 structure and PL emission from a phenomenon called quantum confinement (vide infra). ...
... This quest strongly stimulates the researchers' creativity as illustrated by the exceptional variety of polydentate or macrocyclic ligands published each year, where large brightness has been achieved using very different structures. As selected examples, record brightness has been obtained for (i) Eu(III) using macrocyclic triazacyclononane platform functionalized by a -conjugated chromophore, 18 (ii) Tb(III) thanks to a cryptate ligand [19][20] and (iii) Yb(III) using a sandwich complex between a porphyrin and a Kläui's tripodal ligand. 21 A large variety of chromophores have been tested as antenna among which polyaromatic dyes (anthracene, pyrene), [22][23] xanthene 24 and coumarin derivatives, [25][26] -conjugated chromophores (Michler ketone, 27 tetra-thiafulavalene, 28 dipolar dyes [29][30] ), coordination or organometallic complexes, 31-32 porphyrins, 21 dithienylethene, 33 murexide, [34][35] Herein, we report the use of [2.2]-paracyclophane (pCp) chromophore for the design of tunable antennas. ...
The multi-step synthesis of original antennas incorporating substituted [2.2]paracyclophane (pCp) moieties in the -conjugated skeleton is described. These antennas, functionalized with electron an donor alkoxy fragment (A1) or with a fused coumarin derivative (A2) are incorporated in a triazacyclonane macrocyclic ligand L1 or L2, respectively for the design of Eu(III), Yb(III) and Gd(III) complexes. A combined photophysical/theoretical study reveals that A1 presents a charge transfer character via the through-space paracyclophane conjugation, whereas A2 presents only local excited states centered on the coumarin-paracyclophane moiety, strongly favoring triplet state population via intersystem crossing. The resulting complexes EuL1 and YbL2 are fully emissive in the red and near infrared, respectively whereas the GdL2 complex acts as photosensitizer for the generation of singlet oxygen.
... 37 The hetero-cycle absorption spectrum can be red-shifted through judiciously chosen para-substituents, or by fusion of additional aryl rings onto the pyridine. 21 The push-pull systems obtained by para-functionalization with electron-donating alkynes yield chromophores that are excellent sensitisers for Eu(III), [38][39][40][41][42] and have large two-photon absorption cross-sections. [43][44][45][46][47] Thus, the near infrared-emitting YbL VII and related structures can be excited in the red at λ ex = 760 nm. ...
A series of luminescent lanthanide(iii) complexes consisting of 1,4,7-triazacyclononane frameworks and three secondary amide-linked carbostyril antennae were synthesised. The metal binding sites were augmented with two pyridylcarboxylate donors yielding octadentate ligands. The antennae carried methyl, methoxymethyl or trifluoromethyl substituents in their 4-positions, allowing for a range of excited state energies and antenna electronic properties. The 1H NMR spectra of the Eu(iii) complexes were found to be analogous to each other. Similar results were obtained in the solid-state by single-crystal X-ray crystallography, which showed the structures to have nine-coordinate metal ions with heavily distorted tricapped trigonal prismatic geometries. Steady-state and time-resolved luminescence spectroscopy showed that the antennae could sensitize both Tb(iii) and Eu(iii), however, quantum yields were lower than in other octadentate complexes lacking pyridylcarboxylate. Complexes with more electron-poor pyridines were less emissive even when equipped with the same antenna. The oxidation and reduction potentials of the antennae and the pyridinecarboxylates, respectively, were determined by cyclic voltammetry. The obtained values were consistent with electron transfer from the excited antenna to the pyridine providing a previously unexplored quenching pathway that could efficiently compete with energy transfer to the lanthanide. These results show the crucial impact that photophysically innocent ligand binding sites can have on lanthanide luminescence.
... Depuis, de nombreux dérivés de cette antenne ont été utilisés afin réaliser l'imagerie confocale cellulaire [160], [161], [162], [163] . Triazacyclononane) et de trois antennes de type pyridine-alcyne-aryl comportant en position para un groupement O-alkyl possède un rendement quantique de 25% [164] dans un mélange ...
L’ADN est le support de l’information de tout être vivant. La possibilité de pouvoir cibler et visualiser in vivo une séquence spécifique d’ADN et plus particulièrement un gène est un enjeu de taille pour le suivi médical aussi bien que pour la compréhension du vivant. Pour y parvenir, la détection par luminescence est particulièrement attrayante de par sa facilité de visualisation avec des outils simples.L’objet de cette thèse était d’apporter la preuve de concept de sondes luminescentes pour la détection séquence spécifique d’ADN double brin, basées d’une part sur les propriétés de luminescence des lanthanides, particulièrement intéressantes pour la détection en milieu biologique, et d’autre part sur les propriétés de reconnaissance de l’ADN par les protéines à doigts de zinc. Nous nous sommes intéressé au ciblage d’un duplex d’ADN palindromique de 12 paires de bases par un couple de protéines à doigts de zinc intégrant un système FRET, où un complexe de lanthanide(III) sur une protéine joue le rôle de donneur et un fluorophore organique sur l’autre protéine joue celui d’accepteur.Pour cela, une nouvelle famille de complexes de lanthanide(III) bioconjugables a été élaborée et des protéines à doigts de zinc fonctionnalisée par différents chromophores ont été synthétisés chimiquement par synthèse peptidique supportée sur résine et assemblage par ligation chimique native de trois fragments. Les caractérisations spectroscopiques des systèmes développés ont permis de mettre en évidence l’interaction des sondes avec la séquence d’ADN palindromique et de valider la preuve de concept d’une détection de cette séquence par un FRET basé sur des lanthanides.
... There are essentially three strategies for such a purpose. The first is to use highly pre-organized ligands featuring macrocycles such as triazacyclononane or 1,4,7,10-tetraazacyclododecane (Cyclen) [152][153][154]. The second strategy is to provide numerous negatively charged functions such as carboxylates or phosphonates [154,155] (Figure 11). ...
Current biomedical imaging techniques are crucial for the diagnosis of various diseases. Each imaging technique uses specific probes that, although each one has its own merits, do not encompass all the functionalities required for comprehensive imaging (sensitivity, non-invasive-ness, etc.). Bimodal imaging methods are therefore rapidly becoming an important topic in advanced healthcare. This bimodality can be achieved by successive image acquisitions involving different and independent probes, one for each mode, with the risk of artifacts. It can be also achieved simultaneously by using a single probe combining a complete set of physical and chemical characteristics , in order to record complementary views of the same biological object at the same time. In this scenario, and focusing on bimodal magnetic resonance imaging (MRI) and optical imaging (OI), probes can be engineered by the attachment, more or less covalently, of a contrast agent (CA) to an organic or inorganic dye, or by designing single objects containing both the optical emitter and MRI-active dipole. If in the first type of system, there is frequent concern that at some point the dye may dissociate from the magnetic dipole, it may not in the second type. This review aims to present a summary of current activity relating to this kind of dual probes, with a special emphasis on lantha-nide-based luminescent nano-objects.
... High emission intensity is mandatory when emissive lanthanide complexes are used as tags in bioassays or as optical probes. 6 In addition to optimal emission properties, a luminescent Ln(III) complex must interact selectively with a target bioanalyte in a complex matrix containing competing species, such as proteins. Among the main proteins in the biological fluids, serum albumin (SA), which represents 52% of the protein composition in the circulatory system, has been broadly studied in light of its very important physiological and pharmacological functions. ...
