Sign Reversal of a Large Circularly Polarized Luminescence Signal by the Twisting Motion of a Bidentate Ligand

Molecular Vibrations in Chiral Europium Complexes Revealed by Near-Infrared Raman Optical Activity

Raman optical activity (ROA) is commonly measured with green light (532 nm) excitation. At this wavelength, however, Raman scattering of europium complexes is masked by circularly polarized luminescence (CPL). This can be avoided using near-infrared (near-IR, 785 nm) laser excitation, as demonstrated here by Raman and ROA spectra of three chiral europium complexes derived from camphor. Since luminescence is strongly suppressed, many vibrational bands can be detected. They carry a wealth of structural information about the ligand and the metal core, and can be interpreted based on density functional theory (DFT) simulations of the spectra. For example, jointly with ROA experimental data, the simulations make it possible to determine absolute configuration of chiral lanthanide compounds in solution.

Circularly Polarized Luminescence of Cyclometalated Platinum(II) Complex Excimers: Large Difference between Isomers

A series of platinum(II) complexes bearing a chiral β-diketonato ligand and a cyclometalated ligand have been prepared. The platinum(II) complexes, (SP-4-3)-[Pt(ppy)(D-tac)] (ppy = 2-phenylpyridine, D-tac = 3-trifluoroacetyl-(D)-camphor), (SP-4-4)-[Pt(ppy)(D-tac)], (SP-4-3)-[Pt(ppy)(D-pbc)] (D-pbc = 3-perfluorobutyryl-(D)-camphor), and (SP-4-4)-[Pt(ppy)(D-pbc)], and their enantiomers were isolated and characterized by elemental analysis, NMR, and X-ray structural analysis. Photoisomerization between SP-4-3 (trans) and SP-4-4 (cis) isomers was observed. Green emission due to the monomer was observed in diluted solutions for all complexes. Higher quantum yields and longer lifetimes of green emission were observed in nonpolar solvents compared to polar solvents. The two geometrical isomers had surprisingly different excimer formation efficiencies. For the trans isomers, orange emission due to the excimers was observed in nonpolar solvents at high concentrations, whereas negligible intensities of the excimer emission were observed for the cis isomers. The formation of the excimers was evaluated by emission decay and time-resolved emission spectra. For the trans isomers, the green emission due to the monomer showed negligible CPL signals, but the orange emission gave pronounced CPL signals. The dissymmetry factors, g-values, of the excimer CPL (g_lum = 0.002) were enhanced over those of the circular dichroism (g_abs = 0.0002, g_lum/g_abs = 10). The intensities of the emission and the CPL of the excimer under oxygen were very small, although those under an argon atmosphere were very strong. Therefore, the emission color of the trans-isomers was changed from green to orange by deoxygenation.

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.

Solid-State Near-Infrared Circularly Polarized Luminescence from Chiral YbIII -Single-Molecule Magnet

A field-induced chiral Yb^III Single-Molecule Magnet (SMM) displayed an unprecedented near-infrared circularly polarized luminescence (NIR-CPL) in the solid-state. The bridging bis(1,10-phenantro[5,6b])tetrathiafulvalene triad (L) allowed an efficient sensitization of the NIR ² F_5/2 →² F_7/2 emission while the NIR-CPL is associated to the f-f transitions of the Yb^III ion bearing chiral β-diketonate derived-camphorate ancillary ligands.

Ancillary ligand modulated stereoselective self-assembly of triple-stranded Eu(iii) helicate featuring circularly polarized luminescence

Creating optically pure metal assemblies is a hot research topic in the realms of chiral supramolecules. Here, three new triple-stranded europium(iii) helicates Eu₂L₃(L′)₂ [L = 4,4′-bis(4,4,4-trifluoro-1,3-dioxobutyl)diphenyl sulphide; L′ = 1,10-phenanthroline (Phen) or R/S-2,2′-bis(diphenylphosphinyl)-1,1′-binaphthyl (R/S-BINAPO)] were synthesized in order to investigate the effects of ancillary ligands on controlling the stereoselective self-assembly of lanthanide helicates. X-ray single crystal structure analysis showed that Eu₂L₃(Phen)₂ crystalized in an achiral space group P1̄ with the equivalent amount of P and M helicates in one single cell. The isolated Eu₂L₃(S-BINAPO)₂ and Eu₂L₃(R-BINAPO)₂ were verified to be enantiopure by ¹H, ¹⁹F, ³¹P NMR and DOSY NMR analyses. Additionally, the mirror-image CD spectra also demonstrated the successful syntheses of the enantiomers and the presence of an effective chirality transformation from BINAPO to achiral L. Furthermore, the perfect mirror-image circularly polarized luminescence (CPL) spectra of Eu₂L₃(S-BINAPO)₂ and Eu₂L₃(R-BINAPO)₂ indicated the existence of the excited state chirality of the Eu³⁺ center associated with |g_lum| values reaching 0.112. In addition, the photophysical properties of three helicates were also discussed.

