Luminescent chiral lanthanide(III) complexes as potential molecular probes

Augmentation of NIR Circularly Polarized Luminescence Activity in Shibasaki-Type Lanthanide Complexes Supported by the Spirane Sphenol

We report two new circularly polarized luminescence (CPL)-active lanthanide complexes emissive in the near-infrared (NIR) region; using sphenol as a supporting ligand, we provide the first reported example of an NIR-emissive lanthanide complex supported by a chiral spirane. Inclusion of a quaternary carbon to impart axial chirality results in dramatic augmentation of the CPL strength of the resultant sphenolate complexes (glum ≤ 0.77 for [(sphenol)3ErNa3(thf)6]) compared to that of their contemporary biaryl-based axially chiral analogues (glum ≤ 0.47 for [(binol)3ErNa3(thf)6]). Despite similar structural parameters, the rigid spiro carbon of sphenol enables the strongest dissymmetry factors observed to date from Shibasaki-type complexes for both Yb and Er. We also demonstrate the sensitivity of the reported chiroptical measurements to small variations in instrumental parameters, such as bandpass, and suggest a standardized method or at least that additional detail should be included in future reports to allow for direct comparisons between newly published CPL emitters.

Rapid Synthesis of Chiral Figure-Eight Macrocycles Using a Preorganized Natural Product-Based Scaffold

Chiral D2 -symmetric figure-eight shaped macrocycles are promising scaffolds for amplifying the chiroptical properties of π-conjugated systems. By harnessing the inherent and adaptable conformational dynamics of a chiral C2 -symmetric bispyrrolidinoindoline (BPI) manifold, we developed an enantio-divergent modular synthetic platform to rapidly generate a diverse range of chiral macrocycles, spanning from 14- to 66-membered rings, eliminating the need for optical resolution. Notably, a 32-membered figure-eight macrocycle showed excellent circularly polarized luminescence (CPL: |glum |=1.1×10-2 ) complemented by a robust emission quantum yield (Φfl =0.74), to achieve outstanding CPL brightness (BCPL : ϵ×Φfl ×|glum |/2=480). Using quadruple Sonogashira couplings, this versatile synthetic platform enables precise adjustments of the angle, distance, and length among intersecting π-conjugated chromophores. Our synthetic strategy offers a streamlined and systematic approach to significantly enhance BCPL values for a variety of chiral D2 -symmetric figure-eight macrocycles.

Exchangeable Self-Assembled Lanthanide Antennas for PLIM Microscopy

Lanthanides have unique photoluminescence (PL) emission properties, including very long PL lifetimes. This makes them ideal for biological imaging applications, especially using PL lifetime imaging microscopy (PLIM). PLIM is an inherently multidimensional technique with exceptional advantages for quantitative biological imaging. Unfortunately, due to the required prolonged acquisitions times, photobleaching of lanthanide PL emission currently constitutes one of the main drawbacks of PLIM. In this study, we report a small aqueous-soluble, lanthanide antenna, 8-methoxy-2-oxo-1,2,4,5-tetrahydrocyclopenta[de]quinoline-3-phosphonic acid, PAnt, specifically designed to dynamically interact with lanthanide ions, serving as exchangeable dye aimed at mitigating photobleaching in PLIM microscopy in cellulo. Thus, self-assembled lanthanide complexes that may be photobleached during image acquisition are continuously replenished by intact lanthanide antennas from a large reservoir. Remarkably, our self-assembled lanthanide complex clearly demonstrated a significant reduction of PL photobleaching when compared to well-established lanthanide cryptates, used for bioimaging. This concept of exchangeable lanthanide antennas opens new possibilities for quantitative PLIM bioimaging.

Boosting quantum yields and circularly polarized luminescence of penta- and hexahelicenes by doping with two BN-groups

The incorporation of boron-nitrogen (BN) units into polycyclic aromatic hydrocarbons (PAHs) as an isoelectronic replacement of two carbon atoms can significantly improve their optical properties, while the geometries are mostly retained. We report the first non-π-extended penta- and hexahelicenes comprising two aromatic 1,2-azaborinine rings. Comparing them with their all-carbon analogs regarding structural, spectral and (chir)optical properties allowed us to quantify the impact of the heteroatoms. In particular, BN-hexahelicene BN[6] exhibited a crystal structure congruent with its analog CC[6], but displayed a fivefold higher fluorescence quantum yield (φfl = 0.17) and an outstanding luminescence dissymmetry factor (|glum| = 1.33 × 10-2). Such an unusual magnification of both properties at the same time makes BN-helicenes suitable candidates as circularly polarized luminescence emitters for applications in materials science.