The cationic enantiopure (R,R) and luminescent Eu(III) complex [Eu(bisoQcd)(H2O)2] OTf (with bisoQcd = N,N'-bis(2-isoquinolinmethyl)-trans-1,2-diaminocyclohexane N,N'-diacetate and OTf = triflate) was synthesized and characterized. At physiological pH, the 1:1 [Eu(bisoQcd)(H2O)2]+ species, possessing two water molecules in the inner coordination sphere, is largely dominant. The interaction with bovine serum albumin (BSA) was studied by means of several experimental techniques, such as luminescence spectroscopy, isothermal titration calorimetry (ITC), molecular docking (MD), and molecular dynamics simulations (MDS). In this direction, a ligand competition study was also performed by using three clinically established drugs (i.e., ibuprofen, warfarin, and digitoxin). The nature of this interaction is strongly affected by the type of the involved heteroaromatic antenna in the Eu(III) complexes. In fact, the presence of isoquinoline rings drives the corresponding complex toward the protein superficial area containing the tryptophan residue 134 (Trp134). As the main consequence, the metal center undergoes the loss of one water molecule upon interaction with the side chain of a glutamic acid residue. On the other hand, the similar complex containing pyridine rings ([Eu(bpcd)(H2O)2]Cl with bpcd = N,N'-bis(2-pyridylmethyl)-trans-1,2-diaminocyclohexane N,N'-diacetate) interacts more weakly with the protein in a different superficial cavity, without losing the coordinated water molecules.
Pyridines undergo a facile SNHAr phosphinylation with H-phosphinates under catalyst- and solvent-free conditions (50–55 °C) in the presence of benzoylphenylacetylene to afford 4-phosphinylpyridines in up to 68% yield. In this...
The incorporation of luminescent lanthanide complexes in inorganic matrices opens appealing possibilities in the rational upstream conception of new luminescent materials with easier recyclability. In this regard, the influence of SBA‐15 mesoporous silica, as a host matrix, on the photophysical properties of europium‐trisdipicolinate cesium salt is investigated. The lanthanide complex can be completely and reversibly adsorbed on the silica, using incipient wetness impregnation (IWI) in WATER, without the need of anchoring groups or covalent bonding on the silica surface. This specific procedure allows the homogeneous dispersion of the lanthanide complex into the host silica matrix. Appropriately assessing the photophysical properties of the targeted luminescent material proved remarkably challenging, demanding utmost caution, particularly due to the strong scattering of the mesoporous matrices. Additionally, while the observations confirm an important increase of luminescence lifetimes of lanthanide complexes upon integration into these mediums, this variation is attributed to the differing refractive indexes and not to specific surface interactions or confinement effect. Indeed, it appeared that previous literature precedents depicted erroneous exhalations of the complexes intrinsic quantum yield due to the repeated use of erroneous refractive index values. In the meantime, the luminance of these materials under UV irradiation improves drastically in comparison to what is obtained with the physical dispersion of micro‐crystals of the lanthanide complex onto silica. It is demonstrated that this improvement is due to a reduced inner filter effect in the adsorbed samples. In this regard, IWI of lanthanide complexes in the mesoporous matrices method could be of interest in the development of recyclable emitting layers, less demanding in terms of emitter quantity.
Detailed photophysical studies of luminescent lanthanide complexes are presented and elaborated using a newly developed thioanisolyl‐picolinate antenna and the related tacn macrocyclic ligand. The new ligand proved to sensitise Nd(III), Sm(III), Eu(III) and Yb(III) emission. Eu(III) complex showed complete energy transfer, yielding high quantum yield (44 %) and brightness, while the Tb(III) analogue underwent a thermally activated back‐energy transfer, resulting in a strong oxygen quenching of the triplet excited state. Transient absorption spectroscopy measurements of Gd(III), Tb(III) and Eu(III) compounds confirmed the sensitization processes upon the charge‐transfer antenna excitation. The triplet excited state lifetime of the Tb(III) complex was 5‐times longer than that of the Gd(III) analogue. In contrast, the triplet state was totally quenched by the energy transfer to the 4f‐metal ion in the Eu(III) species. Nonlinear two‐photon absorption highlighted efficient biphotonic sensitization in Eu(III) and Sm(III) complexes. In case of the Nd(III) compound, one‐photon absorption in 4f–4f transitions was predominant, despite the excitation at the antenna two‐photon band. This phenomenon was due to the Nd(III) 4f–4f transitions overlapping with the wavelength‐doubled absorption of the complex.
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.
In the past, Lanthanide Luminescent Bioprobes (LLBs) based on pyclen-bearing π-extended picolinate antennas were synthesized and demonstrated well-adapted optical properties for biphotonic microscopy. The objective of this work is to develop a strategy to design bifunctional analogues of the previously studied LLBs presenting an additional reactive chemical group to allow their coupling to biological vectors to reach deep in vivo targeted two-photon bioimaging. Herein, we elaborated a synthetic scheme allowing the introduction of a primary amine on the para position of the macrocyclic pyridine unit. The photophysical and bioimaging studies demonstrate that the introduction of the reactive function does not alter the luminescent properties of the LLBs paving the way for further applications.
Fluorescent chemosensors have been widely applied in many diverse fields such as biology, physiology, pharmacology, and environmental sciences. The interdisciplinary nature of chemosensor research has continued to grow over the last 25 years to meet the increasing needs of monitoring our environment and health.
More recently, a large range of fluorescent chemosensors have been established for the detection of biologically and/or environmentally important species, and are increasingly being used to solve biological problems. The use of these molecules as imaging probes to diagnose and treat disease is gaining momentum with clear future applications.
This book will bring together world-leading experts to describe the current state of play in the field and introduce the cutting-edge research and possible future directions into fluorescent chemosensors design. Chapters focus on the basic principles involved in the design of chemosensors for specific analytes, problems, and challenges in the field.
Concentrating on advanced techniques and methods, the book will be of use for academics and researchers across a number of disciplines, with international appeal.
Alkyl-H-phosphinic acid alkyl esters are synthesized in 65–71% yield via chemoselective reaction of alkyl bromides with available alkyl-H-phosphinic acids (60–65 °C, Et3N). The latter are prepared, in turn, by direct phosphorylation of alkyl bromides with red phosphorus under phase-transfer conditions.
Pyrroloquinoline quinone (PQQ) is a redox cofactor in calcium- and lanthanide-dependent alcohol dehydrogenases that has been known and studied for over 40 years. Despite its long history, many questions regarding its fluorescence properties, speciation in solution and in the active site of alcohol dehydrogenase remain open. Here we investigate the effects of pH and temperature on the distribution of different PQQ species (H3PQQ to PQQ3- in addition to water adducts and in complex with lanthanides) with NMR and UV-Vis spectroscopy as well as time-resolved laser-induced fluorescence spectroscopy (TRLFS). Using a europium derivative from a new, recently-discovered class of lanthanide-dependent methanol dehydrogenase (MDH) enzymes, we utilized two techniques to monitor Ln binding to the active sites of these enzymes. Employing TRLFS, we were able to follow Eu(III) binding directly to the active site of MDH using its luminescence and could quantify three Eu(III) states: Eu(III) in the active site of MDH, but also in solution as PQQ-bound Eu(III) and in the aquo-ion form. Additionally, we used the antenna effect to study PQQ and simultaneously Eu(III) in the active site.
