Nona-Coordinated Chiral Eu(III) Complexes with Stereoselective Ligand–Ligand Noncovalent Interactions for Enhanced Circularly Polarized Luminescence

Triplet energy transfer from quantum dots increases Ln(iii) photoluminescence, enabling excitation at visible wavelengths

Europium(iii) complexes are promising for bioimaging because of their long-lived, narrow emission. The photoluminescence (PL) from europium(iii) complexes is usually low. Thus, the effective utilization of low-energy light >400 nm and enhancement of PL are long-standing goals. Here, we show for the first time that 1-naphthoic acid triplet transmitter ligands bound to CdS quantum dots (QDs) and europium(iii) complexes create an energy transfer cascade that takes advantage of the strong QD absorption. This is confirmed by transient absorption spectroscopy, which shows hole mediated triplet energy transfer from QDs to 1-NCA, followed by triplet transfer from 1-NCA to europium(iii) complexes with an efficiency of 65.9 ± 7.7%. Smaller CdS QDs with a larger driving force lead to higher triplet transfer efficiency, with Eu(iii) PL intensity enhanced up to 21.4 times, the highest value ever reported. This hybrid QD system introduces an innovative approach to enhance the brightness of europium complexes.

Two-Photon Circularly Polarized Luminescence of Chiral Eu Complexes

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

Towards Fast Circularly Polarized Luminescence in 2-Coordinate Chiral Mechanochromic Copper(I) Carbene Complexes

A series of chiral mechanochromic copper(I) cAAC (cAAC=cyclic (alkyl)(amino)carbene) complexes with a variety of amide ligands have been studied with regard to their photophysical and chiroptical properties to elucidate structure-property relationships for the design of efficient triplet exciton emitters exhibiting circularly polarized luminescence. Depending on the environment, which determines the excited state energies, either thermally activated delayed fluorescence (TADF) from 1/3 LLCT states or phosphorescence from 3 LLCT/LC states occurs. However, neither chiral moieties at the carbene nor at the carbazolate ligands provide detectable luminescence dissymmetries glum . An exception is [Cu(phenoxazinyl)(cAAC)], showing orange to deep red TADF with λmax =601-715 nm in solution, powders and in PMMA. In this case, the amide ligand can undergo distortions in the excited state. This design motif leads to the first linear, non-aggregated CPL-active copper(I) complex with glum of -3.4 ⋅ 10-3 combined with a high radiative rate constant of 6.7 ⋅ 105  s-1 .

Asymmetric Lumino-Transformer: Circularly Polarized Luminescence of Chiral Eu(III) Coordination Polymer with Phase-Transition Behavior

A chiral Eu(III) coordination polymer with phase-transition behavior, [Eu(+tfc)₃(m-dpeb)]_n, (+tfc: (+)-3-trifluoroacetylcamphorato, m-dpeb: 1,3-bis(diphenylphosphorylethynyl)benzene) was reported for understanding the effect of polymer chain arrangement (orientation effect) on the circularly polarized luminescence (CPL) in a solid system. The phase-transition behavior of the transformable Eu(III) coordination polymer was characterized using differential scanning calorimetry and powder X-ray diffraction. The Eu(III) coordination polymer exhibited phase transition at approximately 180 °C. The magnitude of the CPL intensity was drastically changed because of the phase transition, without coordination geometrical change around the Eu(III) ion. In this study, the orientation effect of a chiral Eu(III) coordination polymer on the CPL properties in crystalline solid is demonstrated.

Asymmetric induction in quadruple-stranded europium(III) helicates and circularly polarized luminescence

Chiral supramolecular lanthanide-helicates are regarded as promising chiroptical materials due to their combination of ground and excited state chirality and special luminescence property from Ln3+, named as circularly polarized luminescence...