Chiral ancillary ligands (R/S-BINAPO) modulated the stereoselective self-assembly of lanthanide helicates, which presented strong CPL with |g_lum| values up to 0.112 and high luminescence quantum yield up to 34%.

pH-influenced handedness inversion of circularly polarized luminescence

Supramolecular co-assemblies between tolane-derived Phe–Phe dipeptides and 1,2-diaminoethane were fabricated, and CPL handedness inversion was achieved by regulating the pH value.

Supramolecular chirality transformation driven by monodentate ligand binding to a coordinatively unsaturated self-assembly based on C 3-symmetric ligands

Monodentate ligand binding is facilitated by supramolecular chirality transformations from propeller-shaped chirality into single-twist chirality by altering the self-assembly of C 3-symmetric chiral ligands. The C 3-symmetric chiral ligands (Im R 3Bz and Im S 3Bz) contain three chiral imidazole side arms (Im R  and Im S ) at the 1,3,5-positions of a central benzene ring. Upon coordination to zinc ions (Zn2+), which have a tetrahedral coordination preference, the C 3-symmetric chiral ligands assemble, in a stepwise manner, into a propeller-shaped assembly with a general formula (Im( R or S ) 3Bz)4(Zn2+)3. In this structure each Zn2+ ion coordinates to the three individual imidazole side arms. The resulting assembly is formally coordinatively unsaturated (coordination number, n = 3) and capable of accepting monodentate co-ligands (imidazole: ImH2) to afford a coordinatively saturated assembly [(ImH2)3(Im R 3Bz)4(Zn2+)3]. The preformed propeller-shaped chirality is preserved during this transformation. However, an excess of the monodentate co-ligand (ImH2/Zn2+ molar ratio of ∼1.7) alters the propeller-shaped assembly into a stacked dimer assembly [(ImH2) m (Im R 3Bz)2(Zn2+)3] (m = 4-6) with single-twist chirality. This switch alters the degree of enhancement and the circular dichroism (CD) pattern, suggesting a structural transition into a chiral object with a different shape. This architectural chirality transformation presents a new approach to forming dynamic coordination-assemblies, which have transformable geometric chiral structures.

Off-off-on chiroptical property switching of a pyrene luminophore by stepwise helicate formation

Pyrene-based chiral ligands (LR and LS) consisting of two chiral imidazole sidearms at the 1,8-positions first form pyrene-bridged oligomers [(L(R or S))3(Zn2+)n], which are chiroptically inactive, with a low concentration of zinc ions (Zn2+). Upon addition of more Zn2+, chiroptically active coordination helicates ([(L(R or S))3(Zn2+)2]) are formed. The stepwise assembly process successfully facilitated an "off-off-on" chiroptical switching process of a pyrene luminophore controlled by the concentration of Zn2+. The resulting pyrene-bridged helicates exhibit high performance in terms of a dissymmetry factor of circular dichroism (|gCD| = 0.006) and circularly polarized luminescence (|gCPL| = 0.01).

Circular dichroism and circularly polarised luminescence of bipyrenyl oligopeptides, with piperidines added in the peptide chains

Upon combining chiral peptides (the most basic chiral source) with pyrene moieties, we found that chiral oligopeptides bearing two-pendant pyrenyl units exhibited circularly polarised luminescence (CPL) originating from intramolecular excimers at 450-490 nm in various solvents, and the sign of their CPL signals depended on the type of solvent employed. The CPL and circular dichroism signs and intensities could be tuned by the introduction of a piperidine unit into the chiral peptide chain; thus, the obtained structure could be considered a practical Lock ON-OFF system for oligopeptide luminophores.

