Luminescent chiral lanthanide(III) complexes as potential molecular probes

Ln(iii) complexes (Ln = Eu, Gd, Tb, Dy) with a chiral ligand containing 1,10-phenanthroline and (–)-menthol fragments: synthesis, structure, magnetic properties and photoluminescence

Novel chiral luminescent Ln(iii) complexes are reported.

Steric and Stereochemical Modulation in Pyridyl- and Quinolyl-Containing Ligands

Nitrogen-containing pyridine and quinoline are outstanding platforms on which excellent ionophores and sensors for metal ions can be built. Steric and stereochemical effects can be used to modulate the affinity and selectivity of such ligands toward different metal ions on the coordination chemistry front. On the signal transduction front, such effects can also be used to modulate optical responses of these ligands in metal sensing systems. In this review, steric modulation of achiral ligands and stereochemical modulation in chiral ligands, especially ionophores and sensors for zinc, copper, silver, and mercury, are examined using published structural and spectral data. Although it might be more challenging to construct chiral ligands than achiral ones, isotropic and anisotropic absorption signals from a single chiroptical fluorescent sensor provide not only detection but also differentiation of multiple analytes with high selectivity.

Enabling the Triplet of Tetraphenylethene to Sensitize the Excited State of Europium(III) for Protein Detection and Time-Resolved Luminescence Imaging

A tetraphenylethene (TPE) group that exhibits aggregation-induced emission is incorporated into the ligand of a Eu(III) complex (TPEEu) to sensitize the excited state of Eu(III). In steady-state measurements, TPEEu exhibits weak luminescence when dissolved in aqueous solutions even at a high concentration level, but emits strong fluorescence of TPE and phosphorescence of Eu(III) upon binding with bovine serum albumin. With a delay time of 0.05 ms and a gate time of 1.0 ms in time-resolved measurements, only phosphorescent emission of Eu(III) is observed with a high on/off ratio. Moreover, this probe is successfully used in time-resolved luminescence imaging to eliminate the background signal from biological autofluorescence without a washing process. This work provides a general strategy in designing Ln(III) complexes for detecting a broad range of biological molecules.

Stapled helical o-OPE foldamers as new circularly polarized luminescence emitters based on carbophilic interactions with Ag(i)-sensitivity

ortho-Oligo(phenylene)ethynylenes (o-OPEs) stapled with enantiopure 2,3-dihydroxybutane diethers have highly intense circular dichroism (CD) spectra and excellent circular polarized luminescence (CPL) responses (glum values up to 1.1 × 10-2), which are consistent with homochiral helically folded structures. In the presence of Ag(i), a change in the CPL emission is observed, representing the first example of CPL active small organic molecular emitters, which can be modulated by carbophilic interactions in a reversible manner.

Circularly polarized laser emission induced in isotropic and achiral dye systems

The production of efficient, tunable, and switchable circularly polarized laser emission would have far reaching implications in optical communications or biophotonics. In this work, it is demonstrated the direct generation of circularly polarized (CP) laser emission in achiral and isotropic dye laser systems without the use of extracavity polarizing elements, and without resorting to chiral dyes, chiral liquid crystal matrices, or interferometric methods. The origin of this ellipticity arises from the dynamic birefringence induced by the strong and polarized laser pumping and the subsequent orientation anisotropy of the excited molecular dipoles. A complete polarimetric characterization of the polarization state of conventional dye laser oscillators as a function of different experimental parameters is performed and it is shown that the generated light always possesses a certain level of circularity that changes in a distinctive way with pump energy and polarization. These results demonstrate that it is possible to generate and modulate CP laser light from efficient and photostable conventional laser dyes.

Bis(haloBODIPYs) with Labile Helicity: Valuable Simple Organic Molecules That Enable Circularly Polarized Luminescence

Simple organic molecules (SOM) based on bis(haloBODIPY) are shown to enable circularly polarized luminescence (CPL), giving rise to a new structural design for technologically valuable CPL-SOMs. The established design comprises together synthetic accessibility, labile helicity, possibility of reversing the handedness of the circularly polarized emission, and reactive functional groups, making it unique and attractive as advantageous platform for the development of smart CPL-SOMs.

Circularly polarized luminescence under near-UV excitation and structural elucidation of a Eu complex

A new chiral diketonate ligand based on carvone binds early lanthanides. The Eu complex displays high circularly polarized (g(lum) = 0.82) red luminescence under near-UV excitation in the solid state. This is the first report on such CPL activity in the solid state with a rigorous protocol to exclude artifacts. Paramagnetic NMR spectroscopy revealed the nature and structure of the active species.