Mechanochemical Scholl Reaction on Phenylated Cyclopentadiene Core: One-Step Synthesis of Fluoreno[5]helicenes

In this study, we explore feasibility of the mechanochemical approach in the synthesis of tetrabenzofluorenes (fluoreno[5]helicenes). For this, commercially available phenylated cyclopentadiene precursors are subjected to the Scholl reaction in the solid state using FeCl3 as an oxidant and sodium chloride as the solid reaction medium. This ball milling process gave access to the 5-membered ring containing-helicenes in one synthetic step in high (95-96 %) isolated yields. The solution-phase reactions, however, were found to be moderate to low yielding in this regard (10-40 %).

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.

Calculating the Circular Dichroism of Chiral Halide Perovskites: A Tight-Binding Approach

Chiral metal halide perovskites have emerged as promising optoelectronic materials for the emission and detection of circularly polarized visible light. Despite chirality being realized by adding chiral organic cations or ligands, the chiroptical activity originates from the metal halide framework. The mechanism is not well understood, as an overarching modeling framework is lacking. Capturing chirality requires going beyond electric dipole transitions, which is the common approximation in condensed matter calculations. We present a density functional theory (DFT) parametrized tight-binding (TB) model, which allows us to calculate optical properties including circular dichroism (CD) at low computational cost. Comparing Pb-based chiral perovskites with different organic cations and halide anions, we find that the structural helicity within the metal halide layers determines the size of the CD. Our results mark an important step in understanding the complex correlations of structural, electronic, and optical properties of chiral perovskites and provide a useful tool to predict new compounds with desired properties for novel optoelectronic applications.

Synthesis of Closed-Heterohelicenes Interconvertible between Their Monomeric and Dimeric Forms

Oxidative fusion reaction of cyclic heteroaromatic pentads consisting of pyrrole and thiophene gave closed-heterohelicene monomers and dimers depending on the oxidation conditions. Specifically, oxidation with [bis(trifluoroacetoxy)iodo]benzene (PIFA) gave closed-[7]helicene dimers connected at the β-position of one of the pyrrole units with remarkably elongated C-C bonds of about 1.60 Å. Although this bond was intact against thermal and physical activations, homolytic bond dissociation took place in DMSO upon irradiation with UV light to give the corresponding monomers. Thus, interconversion between the closed-helicene monomer and dimer was achieved. The optically pure dimer was photo-dissociated into the monomers associated with a turn-on of circularly polarized luminescence (CPL).

Two pairs of chiral YbIII enantiomers presenting distinct NIR luminescence and circularly polarized luminescence performances with giant differences in second-harmonic generation responses

By introducing enantiomerically pure mono-bidentate N-donor ligands (LR/LS) into Yb(btfa)3(H2O)2 and Yb(dbm)3(H2O), respectively, two pairs of chiral YbIII enantiomers, namely Yb(btfa)3LR/Yb(btfa)3LS (D-1/L-1) and [Yb(dbm)3LR]·[Yb(dbm)3(C2H5OH)]/[Yb(dbm)3LS]·[Yb(dbm)3(C2H5OH)] (D-2/L-2) were isolated, where btfa- = 3-benzoyl-1,1,1-trifluoroacetonate, dbm- = dibenzoylmethanate, and LR/LS = (-)/(+)-4,5-pinenepyridyl-2-pyrazine. D-1/L-1 possess mononuclear structures in which the YbIII ions are eight-coordinated, while D-2/L-2 show cocrystal structures containing Yb(dbm)3(LR/LS) and Yb(dbm)3(C2H5OH) moieties in which the two YbIII ions are eight and seven-coordinated, respectively. They not only feature different molecular structures but also present distinct linear and nonlinear optical performances. Chiral mononuclear D-1 has better near infrared photo-luminescence (NIR-PL) and circularly polarized luminescence (CPL) performances than chiral cocrystal D-2. More remarkably, D-1/L-1 show large second-harmonic generation (SHG) responses (up to 1.25/1.28 × KDP) 18/16 times those of D-2/L-2 (0.07/0.08 × KDP). In addition, D-2/L-2 represent the first examples of lanthanide cocrystal complexes with NIR-PL, NIR-CPL and SHG properties.

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 .

DNA-induced circularly polarized luminescence of helicene racemates

Enantiopure helicenes have been extensively investigated due to their outstanding chiroptical properties, while helicene racemates are considered as chiroptically silent. Here, we describe a facile method to produce circularly polarized luminescence (CPL) from helicene racemates via supramolecular association with DNA in aqueous solution. Racemic cationic helicene derivatives are immobilized in the grooves of commercially available double-stranded right-handed DNA, and the discrimination of left- and right-handed helicenes by chiral DNA is monitored by single molecule force spectroscopy. This subsequently leads to the generation of prominent CPL with dissymmetric factor |glum | of close to 0.01, which is approximate to enantiopure helicenes. The strategy developed in this work avoids the tedious and expensive chiral resolution process and provides a distinctive insight into the fabrication of CPL-emitting systems.