Self-assembly-induced luminescence of Eu3+-complexes and application in bioimaging

Design and engineering of highly efficient emitting materials with assembly-induced luminescence, such as room-temperature phosphorescence (RTP) and aggregation-induced emission (AIE), have stimulated extensive efforts. Here, we propose a new strategy to obtain size-controlled Eu³⁺-complex nanoparticles (Eu-NPs) with self-assembly-induced luminescence (SAIL) characteristics without encapsulation or hybridization. Compared with previous RTP or AIE materials, the SAIL phenomena of increased luminescence intensity and lifetime in aqueous solution for the proposed Eu-NPs are due to the combined effect of self-assembly in confining the molecular motion and shielding the water quenching. As proof of concept, we also show that this system can be further applied in bioimaging, temperature measurement and HClO sensing. The SAIL activity of the rare-earth (RE) system proposed here offers a further step forward on the roadmap for the development of RE light conversion systems and their integration in bioimaging and therapy applications.

Strong circularly polarized luminescence of mixed lanthanide coordination polymers with control of 4f electronic structures

This paper reports chiral mixed Eu(iii)-Ln(iii) coordination polymers (Ln = Gd and Sm) with a high dissymmetry factor of circularly polarized luminescence (gCPL = 0.15) for the enhancement of the emission quantum yield (Φtot ≥ 50%), achieved via control of 4f electronic structures.

Diastereoselective self-assembly of a triple-stranded europium helicate with light modulated chiroptical properties

Chiroptical photoswitches are of increasing interest for their potential in advanced information technologies. Herein, an achiral bis-β-diketonate ligand (o-L) with a photoresponsive diarylethene moiety as a linker was designed, which co-assembled with Eu³⁺ ions and R- and S-bis(diphenylphosphoryl)-1,10-binaphthyl (R/S-BINAPO) as chiral ancillaries to form dinuclear triple-stranded helicates, [Eu₂(o-L)₃(R/S-BINAPO)₂]. The helicates in the enantiopure form were confirmed by ¹H, ¹⁹F, ³¹P NMR and DOSY NMR analyses. Furthermore, the mirror-image CD and CPL spectra also demonstrate the existence of stable ground- and excited-state chiralities in solution. When exposed to alternate ultraviolet and visible light, the helicates showed reversible color variations from colorless to purple, followed by the presence of light-triggered quadruple optical and chiroptical outputs, named CD, PL, CPL and g_lum switches. With these light-modulated optical outputs, the possibility for the fabrication of IMPLICATION and INHIBIT logic gates was discussed.

Structure-Dependent Photoluminescence of Europium(III) Coordination Oligomeric Silsesquioxane: Synthesis and Mechanism

The coordination environment of Eu3+ is a crucial factor in the optical performance of the complex. Herein, a new kind of oligomeric silsesquioxane was employed to improve the coordination environment of central ions, the luminescence intensity of which was greatly enhanced with an efficient emission peak at 619 nm. More importantly, the photoluminescent properties of the product will be altered because of the formation of the Si-O-Si structure. The relevant mechanism has also been investigated and proposed by a series of characterization analyses. Additionally, the fluorescence lifetime, intrinsic quantum yield, and energy transfer efficiency were calculated. In addition, the observed trend of Judd-Ofelt intensity parameters was used to justify the coordination environment of Eu3+ ions. The experimental results reveal that the sol-gel reaction of the ligands can effectively promote intramolecular energy transfer. In addition, we introduced four theory modules of ligands (LSi, LSi-1, LSi-2, and LSi-3) with certain rules of formation of Si-O-Si, and density functional theory (DFT) and time-dependent DFT (TD-DFT) were used to explore their excited electron transfer process and their electronic absorption spectra, combined with Marcus theory. The calculated results show that the sol-gel reaction will induce the separation of the distribution of excited holes and electrons, leading to an efficient charge-transfer (CT) process. The predictable results were in good accordance with the experimental findings. Consequently, the sol-gel reaction occurring among ligands will be attributed to an efficient CT process, leading to a strong luminescence intensity, as observed experimentally.

Chiral BINAPO Induced Circularly Polarized Luminescence in a Triple-Stranded Eu2L3(BINAPO)2 Helicate