Localizing Perturbations of the Racemic Equilibria Involving Dipicolinate-Derived Lanthanide(III) Complexes

Helical D3 tris(4-amino-2,6-pyridine-dicarboxylate)terbium(III) and europium(III) complexes, which form a racemic equilibrium in aqueous solution, were prepared to study their secondary coordination sphere interactions with chiral amino acids. These interactions were probed using a combination of circularly polarized luminescence (CPL) and 13C NMR spectroscopy. The results indicate that, regardless of the interaction between the chiral molecule and the complex, without an accessible hydrogen-bond donor on the associating molecule, perturbation of the racemic equilibrium cannot occur. A generalized conclusion is established that indicates that the mechanism of chiral recognition by tris(dipicolinate)lanthanide(III) complexes is similar across a variety of analogous ligands.

Circularly Polarized Luminescence from Helically Chiral N,N,O,O-Boron-Chelated Dipyrromethenes

Helically chiral N,N,O,O-boron chelated dipyrromethenes showed solution-phase circularly polarized luminescence (CPL) in the red region of the visible spectrum (λem (max) from 621 to 663 nm). The parent dipyrromethene is desymmetrised through O chelation of boron by the 3,5-ortho-phenolic substituents, inducing a helical chirality in the fluorophore. The combination of high luminescence dissymmetry factors (|glum | up to 4.7 ×10(-3) ) and fluorescence quantum yields (ΦF up to 0.73) gave exceptionally efficient circularly polarized red emission from these simple small organic fluorophores, enabling future application in CPL-based bioimaging.

Induced europium CPL for the selective signalling of phosphorylated amino-acids and O-phosphorylated hexapeptides

Circularly polarised luminescence signals selectively the binding of a range of chiral phosphorylated molecules.

Very bright, enantiopure europium(iii) complexes allow time-gated chiral contrast imaging

Chiral image contrast is reported with very bright enantiopure europium complexes that emit circularly polarized light.

Helical aromatic imide based enantiomers with full-color circularly polarized luminescence

Helical aromatic imide based enantiomers were synthesized, and they exhibited full-color emissions and CPL properties in solution.

Circularly polarized luminescence reveals interaction between commercial stains and protein matrices used in paintings

Here we show that circularly polarized luminescence (CPL) can give unique insight into interactions between fluorescent commercial stains and protein-based materials used in painting. CPL can complement information from ECD and fluorescence.

Circularly Polarized Luminescence from Simple Organic Molecules

This article aims to show the identity of "circularly polarized luminescent active simple organic molecules" as a new concept in organic chemistry due to the potential interest of these molecules, as availed by the exponentially growing number of research articles related to them. In particular, it describes and highlights the interest and difficulty in developing chiral simple (small and non-aggregated) organic molecules able to emit left- or right-circularly polarized light efficiently, the efforts realized up to now to reach this challenging objective, and the most significant milestones achieved to date. General guidelines for the preparation of these interesting molecules are also presented.

Chirality sensing and size recognition of N-Boc-amino acids by cage-type dimeric lanthanide complexes: chirality detection of N-Boc-aspartate anions via luminescence colour change

A mixture of chiral luminescent macrotricyclic-cyclen-Tb–Eu complexes enabled naked-eye discrimination of N-Boc-d- and l-aspartates via luminescence colour change.

Quantifying the formation of chiral luminescent lanthanide assemblies in an aqueous medium through chiroptical spectroscopy and generation of luminescent hydrogels

Herein we present the synthesis and the photophysical evaluation of water-soluble chiral ligands (2·(R,R) and 2·(S,S)) and their application in the formation of lanthanide directed self-assembled structures. These pyridine-2,6-dicarboxylic amide based ligands, possessing two naphthalene moieties as sensitising antennae, that can be used to populate the excited state of lanthanide ions, were structurally modified using 3-propanesultone and caesium carbonate, allowing for the incorporation of a water-solubilising sulfonate motif. We show, using microwave synthesis, that Eu(iii) forms chiral complexes in 1 : 3 (M : L) stoichiometries (Eu·[2·(R,R)]₃ and Eu·[2·(S,S)]₃) with these ligands, and that the red Eu(iii)-centred emission arising from these complexes has quantum yields (Φ_tot) of 12% in water. Both circular dichroism (CD) and circular polarised luminescence (CPL) analysis show that the complexes are chiral; giving rise to characteristic CD and CPL signatures for both the Λ and the Δ complexes, which both possess characteristic luminescence dissymmetry factors (g_lum), describing the structure in solution. The self-assembly process was also monitored in situ by observing the changes in the ligand absorption and fluorescence emission, as well as in the Eu(iii) luminescence. The change, fitted using non-linear regression analysis, demonstrated high binding affinity for Eu(iii) which in part can be assigned to being driven by additional hydrophobic effects. Moreover, using CD spectroscopy, the changes in the chiroptical properties of both (2·(R,R) and 2·(S,S)) were monitored in real time. Fitting the changes in the CD spectra allowed for the step-wise binding constants to be determined for these assemblies; these matched well with those determined from both the ground and the excited state changes. Both the ligands and the Eu(iii) complexes were then used in the formation of hydrogels; the Eu(iii)-metallogels were luminescent to the naked-eye.