In this work, the development of high luminescent europium(III) complexes in water solution is reported, including their synthesis, analyses of their photophysical properties and applications in bioassays. The three Eu(III) complexes are derived from new ligands based on a tripyridinophane platform. There are four distinct sections in the structure of these ligands: an 18-membered polyaminocarboxylic macrocycle to bind efficiently lanthanide ion in aqueous solutions, three chromophoric subunits (4-(phenylethynyl)pyridine moieties) to effectively sensitize the emission of the metal, two peripheral moieties to solubilise the complex in aqueous media (sulfonate, sulfobetaine or glucose groups) and a free NH2 group available for grafting or bioconjugation. In our synthetic procedure, a pivotal macrocyclic platform is obtained with a high yield in the crucial macrocyclization step due to a metal template ion effect (74% yield). In Tris aqueous buffer (pH 7.4), the Eu(III) complexes display a maximum excitation wavelength at 320 nm, a suitable overall quantum yield (14%), a relatively long lifetime (0.80 ms) and a one-photon brightness in the range of 10000 M-1 cm-1. Importantly, these photophysical properties are retained at dilute concentrations, even in the presence of a very large excess of potentially competing species, such as EDTA or Mg2+ ions. Furthermore, we report the bioconjugation of a Eu(III) complex labelled by an N-hydroxysuccinimide ester reactive group with an antibody (anti-Glutathione-S-Transferase) and the successful application of the corresponding antibody conjugate in the detection of GST-biotin in a fluoroimmunoassay. These new complexes provide solution for high sensitivity in Homogenous Time-Resolved Fluorescence (HTRF®) bioassays.
The potential applications in disparate fields led to a rapid evolution of luminescence thermometry. In particular, luminescent thermometry based on trivalent lanthanide ions (Ln³⁺) has become very popular in the past decade due to the unique versatility, stability, and narrow emission band profiles covering a broad spectral range (from ultraviolet to the infrared) with relatively high emission quantum yields. Nevertheless, the reliability of Ln³⁺ ratiometric nanothermometry measurements is recently questioned in a few works reporting fake temperature readouts caused by experimental artifacts and even intrinsic effects. Using NaYF4:Er³⁺/Yb³⁺@NaNdF4@PAA (PAA stands for polyacrylic acid) core–shell nanoparticles, it is shown that how the primary luminescent thermometer concept can be used to correct the thermometric parameter (the intensity ratio of the Er³⁺ ²H11/2 → ⁴I15/2 and ⁴S3/2 → ⁴I15/2 transitions) from the interference of the intruding ²H9/2 → ⁴I13/2 emission ensuring, thereafter, reliable temperature measurements.
Development of fluorescent/luminescent probes for rapid, selective and sensitive detection of reactive oxygen species (ROS) is of great significance for understanding the roles of ROS in pathophysiological processes. In the present research, a visible-light-excitable Eu³⁺ complex-based probe, Eu(L)3 (DPBT), is designed and synthesized for the time-gated luminescence (TGL) determination of hypochlorous acid (HClO) in vitro and in vivo. The proposed probe exhibits a rapid, selective and sensitive TGL response to HClO, and excellent localization of mitochondria in living cells with low cytotoxicity. These features allow the probe to be used for the TGL sensing and imaging of HClO formation in mimic inflammatory cells at a subcellular level, as well as in endotoxin-induced liver injury and rheumatoid arthritis in live mice. In addition, by immobilizing the probe in the PEG hydrogel, the smart sensor films with rapid response to HClO were prepared, and successfully used for the real-time monitoring of HClO generation in mouse wounds, in order to distinguish the infected wounds from acute ones. Overall, this study provides a useful tool for the clinical monitoring and treatment of wound diseases.
A family of three picolinate pyclen-based ligands, previously investigated for the complexation of Y3+ and some lanthanide ions (Gd3+, Eu3+), was studied with 161Tb and 177Lu in view of potential radiotherapeutic applications. The set of six Tb3+ and Lu3+ complexes was synthesized and fully characterized. The coordination properties in the solid state and in solution were thoroughly studied. Potentiometric titrations, supported by density functional theory (DFT) calculations, showed the very high stability constants of the Tb3+ and Lu3+ complexes, associated with remarkable kinetic inertness for up to 30 days in 1 M HCl. A complete radiolabeling study performed with both 161Tb and 177Lu radionuclides, including experiments with regard to the stability with and without scavengers and kinetic inertness using challenging agents, proved that the ligands could reasonably compete with the DOTA analogue. To conclude, the potential of using the same ligand for both radiotherapy and optical imaging was highlighted for the first time.
Near Infrared (NIR) imaging agents are extensively used in the biological or preclinical treatment and diagnosis of a wide range of diseases including cancers and tumors. The current arsenal of NIR compounds are most constituted by organic dyes, polymers, inorganic nanomaterials, whereas Ln molecular complexes explore an alternative approach to design NIR probes that are potentially bring new molecular toolkits into the biomedicine. In this review, NIR imaging agents are categorized according to their molecular sizes, constitution and the key properties and features of each class of compounds are briefly defined wherever possible. To better elucidate the features of Ln complexes, we provide a succinct understanding of sensitization process and molecular Ln luminescence at a mechanistic level, which may help to deliver new insights to design NIR imaging probes. Finally, we used our work on NIR ytterbium (Yb³⁺) probes as an example to raise awareness of exploring biologically relevant chemical space for lanthanide complexes as chemical entities for biological activity.
In addition to the already described ligand L
4a
, two pyclen-based lanthanide chelators, L
4b
and L
4c
, bearing two specific picolinate two-photon antennas (tailor-made for each targeted metal) and one acetate arm arranged in a dissymmetrical manner, have been synthesized, to form a complete family of lanthanide luminescent bioprobes: [EuL
4a
], [SmL
4a
], [YbL
4b
], [TbL
4c
], and [DyL
4c
]. Additionally, the symmetrically arranged regioisomer L
4a'
was also synthesized as well as its [EuL
4a'
] complex to highlight the astonishing positive impact of the dissymmetrical N-distribution of the functional chelating arms. The investigation clearly shows the high performance of each bioprobe, which, depending on the complexed lanthanide, could be used in various applications. Each presents high brightness, quantum yields, and lifetimes. Staining of the complexes into living human breast cancer cells was observed. In addition, in vivo two-photon microscopy was performed for the first time on a living zebrafish model with [EuL
4a
]. No apparent toxicity was detected on the growth of the zebrafish, and images of high quality were obtained.
Upconversion (UC) is the process by which the energy of multiple photons is absorbed by a compound and restored in
the form of a photon of higher energy than the incident light, resulting in an anti-Stokes process. Although studied
theoretically since the middle of the last century and experimentally observed in the 1960’s, the process was up to
recently mainly restricted to solid state devices and ultimately to nanoparticles at the end of the century. At the same
period, different researches were directed towards the possibility to observe UC at the molecular level and it is only
recently that the phenomenon could be observed in discrete molecular entities in solution with still very few examples.
This review aims at explaining the difficulties encountered at the molecular level compared to the solid state and
summarizes the results reported to date on UC at the molecular scale.
Mitochondria are essential organelles in eukaryotic cells, containing various signaling molecules and important enzymes associated with cell growth, death, and proliferation. The visualization of mitochondria and their biochemistry with confocal microscopy, fluorescence (phosphorescence) lifetime microscopy (FLIM, PLIM), and super-resolution microscopy has therefore been of great interest in recent years. In particular, transition metal complexes have emerged as excellent mitochondria-targeting probes, due to their high photostabilities, large Stokes shifts, tunable chemical structures and long luminescence lifetimes. In this review, we focus on platinum, ruthenium and iridium complexes, and their application as detectors of micro-environmental alterations as well as for the imaging of signaling molecules inside mitochondria.