Circular Polarized Light Emission in Chiral Inorganic Nanomaterials

Chiral inorganic nanostructures strongly interact with photons changing their polarization state. The resulting circularly polarized light emission (CPLE) has cross-disciplinary importance for a variety of chemical/biological processes and is essential for development of chiral photonics. However, the polarization effects are often complex and their interpretation is dependent on the several structural parameters of the chiral nanostructure. CPLE in nanostructured media has multiple origins and several optical effects are typically convoluted into a single output. Analyzing CPLE data obtained for nanoclusters, nanoparticles, nanoassemblies, and nanocomposites from metals, chalcogenides, perovskite, and other nanostructures, it is shown here that there are several distinct groups of nanomaterials for which CPLE is dominated either by circularly polarized luminescence (CPL) or circularly polarized scattering (CPS); there are also many nanomaterials for which they are comparable. The following points are also demonstrated: 1) CPL and CPS contributions involve light-matter interactions at different structural levels; 2) contribution from CPS is especially strong for nanostructured microparticles, nanoassemblies, and composites; and 3) engineering of materials with strongly polarized light emission requires synergistic implementation of CPL and CPS effects. These findings are expected to guide development of CPLE materials in a variety of technological fields, including 3D displays, information storage, biosensors, optical spintronics, and biological probes.

Rapid handheld time-resolved circularly polarised luminescence photography camera for life and material sciences

Circularly polarised luminescence (CPL) is gaining a rapidly increasing following and finding new applications in both life and material sciences. Spurred by recent instrumental advancements, the development of CPL active chiral emitters is going through a renaissance, especially the design and synthesis of CPL active luminescent lanthanide complexes owing to their unique and robust photophysical properties. They possess superior circularly polarised brightness (CPB) and can encode vital chiral molecular fingerprints in their long-lived emission spectrum. However, their application as embedded CPL emitters in intelligent security inks has not yet been fully exploited. This major bottleneck is purely hardware related: there is currently no suitable compact CPL instrumentation available, and handheld CPL photography remains an uncharted territory. Here we present a solution: an all solid-state small footprint CPL camera with no moving parts to facilitate ad hoc time-resolved enantioselective differential chiral contrast (EDCC) based one-shot CPL photography (CPLP).

One-Dimensional Chiral Copper Iodide Chain-Like Structure Cu4 I4 (R/S-3-quinuclidinol)3 with Near-Unity Photoluminescence Quantum Yield and Efficient Circularly Polarized Luminescence

Chiral organic-inorganic hybrid metal halide materials have shown great potential for circularly polarized luminescence (CPL) related applications for their tunable structures and efficient emissions. Here, this work combines the highly emissive Cu₄ I₄ cubane cluster with chiral organic ligand R/S-3-quinuclidinol, to construct a new type of 1D Cu-I chains, namely Cu₄ I₄ (R/S-3-quinuclidinol)₃ , crystallizing in noncentrosymmetric monoclinic P2₁ space group. These enantiomorphic hybrids exhibit long-term stability and show bright yellow emission with a photoluminescence quantum yield (PLQY) close to 100%. Due to the successful chirality transfer from the chiral ligands to the inorganic backbone, the enantiomers show intriguing chiroptical properties, such as circular dichroism (CD) and CPL. The CPL dissymmetry factor (g_lum ) is measured to be ≈4 × 10^-3 . Time-resolved photoluminescence (PL) measurements show long averaged decay lifetime up to 10 µs. The structural details within the Cu₄ I₄ reveal the chiral nature of these basic building units, which are significantly different than in the achiral case. This discovery provides new structural insights for the design of high performance CPL materials and their applications in light emitting devices.