Chiral probe development for circularly polarised luminescence: comparative study of structural factors determining the degree of induced CPL with four heptacoordinate europium(iii) complexes

Induced CPL signatures arising from the reversible binding of chiral carboxylates allow their absolute configuration and enantiomeric purity to be assessed.

The application of chiroptical spectroscopy (circular dichroism) in quantifying binding events in lanthanide directed synthesis of chiral luminescent self-assembly structures

The binding of asymmetrical and optically pure tridentate ligands containing one carboxylic group and 2-naphthyl as an antenna to lanthanide ions was studied in CH₃CN.

The binding of asymmetrical and optically pure tridentate ligands (L = 1(S) and 1(R)) containing one carboxylic group and 2-naphthyl as an antenna to lanthanide ions (M = La(iii) and Eu(iii)) was studied in CH₃CN, showing the successive formation of M:L, M:L₂ and M:L₃ stoichiometric species in solution. The europium complexes EuL₃ were also synthesised, structurally characterised and their photophysical properties probed in CH₃OH and CH₃CN. The changes in the chiroptical properties of both 1(S) and 1(R) were used (by circular dichroism (CD) spectroscopy) to monitor the formation of these chiral self-assemblies in solution. While circularly polarised luminescence (CPL) showed the formation of Eu(1(S))₃ and Eu(1(R))₃ as enantiomers, with high luminescence dissymmetry factors (g_lum), fitting the CD changes allowed for binding constants to be determined that were comparable to those seen in the analyses of absorbance and luminescence changes.

Specific chiral sensing of amino acids using induced circularly polarized luminescence of bis(diimine)dicarboxylic acid europium(III) complexes

The circularly polarized luminescence (CPL) from [Eu(pda)2](-) (pda = 1,10-phenanthroline-2,9-dicarboxylic acid) and [Eu(bda)2](-) (bda = 2,2'-bipyridine-6,6'-dicarboxylic acid) in aqueous solutions containing various amino acids was investigated. The europium(III) complexes exhibited bright-red luminescence assignable to the f-f transition of the Eu(III) ion when irradiated with UV light. Although the luminescence was not circularly polarized in the solid state or in aqueous solutions, in accordance with the achiral crystal structure, the complexes exhibited detectable induced CPL (iCPL) in aqueous solutions containing chiral amino acids. In the presence of L-pyrrolidonecarboxylic acid, both [Eu(pda)2](-) and [Eu(bda)2](-) showed similar iCPL intensity (glum ∼ 0.03 for the (5)D0 → (7)F1 transition at 1 mol·dm(-3) of the amino acid). On the other hand, in the presence of L-histidine or L-arginine, [Eu(pda)2](-) exhibited intense CPL (glum ∼ 0.08 for the (5)D0 → (7)F1 transition at 0.10 mol·dm(-3) of the amino acid), whereas quite weak CPL was observed for [Eu(bda)2](-) under the same conditions (glum < 0.01). On the basis of analysis of the iCPL intensities in the presence of 12 amino acids, [Eu(pda)2](-) was found to be a good chiral CPL probe with high sensitivity (about 10(-2) mol·dm(-3)) and high selectivity for L-histidine at pH 3 and for L-arginine at pH 7. The mechanism of iCPL was evaluated by analysis of the fine structures in the luminescence spectra and the amino acid concentration dependence of glum. For the [Eu(pda)2](-)-histidine/arginine systems, the europium(III) complexes possess coordination structures similar to that in the crystal with slight distortion to form a chiral structure due to specific interaction with two zwitterionic amino acids. This mechanism was in stark contrast to that of the europium(III) complex-pyrrolidonecarboxylic acid system in which one amino acid coordinates to the Eu(III) ion to yield an achiral coordination structure.