Chiral lanthanide helicates with circularly polarized luminescence (CPL) properties have found potential application in bioanalyses and chirality sensing. However, the preparation of the enantiopure helicates through a coordination-directed self-assembly strategy is challenging due to the greatly labile coordination geometries of lanthanide ions. Herein, a pair of enantiopure triple-stranded EuIII helicates [(Eu2L3)(R/S-BINAPO)2] are assembled by three achiral C2-symmeric bis-β-diketones (4,4′-bis(4,4,4-trifluoro-1,3-dioxobutyl)(phenoxyl)-1,1′-biphenyl, L) helically twisting around two EuIII ions, and two chiral R/S-bis(diphenylphosphoryl)-1,1′-binaphthyl (R/S-BINAPO) as ancillary ligands. Electrospray ionization–mass spectrometry, NMR, and circular dichroism (CD) spectra confirm the formation of a pair of enantiopure chiral topological helicates (Eu2L3)(R-BINAPO)2 and (Eu2L3)(S-BINAPO)2. As expected, the helicates present strong CPL with the |glum| up to 0.09, and the higher luminescent quantum yields (QYs) of up to 51%.

Circularly polarized lanthanide luminescence for advanced security inks

Authenticating products and documents with security inks is vital to global commerce, security and health. Lanthanide complexes are widely used in luminescent security inks owing to their unique and robust photophysical properties. Lanthanide complexes can also be engineered to undergo circularly polarized luminescence (CPL), which encodes chiral molecular fingerprints in luminescence spectra that cannot be decoded by conventional optical measurements. However, chiral CPL signals have not yet been exploited as an extra security layer in advanced security inks. This Review introduces CPL and related concepts that are necessary to appreciate the challenges and potential of lanthanide-based, CPL-active security inks. We describe recent advances in CPL analysis and read-out technologies that have expedited CPL-active security ink applications. Further, we provide a systematic meta-analysis of strongly CPL-active Euⁱⁱⁱ, Tbⁱⁱⁱ, Smⁱⁱⁱ, Ybⁱⁱⁱ, Cmⁱⁱⁱ, Dyⁱⁱⁱ and Crⁱⁱⁱ complexes, discussing the suitability of their photophysical properties and highlighting promising candidates. We conclude by providing key recommendations for the development and advancement of the field.

Visible Circularly Polarized Luminescence of Octanuclear Circular Eu(III) Helicate

This work reports on the structural and photophysical characterization of D₄-symmetrical octanuclear circular Ln^III helicates, [(R)- or (S)-ⁱPr-Pybox]₈(Ln^III)₈(THP)₈ (where Ln = Eu and Tb, THP = trianionic tris-β-diketonate, and ⁱPr-Pybox = chiral bis(4-isopropyl-2-oxazolinyl)pyridine). X-ray crystallographic analysis revealed that the octanuclear circular helicate possesses square antiprism architecture and consists of four [(R)- or (S)-ⁱPr-Pybox]₂Ln^III₂(THP)₂ asymmetric units arranged in a closed ring form. Ligand-to-ligand interactions between the THP and the ⁱPr-Pybox ligands have successfully directed formation of enantiopure, homoconfigurational (Δ,Δ,Δ,Δ,Δ,Δ,Δ,Δ)-R and (Λ,Λ,Λ,Λ,Λ,Λ,Λ,Λ)-S isomers. All of the nonacoordinated Ln^III ions are identical and exhibit a distorted capped square antiprism (CSAP) geometry. Upon excitation of the ligand absorption band (λ = 360 nm), the circular helicates display characteristic Eu^III (red, ⁵D₀ → ⁷F_J, J = 0-4) or Tb^III (green, ⁵D₄ → ⁷F_J, J = 6-3) core f-f luminescence. The overall emission quantum yields of the circular Eu^III and Tb^III helicates are 0.145 and 0.0013, respectively, in chloroform. The Eu^III and Tb^III complexes exhibit remarkable circularly polarized luminescence (CPL) activity at their magnetic dipole transition with observed luminescence dissymmetry factors |g_lum| of 1.25 (⁵D₀ → ⁷F₁, λ = 592 nm) and 0.25 (⁵D₄ → ⁷F_5, λ = 541 nm), respectively. Exceptional |g_lum| values of the circular Eu^III helicates highlight the visible intensity difference between left and right circularly polarized emissions of R and S isomers in chloroform and PMMA thin film.