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.

Europium amphiphilic naphthalene based complex for the enhancement of linearly polarized luminescence in Langmuir–Blodgett films

The complexation in LB films induces the linearly polarized luminescence of europium by the excited 1-naphtoate ligand.

Electronic chirality inversion of lanthanide complex induced by achiral molecules

A novel mechanism for chiroptical activity inversion based on the electronic structure of metal complexes without Λ- or Δ-type structure change was demonstrated spectroscopically and theoretically. To demonstrate the mechanism, a europium (Eu(III)) complex with chiral (+)-3-(trifluoroacetyl)camphor (+tfc) and achiral triphenylphosphine oxide (tppo) was prepared. The steric and electronic structures of the Eu(III) complex were adjusted by additional achiral tppo and coordinating acetone molecules, and were characterised by 1H NMR, photoluminescence, and emission lifetime measurements. The optical activity of the Eu(III) complex in solution was evaluated by circularly polarized luminescence (CPL) measurements. CPL sign inversion, which was independent of Λ- or Δ-type structure changes from the spectroscopic viewpoint, and a drastic CPL intensity enhancement were observed depending on the external achiral molecules around Eu(III) ion. These phenomena provide the first clarification of optical activity change associated with electronic structure rather than chiral coordination structure-type (Λ or Δ) under external environments.

A Smart Sensing Method for Object Identification Using Circularly Polarized Luminescence from Coordination-Driven Self-Assembly

The potential use of circularly polarized luminescence for object identification in a sensor application is demonstrated. New luminescence probes using pyrene derivatives as sensor luminophores were developed. (R,R)-Im₂ Py and (S,S)-Im₂ Py contain two chiral imidazole moieties at 1,6-positions through ethynyl spacers (angle between spacers ca. 180°). The probe molecules spontaneously self-assemble into chiral stacks (P or M helicity) upon coordination to metal ions with tetrahedral coordination (Zn²⁺ ). The chiral probes display neither circular dichroism (CD) nor circularly polarized luminescence (CPL) without metal ions. However, (R,R)-Im₂ Py and (S,S)-Im₂ Py exhibit intense chiroptical activity (CD and CPL) upon self-assembly with Zn²⁺ ions. (R,R)-Im₂ Py and (S,S)-Im₂ Py with chemical stimuli-responsibility allow sensing using the CPL signal as detection output, enabling us to discriminate between a signal from the target analyte and that from non-target species.

The effect of ligand symmetry on the ratiometric luminescence characteristics of lanthanides

This work demonstrated that the ligand symmetry of europium(iii) complexes controls the ratiometric luminescence characteristics of lanthanides. Nona-coordinated europium(iii) complexes having unsymmetrical β-diketonate ligands (Cs) exhibit distinctive ratiometric spectral variations in the extremely narrow f-f transition bands over the temperature range from 253 to 323 K. In contrast, no such ratiometric change can be observed in a series of nona-coordinated europium(iii) complexes containing symmetrical β-diketonate ligands (C2v). The remarkable difference depending on the ligand symmetry (Csvs. C2v) suggests that the coordination rearrangement of β-diketonate in the complex causes ratiometric spectral variations in extremely narrow f-f transition bands, where two europium(iii) complex isomers exist in the solution equilibrium. A self-calibration method using dual iso-emissive points is reported, where self-calibration using the two emission intensities at the iso-emissive points reduces the coefficient of variation in luminescence thermometry.

Monitoring of the ADP/ATP Ratio by Induced Circularly Polarised Europium Luminescence

A series of three europium complexes bearing picolyl amine moieties was found to possess differing binding affinities towards Zn²⁺ and three nucleotides: AMP, ADP, and ATP. A large increase in the total emission intensity was observed upon binding Zn²⁺ , followed by signal amplification upon the addition of nucleotides. The resulting adducts possessed strong induced circularly polarised emission, with ADP and ATP signals of opposite sign. Model DFT geometries of the adducts suggest the Δ diastereoisomer is preferred for ATP and the Λ isomer for ADP/AMP. This change in sign allows the ADP/ATP (or AMP/ATP) ratio to be assessed by monitoring changes in the emission dissymmetry factor, g_em .