Based on our previous works involving two 1,4,7-triazacyclononane (tacn)-based ligands Hno2py1pa (1-Picolinic acid-4,7-bis(pyridin-2-ylmethyl)-1,4,7-triazacyclononane) and Hno1pa (1-Picolinic acid-1,4,7-triazacyclononane), we report here the synthesis of analogues bearing picolinate-based π-conjugated ILCT (Intra-Ligand Charge Transfer) transition antenna (HL1, HL2), using regiospecific N-functionalization of the tacn skeleton and their related transition metal complexes (e.g. Cu2+, Zn2+ and Mn2+). Coordination properties as well as their photophysical and electrochemical properties were investigated in order to quantify the impact of such antenna on the luminescent or relaxometric properties of the complexes. The spectroscopic properties of the targeted ligands and metal complexes have been studied using UV-Vis absorption and fluorescence spectrocopies. While the zinc complex formed with HL1 possesses a moderate quantum yield of 5%, complexation of Cu2+ led to an extinction of the luminescence putatively attributed to a photo-induced electron transfer, as supported by spectroscopic and electrochemical evidences. The [Mn(L2)]+ complex is characterized by a fluorescence quantum yield close to 8% in CH2Cl2. The potential interest of such systems as bimodal probes has been assessed from radiolabeling experiments conducted on HL1 and 64Cu2+ as well as confocal microscopy analyses and from relaxometric studies carried out on the cationic [Mn(L2)]+ complex. These results showed that HL1 can be used for radiolabeling, with a radiochemical conversion of 40% in 15 min at 100 °C. Finally, the relaxivity values obtained for [Mn(L2)]+, r1p = 4.80 mM-1·s-1 and r2p = 8.72 mM-1·s-1, make the Mn(II) complex an ideal candidate as a probe for Magnetic Resonance Imaging.
The syntheses and photophysical behaviour of nine, strongly luminescent nonadentate Eu(III) complexes are reported. Each complex is based on N-functionalised 1,4,7- triazacyclononane, and linkage to other groups or targeting vectors can occur either via amide bond formation to a coordinated pyridine p-aminopropyl group or via a nucleophilic substitution reaction involving thiol attack on a metal coordinated p-nitropyridyl moiety. Evidence is presented in favour of the latter conjugation strategy, as parallel work with maleimide conjugates was complicated or compromised by the propensity to undergo postconjugation thiol exchange or succinimide ring hydrolysis reactions. Confocal microscopy and spectral imaging studies revealed that the peptide conjugate of AcCFFKDEL was found to localise selectively in the endoplasmic reticulum of mouse fibroblast cells, whereas the related maleimide conjugate was only observed in cellular lysosomes.
Azide- and alkyne-functionalized bioconjugable luminescent lanthanide complexes are reported. Reactive handles were introduced into the complexes by the late-stage modification of a methylenecarboxylic acid antenna pendent group. Tb and Eu quantum yields (11-13% and 3.4-3.6%, respectively) were not greatly affected by the presence of the azide or the alkyne compared to the parent complex (ΦTb = 10%, ΦEu = 2.8%). Two avenues were explored for improving the luminescence of the lanthanide (Ln) complexes: (1) attaching the antenna through a tertiary amide linker and (2) replacing a monodentate carboxylate ligand with a bidentate pyridylcarboxylate donor, which yielded a nonadentate ligand that could saturate the lanthanide coordination sphere and eliminate the quenching metal-bound water molecule that was present in the octadentate complexes. The combination of both approaches yielded Eu and Tb emitters with 5.8% and 46% quantum yields. For the Eu complex, this value was the same as ΦEu in the octadentate parent complex. We attribute this to increased photoinduced electron transfer quenching in the nonadentate species, which compensates for the reduced O-H quenching.
The spectroscopy of nitrate complexes of Eu(III) and Tb(III) with chiral and racemic imine-based [L1 = (N,N’-bis(2-pyridylmethylidene)-1,2-(R,R + S,S)-cyclohexanediamine) and L3 = N, N’-bis(2-quinolylmethylidene)-1,2-(R,R + S,S)-cyclohexanediamine] and amine-based [L2 = N,N’-bis(2-pyridylmethyl)-1,2-(R,R + S,S)-cyclohexanediamine) and L4 = N,N’-bis(2-quinolylmethyl)-1,2-(R,R + S,S)-cyclohexanediamine] ligands has been studied under high hydrostatic pressure (above 100 kbar). With the increasing pressure, a reduction of the Tb(III) and Eu(III) luminescence intensity is detected for all the complexes, whilst a significant reduction of the Tb(III) and Eu(III) excited state lifetimes has been observed for all Tb-based complexes [L1Tb(NO3)3 → L4Tb(NO3)3] and only for the Eu(III) complexes containing the imine-based ligands [L1Eu(NO3)3 and L3Eu(NO3)3]. This behavior has been rationalized taking into account two main aspects: i) the relative position of the energy levels of the ligands and the metal ions and ii) the change of these position upon compression DFT calculations have been also performed to elucidate the nature of the orbitals involved in the UV electronic absorption transitions (NTO orbitals) upstream of the energy transfer process to the metal ion.
Luminescent lanthanides provide a promising alternative to organic chromophores for cellular bioimaging and bioassay applications; efficacy is closely governed by their respective quantum yields. Conventionally utilized quantum-yield measurements for lanthanides are laborious and not amenable to rapid relative comparison of compound performance. Here, we introduce a high-throughput optical imaging method to determine and directly compare relative quantum yield using Cherenkov-radiation-mediated excitation of luminescent lanthanide complexes.
The f-block elements, which comprise both the lanthanide and actinide series, possess interesting spectroscopic, magnetic, and nuclear properties that make them uniquely suited for a range of biomedical applications. In this Forum Article, we provide a concise overview on the different ways that these elements are employed in medicine, highlighting their dual implementation in both diagnostic and therapeutic applications. A key requirement for the use of these labile metal ions in medicine is a suitable chelating agent that controls their in vivo biodistribution. Toward this goal, we also report our research describing the synthesis and characterization of a rigid 18-membered macrocycle called CHX-macropa, an analogue of the previously reported nonrigid ligand macropa ( J. Am. Chem. Soc. 2009, 131, 3331). The lanthanide coordination chemistry of CHX-macropa is explored in detail by pH potentiometry and density functional theory (DFT) calculations. These studies reveal that CHX-macropa exhibits an enhanced thermodynamic selectivity for large over small lanthanides in comparison to its nonrigid analogue macropa. DFT calculations suggest that a key factor in the enhanced selectivity of this ligand for the large f-block ions is its rigid macrocyclic core, which cannot adequately distort to interact effectively with small ions. On the basis of its high affinity for large f-block ions, the design strategies implemented in CHX-macropa may be valuable for applying these elements in the diagnosis or treatment of disease.
A series of highly luminescent europium(III) complexes which exhibit photoluminescence from the Eu(III) center following energy transfer from the UV absorbing organic sensitizer have been investigated using a combination of ultrafast optical transient absorption and Eu L3 X-ray transient absorption techniques. We have previously demonstrated that the latter can be used as a signature of 4f-4f excitation responsible for the photoluminescence in these Eu(III) coordination complexes, but the long timescale of the earlier measurements did not allow direct observation of the ligand-to-metal energy transfer step, preventing a determination of the sensitization mechanism. Here, we provide the first direct experimental verification that Dexter electron exchange from the ligand triplet state is the dominant energy transfer mechanism in these photoluminescent systems. Moreover, the optical transient absorption results obtained herein imply that energy transfer for all three compounds has near unity yield, regardless of differences in the sensitization efficiencies, suggesting that the variations in the sensitization efficiencies are determined almost entirely by differences in the ligand-centered intersystem crossing rates. The implications for the rational design of more effective photoluminescent lanthanide complexes are discussed.