Self-assembled Tetranuclear EuIII Complexes with D2- and C2h-Symmetrical Square Scaffold

Three new tetranuclear europium(III) alternating circular helicates, (2-^PhRRRR/2-^PhSSSS) [(R)- or (S)-Ph-Pybox]₄Eu^III₄(BPP)₆, (2-^iPrRRRR/2-^iPrSSSS) [(R)- or (S)-ⁱPr-Pybox]₄Eu^III₄(BPP)₆, and (3-^PhRRRR/3-^PhSSSS) [(R)- or (S)-Ph-Pybox]₄Eu^III₄(BHP)₆, are presented with their structural and chiroptical characterization (BPP and BHP = bis-β-diketonates; Pybox = chiral bis(oxazolinyl) pyridine ligand). X-ray crystallographic analysis revealed that different extents of interligand-contacting interactions between bis-β-diketonates and Pybox ligands produce different bis-β-diketonates arrangements around four Eu^III ions and, thus, their specific symmetry in the final tetranuclear complexes. 2-^PhRRRR/2-^PhSSSS and 3-^PhRRRR/3-^PhSSSS are recognized as D₂-symmetry, while 2-^iPrRRRR/2-^iPrSSSS self-assemblies possess pseudo C_2h-symmetry. Due to different molecular symmetry, 2-^PhRRRR/2-^PhSSSS and 2-^iPrRRRR/2-^iPrSSSS crystals display different ligands orientation in their Eu^III coordination spheres. The presence of pseudo σ_h-mirror symmetry in 2-^iPrRRRR/2-^iPrSSSS promotes a pair of distinguishable Eu^III geometries. All the chiral self-assemblies exhibit almost identical photoluminescence emission spectra patterns of f-f transitions in the Eu^III core. The Eu^III self-assemblies exhibit intense CPL with different observed |g_lum| values (2-^PhRRRR/2-^PhSSSS ∼ |0.31|, 2-^iPrRRRR/2-^iPrSSSS ∼ |0.08|, 3-^PhRRRR/3-^PhSSSS ∼ |0.31| in chloroform).

Electronic strain effect on Eu(iii) complexes for enhanced circularly polarized luminescence

The structural and electronic strain of ligands promotes the enhancement of the magnitude of circularly polarized luminescence in chiral Eu(iii) complexes.

Modulating ICT emission: a new strategy to manipulate the CPL sign in chiral emitters

A new strategy to manipulate the circularly polarized luminescence (CPL) handedness in chiral emitters, based on modulating the population of an emissive ICT state, is proposed. Such a strategy is particularly interesting for conformationally rigid and non-aggregating chiral organic emitters, opening up new perspectives for the development of CPL applications based on organic molecules.

Sensitized near infrared emission through supramolecular d → f energy transfer within an ionic Ru(ii)-Er(iii) pair

The newly synthesized ionic triple salt Ru-Er, {[Ru^II(bpy)₂(dbim)][Er^III(hfac)₄][CF₃COO]·H₂O} (bpy = 2,2'-bipyridine; hfac^- = hexafluoroacetylacetonate; dbim = 2,2'-dibenzimidazole) exhibits near-infrared (NIR) emission at 1535 nm by intermolecular Ru → Er (d → f) energy transfer across supramolecular interactions when pumped within the Ru(ii) ³MLCT band. It is the first such observation for a transition metal-lanthanide ionic pair.

Complexes of Eu(iii)(hfa)3 with a planar chiral P(iii) ligand (Phanephos): solvent-sensitive sign inversion of circularly polarised luminescence

Phanephos coordinates with Eu(iii)(hfa)₃ in chloroform and acetone, but with the opposite CPL sign.

Can chiral P(iii) coordinate Eu(iii)? Unexpected solvent dependent circularly polarised luminescence of BINAP and Eu(iii)(hfa)3 in chloroform and acetone

The coordination of Eu(iii) (from Eu(iii)(hfa)₃) by chiral P(iii) (from BINAP) led to circularly polarised luminescence in dilute solutions on the order of |g_em| ≈ 10⁻² at ⁵D₀ → F₁ and |g_em| ≈ 10⁻² at ⁵D₀ → F₂ transitions, i.e., in chloroform and acetone.

Red colored CPL emission of chiral 1,2-DACH-based polymers via chiral transfer of the conjugated chain backbone structure

Chiral polymers incorporating a chiral 1,2-DACH moiety and BODIPY can exhibit strong mirror image Cotton effects and emit red colored CPL signals via the effective chiral transfer of the conjugated polymer chain backbone.