Hierarchical Emergence and Dynamic Control of Chirality in a Photoresponsive Dinuclear Complex

Chiroptical photoswitches are of interest from a viewpoint of applications in advanced information technologies. We report dynamic on-off photoswitching of circularly polarized luminescence (CPL) in a binuclear europium complex system. Two coordination units are arranged closely in a chiral fashion by a photoresponsive ligand with a one-handed helical structure. The chirality in the helical scaffold is hierarchically transferred to the chirality in nine-coordinate complex sites. The chiral close arrangement of complex units induces the enrichment of a specific chiral coordination structure in the nine-coordinate europium sites. The chiral arrangement of complex units is switched in conjunction with the photoinduced helix-nonhelix structural change in the photoresponsive framework, demonstrating on-off switching of CPL with high contrast.

Ligand-to-Ligand Interactions That Direct Formation of D2-Symmetrical Alternating Circular Helicate

This work demonstrates that ligand-to-ligand interactions between achiral bis-β-diketonate (BTP) and chiral bis(4-phenyl-2-oxazolinyl)pyridine [( R)- or ( S)-Ph-Pybox] are successfully directed to the fabrication of a D₂-symmetrical alternating circular helicate with the general formula [( R)- or ( S)-Ph-Pybox]₄(Ln^III)₄(BTP)₆. The lanthanide(III) Ln^III assemblies (Ln^III₄- RRRR and Ln^III₄- SSSS) have a nanometer-size squarelike grid (interatomic distances > 10 Å). X-ray structure analysis revealed that the circular helicate contains two double helicate Ln^III₂L₂ units, where both show ( M)-helicity for Ln^III₄- RRRR and ( P)-helicity for Ln^III₄- SSSS, where π-π stacking interaction is present between the side arm of ( R)-Ph-Pybox (Ph₁) and the adjacent BTP ligand around the Eu^III metal center ( d_ππ = 3.636 Å: the diketonate plane···Ph₁ distance). The newly obtained circular lanthanide(III) helicate exists as single and homochiral diastereomers in solution (Ln^III₄- RRRR and Ln^III₄- SSSS), exhibiting circularly dichroism (CD) and circularly polarized luminescence (CPL). Conversely, the circular helicate favors the heterochiral arrangement (i.e., Ln^III₄- RRRR/Ln^III₄- SSSS).

Ambidextrous Chirality Transfer Capability from Cellulose Tris(phenylcarbamate) to Nonhelical Chainlike Luminophores: Achiral Solvent-Driven Helix-Helix Transition of Oligo- and Polyfluorenes Revealed by Sign Inversion of Circularly Polarized Luminescence and Circular Dichroism Spectra

We investigated whether helicity and/or chirality of cellulose tris(phenylcarbamate) (CTPC) can transfer to noncharged, nonhelical oligo- and polyfluorenes when CTPC was employed as a solution processable homochiral platform of a D-glucose-skeletal polymer. Noticeably, CTPC revealed the solvent-driven, ambidextrous intermolecular helicity/chirality transfer capability to these fluorenes. The chiroptical inversion characteristics of circularly polarized luminescence (CPL) and the corresponding CD spectra were realized by solely choosing a proper achiral solvent and/or achiral cosolvent. When the solution of PF6 and CTPC in tetrahydrofuran (THF) was cast on a quartz substrate, the dissymmetry ratio of CPL (g_CPL) from the polymer film showed g_CPL = +2.1 × 10^-3 at 429 nm. Conversely, when dichloromethane (DCM) was used as the solvent, the CPL sign was inverted to g_CPL = -2.4 × 10^-3 at 429 nm. The dissymmetry ratio of Cotton CD band (g_CD) from the THF solution was g_CD = +3.2 × 10^-3 at 392 nm; conversely, from the DCM, the CD sign inverted to g_CD = -0.8 × 10^-3 at 371 nm. The sign and magnitude of the g_CD values were interpreted to a London dispersion term (δ_d) of Hansen solubility parameter (δ) of the casting solvents rather than a dipole-dipole interaction term (δ_p) and a hydrogen bonding interaction term (δ_h) of the δ values and dielectric constant (ε). Analysis of solvent-driven changes in FTIR spectra, wide-angle X-ray diffraction profiles, and differential scanning calorimetry diagrams indicated that solvent driven on-off switching of multiple hydrogen bonds due to three urethane groups of CTPC play the key for the inversion. Intermolecular CH/π and π-π interactions among phenyl rings and alkyl groups were assumed to be crucial for helicity/chirality transfer capability based on molecular mechanics and molecular dynamics simulations of PF6-CTPC hybrids. These chiroptical inversion characteristics arose from solvent-driven order-disorder transition characteristics of the CTPC helix rather than a helix-helix transition of CTPC itself.