The design, synthesis, and application of a nine-coordinate gadolinium(III)-containing spin label, [Gd.sTPATCN]-SL, for use in nanometer-distance measurement experiments by EPR spectroscopy is presented. The spin label links to cysteines via a short thioether tether and has a narrow central transition indicative of small zero-field splitting (ZFS). A protein homodimer, TRIM25cc, was selectively labeled with [Gd.sTPATCN]-SL (70%) and a nitroxide (30%) under mild conditions and measured using the double electron electron resonance (DEER) technique with both commercial Q-band and home-built W-band spectrometers. The label shows great promise for increasing the sensitivity of DEER measurements through both its favorable relaxation parameters and the large DEER modulation depth at both Q- and W-band for the inter-Gd(III) DEER measurement which, at 9%, is the largest recorded under these conditions.
The synthesis, photophysical properties, DNA binding, DNA cleavage and cellular imaging behaviour of a range of complexes of the type [Re(CO)3(dppz)(PyR)]+ are reported, where PyR represents a range of substituted pyridines which have previously been studied for cellular localisation of related complexes. Confocal imaging experiments confirm that the complexes retain the variety of cellular localisation behaviour associated with the PyR units in other complexes, and suggest applications as probes for oligonucleotides in specific cellular compartments (e.g. mitochondrial DNA). This study illustrates the importance of considering cellular localisation as a prime consideration in the design of probes for in vivo application.
In this article, imaging applications of luminescent complexes and recent advances in the design and photophysical behaviour of near-IR responsive complexes are reviewed. Various properties of the luminescent lanthanide complexes are also discussed in detail.
Lanthanide complexes of a pyridylphenylphosphinate ligand based on triazacyclononane form an isostructural series. The C(3)-symmetric Δ and Λ complexes of Eu and Tb are strongly emissive and can be resolved by chiral HPLC; the absolute configuration of each complex has been assigned using CD and CPL measurements.
The synthesis and photo-physical properties of an original bis-pyridinylpyrazine chromophore efficiently sensitising europium(III) and samarium(III) are described. The corresponding lanthanide(III) complexes display in aqueous solutions a maximum excitation wavelength which is significantly red-shifted compared to the usual terpyridine-based chelates, and a valuable luminescence brightness above 2,000 dm(3) mol(-1) cm(-1) at 345 nm was obtained with a europium(III) derivative. Further functionalisation with three different bioconjugatable handles was also investigated and their ability to efficiently label a model hexapeptide was evaluated and compared. Finally, the best bioconjugatable europium(III) chelate was used in representative labelling experiments involving monoclonal antibodies and the luminescence features of the corresponding bioconjugates remained satisfactory.
The synthesis of the potentially nonadentate ligand 1,4,7-tris[(6-carboxypyridin-2-yl)methyl]-1,4,7-triazacyclononane (H3tpatcn), a new derivative of 1,4,7-triazacyclononane N-functionalised with three pyridinecarboxylate arms, is described. The complexes of four lanthanide ions (Nd, Eu, Gd, Lu) of this ligand have been prepared and structurally characterised. These complexes, which have high water solubility, show highly rigid C3 symmetric solution structures. All the complexes present mononuclear nine-coordinated solid-state structures. The coordination polyhedron is a slightly distorted tricapped trigonal prism. The NMRD (Nuclear Magnetic Relaxation Dispersion) profiles measured for the [Gd(tpatcn)] complex indicate that the second-sphere contribution arising from the presence of water molecules tightly hydrogen-bonded to the carboxylate moieties on the surface of the complex are not large enough to explain the very high relaxivity of the previously reported [Gd(tpaa)(H2O)2] complex (H3tpaa =
α,α′,α″-nitrilotri(6-methyl-2-pyridinecarboxylic acid)). In fact the low-field relaxivity of [Gd(tpatcn)] more likely points to a favorable electronic relaxation rate.
The synthesis and photophysical characterisation are reported of a series of cationic, neutral and anionic europium and terbium complexes based on structurally related, nonadentate ligands based on the cyclen macrocycle. Each complex incorporates a tetraazatriphenylene moiety and overall absolute emission quantum yields are in the range 15-40% in aerated aqueous media. Dynamic quenching of the lanthanide excited state occurs with electron-rich donors, e.g. iodide, ascorbate and urate, and a mechanistic interpretation is put forward involving an electron transfer process. The cationic lanthanide complexes are taken up by NlH/3T3 cells and tend to localise inside the cell nucleus.
A europium complex selectively staining the nucleolus of NIH 3T3, HeLa, and HDF cells is reported. This complex possesses not only the advantage of the long lifetime of europium emission (0.3 ms), but also a chromophore that allows excitation at a relatively long wavelength (lambda(max) = 384 nm) and gives rise to an acceptable quantum yield (9%). The complex can be used both in live cell and fixed cell imaging, giving an average intracellular concentration on the order of 0.5 microM. Strong binding to serum albumin has been demonstrated by examination of the analogous gadolinium complex, studying relaxivity changes with increasing protein concentration. The intracellular speciation of the complex has been examined by circularly polarized emission spectroscopy and is consistent with the presence of more than one europium species, possibly protein bound.
The synthesis, characterization and luminescent behavior of trivalent Sm, Eu, Dy and Tb complexes of two enantiomeric, octadentate, chiral, 2-hydroxyisophthalamide ligands are reported. These complexes are highly luminescent in solution. Functionalization of the achiral parent ligand with a chiral 1-phenylethylamine substituent on the open face of the complex in close proximity to the metal center yields complexes with strong circularly polarized luminescence (CPL) activity. This appears to be the first example of a system utilizing the same ligand architecture to sensitize four different lanthanide cations and display CPL activity. The luminescence dissymmetry factor, g{sub lum}, recorded for the Eu(III) complex is one of the highest values reported, and this is the first time the CPL effect has been demonstrated for a Sm(III) complex with a chiral ligand. The combination of high luminescence intensity with CPL activity should enable new bioanalytical applications of macromolecules in chiral environments.
The synthesis of novel 4-(phenylethynyl)pyridine subunits containing H2O-soluble complexing agents and their luminescence with EuIII ions are reported. Ligands with high luminescence intensities as well as quantum yields were obtained. Also the prepared labeling reagents as antibody conjugates gave the highest quantum and luminescence yields reported for H2O-soluble EuIII labels.
A series of Eu and Tb complexes of four different chiral ligands incorporating an azaxanthone sensitiser has been evaluated as probes for the bicarbonate anion. Their binding affinities were assessed at ambient pH with bicarbonate, lactate, citrate, phosphate and serum albumin. Binding was signalled by modulation of circularly polarised luminescence and apparent affinity constants were measured by examining changes in emission intensity ratios. Competition experiments show that with these species and ATP present at normal physiological values, bicarbonate can be determined selectively over the concentration range 10 to 35 mM. Bicarbonate levels are also reported by using a mixture of Eu and Tb complexes of a common ligand, examining the ratio of red/green emitted light. These methods have been adapted for the determination of bicarbonate in human serum and used for the assessment of mitochondrial levels of bicarbonate in several different cell types with confocal microscopy.
A series of seven emissive europium(III) and terbium(III) complexes was prepared, incorporating a 3-pyridyl-4-azaxanthone or 3-pyrazolyl-4-azaxanthone sensitising moiety within a polydentate macrocyclic ligand. High overall emission quantum yields in aqueous media are attenuated in the presence of protein or certain oxy anions due to displacement of the N,N'-chelated sensitiser. Nevertheless, these complexes are taken into cells and tend to localise over the first few hours in mitochondria before being trafficked to endosomal compartments. Cell uptake studies, in the presence of competitive inhibitors or promoters of well-defined uptake pathways, reveal a common uptake mechanism involving macro-pinocytosis.