Luminescent lanthanide coordination polymers for photonic applications

Luminescent lanthanide coordination polymers composed of lanthanide ions and organic joint ligands exhibit characteristic photophysical and thermostable properties that are different from typical organic dyes, luminescent metal complexes, and semiconductor nanoparticles.

Formation of one-dimensional helical columns and excimerlike excited states by racemic quinoxaline-fused [7]carbohelicenes in the crystal

A series of quinoxaline-fused [7]carbohelicenes (HeQu derivatives) was designed and synthesized to evaluate their structural and photophysical properties in the crystal state. The quinoxaline units were expected to enhance the light-emitting properties and to control the packing structures in the crystal. The electrochemical and spectroscopic properties and excited-state dynamics of these compounds were investigated in detail. The first oxidation potentials of HeQu derivatives are approximately the same as that of unsubstituted reference [7]carbohelicene (Heli), whereas their first reduction potentials are shifted to the positive by about 0.7 V. The steady-state absorption, fluorescence, and circular dichroism spectra also became redshifted compared to those of Heli. The molecular orbitals and energy levels of the HOMO and LUMO states, calculated by DFT methods, support these trends. Moreover, the absolute fluorescence quantum yields of HeQu derivatives are about four times larger than that of Heli. The structural properties of the aggregated states were analyzed by single-crystal analysis. Introduction of appropriate substituents (i.e., 4-methoxyphenyl) in the HeQu unit enabled the construction of one-dimensional helical columns of racemic HeQu derivatives in the crystal state. Helix formation is based on intracolumn π-stacking between two neighboring [7]carbohelicenes and intercolumn CH⋅⋅⋅N interaction between a nitrogen atom of a quinoxaline unit and a hydrogen atom of a helicene unit. The time-resolved fluorescence spectra of single crystals clearly showed an excimerlike delocalized excited state owing to the short distance between neighboring [7]carbohelicene units.

Tools for studying aqueous enantioselective lanthanide-catalyzed Mukaiyama aldol reactions

Use of multiple techniques to study lanthanide-catalyzed reactions in the presence of water.

Controlled self-assembly and luminescence characteristics of Eu(III) complexes in binary aqueous/organic media

Luminescence of sodium tetrakis(naphthoyl trifluoroacetonato) europium(III) (Na[Eu(nta)4]) in binary aqueous-ethanol media is quenched continuously with an increase in the water content, which is ascribed to commonly observed relaxation of photoexcited lanthanide complexes through vibrational coupling with coordinating water. Meanwhile, replacement of sodium ion with an ammonium amphiphile 1 gives a lipid complex 1[Eu(nta)4] which shows distinct changes: its luminescence quantum yield Φ is remarkably increased to ~0.6 above the water content of ~60 vol. %. This unusual enhancement in luminescence intensity occurs in response to self-assembly of 1[Eu(nta)4] into nanoparticles. The lipid counterions provide a hydrophobic atmosphere inside nanoparticles, and they simultaneously form monolayers on the nanoparticle surface that enhance dispersion stability. The size of nanoparticles is tunable depending on the volume fraction of water in the binary media. The lipid-assisted self-assembly of lanthanide complexes provides a unique means to fabricate luminescent nanomaterials, and this approach will be widely applied to fabricate functional coordination nanomaterials.

The structure of MLn(hfbc)4 and a key to high circularly polarized luminescence

The heterobimetallic complex CsEu[(+)-hfbc](4) (hfbc = 3-heptafluorobutyryl camphorate), prepared by Kaizaki and co-workers, displays the highest ratio of polarization versus total luminescence (measured by the g(lum) factor), i.e., ~85% of the emitted photons at 595 nm are left-circularly polarized. We present a detailed structural analysis in solution, based on paramagnetic nuclear magnetic resonance (NMR), and discuss the possible dynamic processes, where its analogues are involved. We demonstrate that the first coordination sphere is very close to the achiral regular square antiprism (SAPR) with D(4d) symmetry, which rules out the intrinsic dissymmetry of the Eu environment for rationalizing the g(lum). In contrast, the dynamic coupling between the f-f transitions, responsible for the emission, to the ligand-centered π-π* transition at 310 nm displays almost ideal geometry to justify g(lum) and discloses a key to high circularly polarized emission.