Unveiling controlled breaking of the mirror symmetry of Eu(fod)3 with α-/β-pinene and BINAP by circularly polarised luminescence (CPL), CPL excitation, and 19F-/31P{1H}-NMR spectra and Mulliken charges

The first CPL signals at f–f transitions of Eu(fod)₃ in α- and β-pinene as chiral solvents and with BINAP (1 : 1) in chloroform were detected.

Solvent-sensitive signs and magnitudes of circularly polarised luminescence and circular dichroism spectra: probing two phenanthrenes as emitters endowed with BINOL derivatives

A C₂-symmeric binaphthyl framework bearing phenanthrene as the emitter exhibited circularly polarised luminescence (CPL) in dilute solutions.

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.

OFF-ON-OFF Dual Emission at Visible and UV Wavelengths from Carbazole Functionalized β-Diketonate Europium(III) Complex

This work demonstrates dual emission "OFF-ON-OFF" switching at visible and UV wavelengths of a carbazole functionalized β-diketone (LH) by a simple change of a europium(III) ion (Eu(3+)) concentration in the submicromolar concentration range. In the presence of 0.25 equiv of Eu(3+) (5 μM), LH forms a luminescent 4:1 complex ([Eu(3+)(L(-))4](-)) exhibiting dual emission at 357 and 613 nm resulting from the local excitation of the carbazole ring and ligand-sensitized luminescence from the Eu(3+)-β-diketonate unit, respectively. The 4:1 complex begins to convert into a 2:1 complex ([Eu(3+)(L(-))2](+)) via a 3:1 complex [Eu(3+)(L(-))3] above a molar ratio ([Eu(3+)]/[LH]) of 0.25, which provides the opportunity for binding of solvent methanol molecules to the vacant site of the Eu(3+) ion in the complex ([Eu(3+)(L(-))2(MeOH)n](+)). The OH oscillators of coordinated methanol molecules facilitate the nonradiative pathway of the Eu(3+) emission; hence the emission at 613 nm almost disappears above the 0.50 equivalent of Eu(3+) (11 μM), while the UV emission at 357 nm remains mostly constant over the whole concentration range.

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.

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.

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.

Potential switchable circularly polarized luminescence from chiral cyclometalated platinum(II) complexes

A series of chiral cyclometalated platinum(II) complexes, [Pt((-)-L1)(Dmpi)]Cl ((-)-1), [Pt((+)-L1)(Dmpi)]Cl ((+)-1), [Pt((-)-L2)(Dmpi)]Cl ((-)-2), [Pt((+)-L2)(Dmpi)]Cl ((+)-2), [Pt3((-)-L2)2(Dmpi)4](ClO4)4 ((-)-3), and [Pt3((+)-L2)2(Dmpi)4](ClO4)4 ((+)-3) [(-)-L1 = (-)-4,5-pinene-6'-phenyl-2,2'-bipyridine, (+)-L1 = (+)-4,5-pinene-6'-phenyl-2,2'-bipyridine), (-)-L2 = (-)-1,3-bis(2-(4,5-pinene)pyridyl)benzene, (+)-L2 = (+)-1,3-bis(2-(4,5-pinene)pyridyl)benzene, Dmpi = 2,6-dimethylphenyl isocyanide], have been designed and synthesized. In aqueous solutions, (-)-1 and (+)-1 aggregate into one-dimensional helical chain structures through Pt···Pt, π-π, and hydrophobic-hydrophobic interactions. (-)-3 and (+)-3 represent a novel helical structure with Pt-Pt bonds. The formation of helical structures results in enhanced and distinct chiroptical properties as evidenced by circular dichroism spectra. Circularly polarized luminescence (CPL) was observed from the aggregates of (-)-1 and (+)-1 in water, as well as (-)-3 and (+)-3 in dichloromethane. The CPL activity can be switched reversibly (for (-)-1 and (+)-1) or irreversibly (for (-)-3 and (+)-3) by varying the temperature.