A series of europium(III) and terbium(III) complexes of three 1,4,7-triazacyclononane-based pyridine containing ligands were synthesized. The three ligands differ from each other in the substitution of the pyridine pendant arm, namely they have a carboxylic acid, an ethylamide, or an ethyl ester substituent, i.e., these ligands are 6,6′,6″-[1,4,7-triazacyclononane-1,4,7-triyltris(methylene)]tris[pyridine-2-carboxylic acid] (H3tpatcn), -tris[pyridine-2-carboxamide] (tpatcnam), and -tris[pyridine-2-carboxylic acid] triethyl ester (tpatcnes) respectively. The quantum yields of both the europium(III) and terbium(III) emission, upon ligand excitation, were highly dependent upon ligand substitution, with a ca. 50-fold decrease for the carboxamide derivative in comparison to the picolinic acid (=pyridine-2-carboxylic acid) based ligand. Detailed analysis of the radiative rate constants and the energy of the triplet states for the three ligand systems revealed a less efficient energy transfer for the carboxamide-based systems. The stability of the three ligand systems in H2O was investigated. Although hydrolysis of the ethyl ester occurred in H2O for the [Ln(tpatcnes)](OTf)3 complexes, the tripositive [Ln(tpatcnam)](OTf)3 complexes and the neutral [Ln(tpatcn)] complexes showed high stability in H2O which makes them suitable for application in biological media. The [Tb(tpatcn)] complex formed easily in H2O and was thermodynamically stable at physiological pH (pTb 14.9), whereas the [Ln(tpatcnam)](OTf)3 complexes showed a very high kinetic stability in H2O, and once prepared in organic solvents, remained undissociated in H2O.
Nonadentate ligands based on triazacyclononane incorporating pyridyl-2-phosphinate groups form an isostructural series of complexes with Ln ions in the solid state and in solution. The Ln ion is effectively shielded from the solvent environment. Crystal structures reveal a rigid C(3)-symmetric tricapped trigonal-prismatic coordination geometry that is maintained in solution for the methyl and phenylphosphinate series, as shown by multinuclear NMR analysis. Variable-temperature measurements of the field dependence of the water proton relaxivity in gadolinium complexes indicate that these systems exclude solvent from the primary coordination environment and minimize the second sphere of solvation. The electronic relaxation time for the gadolinium methylphosphinate complex has been estimated to be 550 (±150) ps by EPR and NMR methods, compared to values of around 0.30-0.05 ps for the terbium-ytterbium series, deduced by analyzing the field dependence (4.7-16.5 T) of the (31)P NMR longitudinal relaxation times. Values are compared with analogous azacarboxylate ligand complexes, supporting a key role for donor atom polarizability in determining the electronic relaxation. Spectral emission behavior in solution of samarium, europium, terbium, and dysprosium complexes is compared, and the resolved RRR-Λ and SSS-Δ complexes show strong circularly polarized luminescence. The molecular quadratic hyperpolarizability 〈β(HLS)〉 has been measured in solution using hyper-Raleigh light-scattering methods, for the whole series of lanthanide complexes of one ligand. The values of 〈β(HLS)〉 reach a maximum around the center of the series and are not simply dependent on the number of f electrons, suggesting a dominant contribution from the octupolar rather than the dipolar term.
A series of experiments has been undertaken in order to gain a greater understanding of the cellular uptake and localisation behaviour of emissive lanthanide complexes as cellular stains or probes. Out of a large number of structurally related complexes characterised recently, a set of seven representative examples has been examined in detail, containing either tetraazatriphenylene or azaxanthone-based sensitising chromophores. Intracellular localisation profiles and cellular uptake and egress behaviour have been studied by microscopy and flow cytometry. Typically, the maximum intracellular concentration was of the order of 0.4 mM, or about 10(9) complexes per cell. The complexes studied were generally not toxic and did not perturb the mitochondrial membrane potential. A common uptake mechanism of macropinocytosis has been identified. A generalisation of trends in behaviour, and structure-activity relationships is presented, and the implications for future probe design discussed.
Cleanly separated by two photons: The design and photophysical characterization of a new highly stable macrocyclic ytterbium complex featuring a two-photon antenna ligand is described. The biphotonic sensitization of the near-infrared ytterbium(III) luminescence and the conception of an unconventional near-infrared biphotonic microscope allow performing in-depth imaging of thick tissues.
In this paper, recent advances in the synthesis, mechanism of sensitized emission, and luminescent properties of organic lanthanide complexes are reviewed. Stress is put on the progress in the development of organic europium complexes and their nanoparticles with excellent visible-light-sensitized and two-photon-sensitized EuIII luminescence properties. These are of increasing importance because bioanalysis or bioimaging techniques based on such labeled materials will combine the advantages of high sensitivity, high signal-to-noise ratio, deep penetration, and low photodamage to biological samples. In addition, the application of long-wavelength-sensitized luminescence of organic lanthanide complexes and their nanoparticles in bioimaging is discussed.
A synthetic approach is developed to obtain families of luminescent lanthanide complexes and markers from a generic family of precursors built from nonadentate coordination sites. The syntheses of the precursors, based on a directed regioselective nucleophilic aromatic substitution on polyfluoropyridines, are described. Functionalisation of the synthons on the aromatic moieties allowed the introduction of labelling functions and/or the extension of the electronic delocalisation, with concomitant changes in the spectroscopic properties. The synthesis of two such families of ligands and of some of their complexes of Eu(III) and Tb(III) are described, and the photo-physical properties of the complexes were measured, revealing excellent luminescence quantum yields reaching unity in some cases. For some of these complexes, the emphasis was further put on the preparation of an N-hydroxylsuccinimide (NHS) ester as activated function for labelling. The Tb and La complexes in the NHS activated form were synthesized and fully characterized. The labelling was first demonstrated on amino functionalized polymer beads and characterized by time-resolved luminescence microscopy. In a second step, the activated Tb complex was used for the labelling of GFR44 monoclonal antibody, and was applied to the detection of carcinoembryonic antigene (CEA) within the frame of a time-resolved fluoroimmunoassay. Comparison with a commercially available kit based on a europium cryptate as energy donor confirms the efficiency of Tb to act as an energy donor with an unoptimised 35% increase of the detection efficiency.
Responsive linear and two-photon induced europium emissive probes have been synthesised with a tailor made peptide for the detection of Cyclin A, the hypersensitive Eu emission (Eu-2) gave the real time signalling and also enhanced the two-photon absorption cross section from 12 GM to 68 GM after Cyclin A binding.
Image and spectral intensity from bicarbonate-selective europium(III) probes localised in the mitochondria of cells is modulated reversibly by variation of external pCO(2), and is suppressed by addition of the carbonic anhydrase inhibitor, acetazolomide.
Two europium complexes with bis(bipyridine) azamacrocyclic ligands featuring pendant arms with or without π-conjugated donor groups are synthesized and fully characterized by theoretical calculations and NMR spectroscopy. Their photophysical properties, including two-photon absorption, are investigated in water and in various organic solvents. The nonfunctionalized ligand gives highly water-stable europium complexes featuring bright luminescence properties but poor two-photon absorption cross sections. On the other hand, the europium complex with an extended conjugated antenna ligand presents a two-photon absorption cross section of 45 GM at 720 nm but is poorly luminescent in water. A detailed solvent-dependent photophysical study indicates that this luminescence quenching is not due to the direct coordination of O-H vibrators to the metal center but to the increase of nonradiative processes in a protic solvent induced by an internal isomerization equilibrium.
Recent startling interest for lanthanide luminescence is stimulated by the continuously expanding need for luminescent materials meeting the stringent requirements of telecommunication, lighting, electroluminescent devices, (bio-)analytical sensors and bio-imaging set-ups. This critical review describes the latest developments in (i) the sensitization of near-infrared luminescence, (ii) ‘‘soft’’ luminescent materials (liquid crystals, ionic liquids, ionogels), (iii) electroluminescent materials for organic light emitting diodes, with emphasis on white light generation, and (iv) applications in luminescent bio-sensing and bio-imaging based on time-resolved detection and multiphoton excitation (500 references).
A series of seven new tetrazole-based ligands (L1, L3-L8) containing terpyridine or bipyridine chromophores suited to the formation of luminescent complexes of lanthanides have been synthesized. All ligands were prepared from the respective carbonitriles by thermal cycloaddition of sodium azide. The crystal structures of the homoleptic terpyridine-tetrazolate complexes [Ln(Li)(2)]NHEt(3) (Ln = Nd, Eu, Tb for i = 1, 2; Ln = Eu for i = 3, 4) and of the monoaquo bypyridine-tetrazolate complex [Eu(H(2)O)(L7)(2)]NHEt(3) were determined. The tetradentate bipyridine-tetrazolate ligand forms nonhelical complexes that can contain a water molecule coordinated to the metal. Conversely, the pentadentate terpyridine-tetrazolate ligands wrap around the metal, thereby preventing solvent coordination and forming chiral double-helical complexes similarly to the analogue terpyridine-carboxylate. Proton NMR spectroscopy studies show that the solid-state structures of these complexes are retained in solution and indicate the kinetic stability of the hydrophobic complexes of terpyridine-tetrazolates. UV spectroscopy results suggest that terpyridine-tetrazolate complexes have a similar stability to their carboxylate analogues, which is sufficient for their isolation in aerobic conditions. The replacement of the carboxylate group with tetrazolate extends the absorption window of the corresponding terpyridine- (approximately 20 nm) and bipyridine-based (25 nm) complexes towards the visible region (up to 440 nm). Moreover, the substitution of the terpyridine-tetrazolate system with different groups in the ligand series L3-L6 has a very important effect on both absorption spectra and luminescence efficiency of their lanthanide complexes. The tetrazole-based ligands L1 and L3-L8 sensitize efficiently the luminescent emission of lanthanide ions in the visible and near-IR regions with quantum yields ranging from 5 to 53% for Eu(III) complexes, 6 to 35% for Tb(III) complexes, and 0.1 to 0.3% for Nd(III) complexes, which is among the highest reported for a neodymium complex. The luminescence efficiency could be related to the energy of the ligand triplet states, which are strongly correlated to the ligand structures.
Ligand-sensitized, luminescent lanthanide(III) complexes are of considerable importance because their unique photophysical properties (microsecond to millisecond lifetimes, characteristic and narrow emission bands, and large Stokes shifts) make them well suited as labels in fluorescence-based bioassays. The long-lived emission of lanthanide(III) cations can be temporally resolved from scattered light and background fluorescence to vastly enhance measurement sensitivity. One challenge in this field is the design of sensitizing ligands that provide highly emissive complexes with sufficient stability and aqueous solubility for practical applications.
To understand better the structure and function of biological systems, cell biologists and biochemists would like to have methods that minimally perturb living systems. The development of emissive optical probes is essential for improving our observation of intracellular signaling and recognition processes. Following excitation of the probe, photons emitted from the probe may be observed by spectroscopy or microscopy and encode information about their environments in their energy, lifetime, and polarization. Such optical probes may be based on organic fluorophores, quantum dots, recombinant proteins, or emissive metal complexes.
The use of long-lifetime emitting lanthanide(III) chelate labels or probes together with time-resolved fluorometry in detection provides a method to generate sensitive bioaffinity assays. However, the use of stable chelates demands very complicated optimization of the chelate structure. A great number of chelates have been synthesized, but usually, only the most prominent structures were then converted to corresponding biomolecule labeling reactants. This review covers the syntheses of luminescent lanthanide chelates comprising a pyridine subunit that allow solution and solid-phase bioconjugation.
The multistep synthesis of an extensive series of push-pull donor-pi-conjugated dipicolinic acid ligands is described. The charge transfer character of the ligand can be tuned by changing the donor group (CH 2R, OR, SR, or NR 2) or the nature of the conjugated backbone (phenyl, phenylethynyl, naphtylethynyl, bis(phenylethynyl), or chalcone). The photophysical properties of related D 3 symmetric europium complexes (absorption and luminescence) were measured. Experiments using two-photon sensitized luminescence of a Eu (III) complex reveal large two-photon absorption (TPA) cross-section values (775 GM at 740 nm) in dichloromethane. Furthermore, some structure-property relationships can be derived from this systematic study, allowing an optimization of TPA properties of lanthanide complexes.
A series of push-pull donor-pi-conjugated dipicolinic acid ligands and related tris-dipicolinate europium and lutetium complexes have been prepared. The ligands present broad absorption and emission transitions in the visible spectral range unambiguously assigned to charge-transfer transitions (CT) by means of time-dependent density functional theory calculations. The photophysical properties (absorption, emission, luminescence quantum yield, and lifetime) of the corresponding europium complexes were thoroughly investigated. Solvatochromism and temperature effects clearly confirm that Eu(III) sensitization directly occurs from the ligand CT state. In addition, modulation of the energy of the CT donating state by changing the nature of the donor fragment allows the optimal energy of the antennae for europium sensitization to be determined, and this optimal energy was found to be close to the (5)D 1 accepting state. Finally, this CT sensitization process has been successfully extended to near-infrared emitters (neodymium and ytterbium).
Several azaxanthone and azathioxanthone sensitizing chromophores have been incorporated into macrocyclic ligands and formed well-defined Eu and Tb complexes in polar media. Ecitation of the heterocyclic chromophore in the range 330 to 382 nm leads to modest amounts of aromatic fluorscence. Substituted acetophenones, benzophenons and acridones have been incorporated into ligand structures to promote lanthanide sensitization. A small library of substituted 1-azaxanthones and 1-azathioxanthones was prepared, in order to analyze the effect of varying the nature and position of substituents in the benzenoid ring on both their singlet and triplet energies. Fluoroscence emission for the 1-azathioxanthone examined was weak unless donor N or O lone-pairs were introduced into the chromophores.
The new potentially octadentate ligand, 1-(carboxymethyl)-4,7-bis[(6-carboxypyridin-2-yl)methyl]-1,4,7-triazacyclononane (H(3)bpatcn), in which two picolinate arms and one acetate arm are connected to the 1,4,7-triazacyclonane core, has been prepared. Potentiometric studies show an increased stability of the Gd(III) complex of H(3)bpatcn (logK(GdL)=15.8(2)) with respect to the Gd(III) complex of the analogous ligand 1,4,7-triazacyclononane-N,N',N''-triacetic acid (H(3)nota) (logK(GdL)=13.7), associated with an increased selectivity of H(3)bpatcn for gadolinium over calcium. The H(3)bpatcn ligand sensitises the terbium ion very efficiently, leading to a long-lived and highly luminescent terbium complex (quantum yield=43 %), in spite of the presence of a coordinated water molecule. (1)H proton NMR studies indicate that the metal ion is rigidly encapsulated by the three arms of the octadentate ligand H(3)bpatcn and that the macrocycle framework remains bound (through the five nitrogen and the three oxygen atoms) even at high temperature. A new theoretical method for interpreting the water proton relaxivity is presented. It is based on recent progresses in the description of the electronic spin relaxation and on an auxiliary probe solute. It replaces the Solomon, Bloembergen and Morgan (SBM) framework, which is questionable at low field, while avoiding resorting to simulations and/or sophisticated theories with additional unknown zero-field splitting (ZFS) parameters. The inclusion of two picolinate groups on a triazacyclononane framework affords the mono-aquo gadolinium complex [Gd(bpatcn)(H(2)O)] with favourable electron-relaxation properties (tau(eff)(S0)=125 ps). The optimisation of the electronic relaxation by ligand design is especially important to achieve high relaxivity in the new generation macromolecular complexes with long rotational correlation times.
Phospholipase C beta (PLC-beta)-coupled G protein-coupled receptor (GPCR) activities traditionally are assessed by measuring Ca2+ triggered by D-myo-inositol 1,4,5-trisphosphate (IP3), a PLC-beta hydrolysis product, or by measuring the production of inositol phosphate using cumbersome radioactive assays. A specific detection of IP3 production was also established using IP3 binding proteins. The short lifetime of IP3 makes this detection very challenging in measuring GPCR responses. Indeed, this IP3 rapidly enters the metabolic inositol phosphate cascade. It has been known for decades that lithium chloride (LiCl) leads to D-myo-inositol 1-phosphate accumulation on GPCR activation by inhibiting inositol monophosphatase, the final enzyme of the IP3 metabolic cascade. We show here that IP1 can be used as a surrogate of IP3 to monitor GPCR activation. We developed a novel homogeneous time-resolved fluorescence (HTRF) assay that correlates perfectly with existing methods and is easily amenable to high-throughput screening. The IP-One assay was validated on various GPCR models. It has the advantage over the traditional Ca2+ assay of allowing the measurement of inverse agonist activity as well as the analysis of PLC-beta activity in any nontransfected primary cultures. Finally, the high assay specificity for D-myo-inositol 1 monophosphate (IP1(1)) opens new possibilities in developing selective assays to study the functional roles of the various isoforms of inositol phosphates.
The homoditopic ligand 6,6'-[methylenebis(1-methyl-1H-benzimid- azole-5.2-diyl)]bis(4-{2-[2-(2-methoxyethoxy)ethoxy]ethoxylpyridine-2-carboxylic acid) (H2LC2) has been tailored to self-assemble with lanthanide ions (Ln(III)), which results in the formation of neutral bimetallic helicates with the overall composition [Ln(2)(L-C2)(3)] and also provides a versatile platform for further derivatization. The grafting of poly(oxyethylene) groups onto the pyridine units ensures water solubility, while maintaining sizeable thermodynamic stability and adequate antenna effects for the excitation of both visible- and NIR-emitting Ln(III) ions. The conditional stability constants (109023) are close to 25 at physiological pH and under stoichiometric conditions. The ligand triplet state features adequate energy (0-phonon transition at approximate to 21900 cm(-1)) to sensitize the luminescence of Eu-III (Q = 21%) and Tb-III (11%) in aerated water at pH 7.4. The emission of several other VIS- and NIR-emitting ions, such as Sm-III (Q = 0.38%) or Yb-III (0.15%), for which in cellulo luminescence is evidenced for the first time, is also sensitized. The Eu-III emission spectrum arises from a main species with pseudo-D-3 symmetry and without coordinated water. The cell viability of several cancerous cell lines (MCF-7, HeLa, Jurkat and 5D10) is unaffected if incubated with up to 500 mu m [Eu-2(L-C2)(3)] during 24 h. Bright Eu-III emission is seen for incubation concentrations above 10 mu m and after a 15-minute loading time; similar images are obtained with Tb-III and Sm-III. The helicates probably permeate into the cytoplasm of HeLa cells by endocytosis. The described luminescent helical stains are robust chemical species which remain undissociated in the cell medium and in presence of other complexing agents, such as edta, dtpa, citrate or L-ascorbate. Their derivatization, which would open the way to the sensing of targeted in cellulo phenomena, is currently under investigation.
Tetrazolate groups have been included by a convenient synthetic route in diverse ligand topologies, which have allowed the incorporation of lanthanide ions into highly luminescent double- and triple-helical complexes, demonstrating their potential for the expansion of lanthanide chemistry and the development of lanthanide-based applications.
12 Examples of the use of emissive complexes of the d-block elements, e.g. Re, Pt, Ir, are increasingly common; e.g ChemComm Communication Downloaded by University of California -San Diego on 07
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79, 789. 5 Recent examples of the use of aryl–alkynylpyridines as sensitizing groups for Eu emission: (a)
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Details of complex synthesis and characterization are given in the ESI †. 7 E Similar methods have been reported earlier
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47, 12289. 4 Polydentate ligands based on triazacyclononane with picolinate donors: (a)
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10 In the rapid diffusion limit, energy transfer from a Eu donor to the dye acceptor (Q) obeys pseudo-first order kinetics
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358, 126.
10 In the rapid diffusion limit, energy transfer from a Eu donor to the
dye acceptor (Q) obeys pseudo-first order kinetics, where 1/t o = k o ;
1/t = k obs and k obs = k o + k 2 [Q]; hence t 0 /t = k obs /k o = 1 + k 2 /k o [Q].
d, 3 J C-P 19 Hz, C 6
- Hz
Hz, H 22 ), 1.36 (3H, t, 3 J H-H 7.1 Hz, H 13 ); 13 C NMR (151 MHz, CDCl 3, δ): 165.9 (C 20 ),
161.2 (d, 3 J C-P 19 Hz, C 6 ), 153.7 (d, 1 J C-P 166 Hz, C 2 ), 132.8 (d, 4 J C-P 3 Hz, C 11 ), 132.4
(d, 2 J C-P 10 Hz, C 9 ), 132.0 (d, 3 J C-P 12 Hz, C 4 ), 131.9 (C 17 ), 131.1 (C 19 ), 129.7 (C 18 ),
129.6 (d, 1 J C-P 139 Hz, C 8 ), 128.6 (d, 3 J C-P 13 Hz, C 10 ), 128.5 (d, 2 J C-P 23
(d, 3 J C-P 6 Hz, C 13 ), 14.4 (C 22 ); 31 P NMR (243 MHz, CDCl 3 , Electronic Supplementary Material (ESI) for
Hz, C 12 ), 61.4 (C 21 ), 16.6 (d, 3 J C-P 6 Hz, C 13 ), 14.4 (C 22 ); 31 P NMR (243 MHz, CDCl 3,
Electronic Supplementary Material (ESI) for Chemical Communications
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6 μmol) was dissolved in a mixture of CD 3 OD/D 2 O (1.5 mL, 2:1 v/v) and KOH (3.8 mg, 67.5 μmol) was added. The solution was stirred at 60 °C under argon for 18 h. The reaction was monitored by 1 H-NMR spectroscopy
The triethyl ester of L 3 (8 mg, 5.6 μmol) was dissolved in a mixture of CD 3 OD/D 2 O
(1.5 mL, 2:1 v/v) and KOH (3.8 mg, 67.5 μmol) was added. The solution was stirred
at 60 °C under argon for 18 h. The reaction was monitored by 1 H-NMR spectroscopy
(400 MHz; loss of CH 3 CH 2 signals at 4.39, 4.13, 1.41 and 1.36 ppm) and 31 P-NMR
spectroscopy (162 MHz; reactant = +25.0 ppm, product = +15.4 ppm). The organic
Electronic Supplementary Material (ESI) for Chemical Communications
This journal is © The Royal Society of Chemistry 2013
94 (2H, dd, 3 J H-P 12.4 Hz, 3 J H-H 8.6 Hz
- O V Dolomanov
- L J Bourhis
- R J Gildea
07 (1H, dd, 3 J H-P 6.2 Hz, 4 J H-H 1.4 Hz, H 3 ), 7.94 (2H, dd, 3 J H-P 12.4 Hz, 3 J H-H 8.6 Hz,
H 9 ), 7.62 (4H, s, H 17 and H 18 ), 7.53 (2H, m, H 5 and H 11 ), 7.45 (2H, dt, 3 J H-P 4.0 Hz,
1.
O. V. Dolomanov, L. J. Bourhis, R. J. Gildea, J. A. K. Howard and H.