Two Chiral YbIII Enantiomeric Pairs with Distinct Enantiomerically Pure N-Donor Ligands Presenting Significant Differences in Photoluminescence, Circularly Polarized Luminescence, and Second-Harmonic Generation

Using enantiomerically pure bidentate and tridentate N-donor ligands (¹L_R/¹L_S and ²L_R/²L_S) to replace two coordinated H₂O molecules of Yb(tta)₃(H₂O)₂, respectively, two eight- and nine-coordinated Yb^III enantiomeric pairs, namely, Yb(tta)₃¹L_R/Yb(tta)₃¹L_S (Yb-R-1/Yb-S-1) and [Yb(tta)₃²L_R]·CH₃CN/[Yb(tta)₃²L_S]·CH₃CN (Yb-R-2/Yb-S-2), were isolated, in which Htta = 2-thenoyltrifluoroacetone, ¹L_R/¹L_S = (-)/(+)-4,5-pinene-2,2'-bipyridine, and ²L_R/²L_S = (-)/(+)-2,6-bis(4',5'-pinene-2'-pyridyl)pyridine. Interestingly, they not only present distinct degrees of chirality but also show large differences in near-infrared (NIR) photoluminescence (PL), circularly polarized luminescence (CPL), and second-harmonic generation (SHG). Eight-coordinated Yb-R-1 with an asymmetric bidentate ¹L_R ligand has a high NIR-PL quantum yield (1.26%) and a long decay lifetime (20 μs) at room temperature, being more than two times those (0.48%, 8 μs) of nine-coordinated Yb-R-2 with a C₂-symmetric tridentate ²L_R ligand. In addition, Yb-R-1 displays an efficient CPL with a luminescence dissymmetry factor g_lum = 0.077, being 4 × Yb-R-2 (0.018). In particular, Yb-R-1 presents a strong SHG response (0.8 × KDP), which is 8 × Yb-R-2 (0.1 × KDP). More remarkably, the precursor Yb(tta)₃(H₂O)₂ exhibits a strong third-harmonic generation (THG) response (41 × α-SiO₂), while the introduction of chiral N-donors results in the switching of THG to SHG. Our interesting findings provide new insights into both the functional regulation and switching in multifunctional lanthanide molecular materials.

Bulky anion effect on the architecture of chiral dysprosium single-molecule magnets

The interest for chiral tris(β-diketonato)lanthanide complexes in coordination chemistry is huge due to its Lewis acid character, optical activity, and the control of the final compound architecture. The reaction of equimolar quantities of [Dy((-)/(+)hfc)₃ (H₂ O)] (hfc^-  = 3-(heptafluoropropylhydroxymethylene)-(+/-)-camphorate) and L led to the formation of a pair of enantiomers for dinuclear complexes [Dy((-)/(+)hfc)₃ (L)]₂ ⋅C₇ H₁₆ ([(-)/(+)1]⋅C₇ H₁₆ ) (L = 4'-(4'''-pyridyl-N-oxide)-1,2':6'1''-bis-(pyrazolyl)pyridine]). Starting from the previous experimental protocol with the addition of bulky BArF anions, a partial dissociation of the chiral [Dy((-)/(+)hfc)₃ (H₂ O)] was observed leading to the isolation of a mono-dimensional cationic chiral polymer {[Dy((-)/(+)hfc)₂ (L)][BarF]}_n ⋅nCH₃ NO₂ ([(-)/(+)2]_n ⋅nCH₃ NO₂ ). Natural circular dichroism (NCD) highlighted an exciton CD couplet for [(-)/(+)2]_n but not for (-)/(+)1. The latter behaves as a single-molecule magnet (SMM) with a blocking temperature up to 4 K, whereas [(-)/(+)2]_n is a 1D assembly of field-induced SMMs with a magnetic relaxation occurring through a Raman process only.

Circularly polarized near-infrared phosphorescence of chiral chromium(III) complexes

Homoleptic Cr(III) complexes containing anionic tridentate 1,8-(bisoxazolyl)carbazolide ligands are phosphorescent in deaerated solutions with peak maxima in the range of 813-845 nm. The ligand carbon-centred chirality has been transferred to the helical chirality of the complexes and hence induced circularly polarized NIR-emissions with dissymmetry factor in the scale of 2.0 × 10^-3.

Strong Coupling of Chiral Frenkel Exciton for Intense, Bisignate Circularly Polarized Luminescence

We show that chiral Frenkel excitons yield intense circularly polarized luminescence with an intrinsic dissymmetry factor in emission g_lum as high as 0.08. This outstanding value is measured through thin films of cyanine J-aggregates that form twisted bundles. Our measurements, obtained by a Mueller polarization analysis, are artifact-free and reveal a quasi-perfect correlation between the dissymmetry factors in absorption, g_abs , and in emission g_lum . We interpret the bisignate dissymmetry factors as the signature of a strong coupling between chiral Frenkel excitons longitudinally excited along the bundles. We further resolve by polarimetry analysis the split in energy between the excited states with a Davydov splitting as small as 28 meV. We finally show the anti-Kasha nature of the chiral emission bands with opposite optical chirality. These mirror-imaged emissive chiroptical features emerge from the structural rigidity of the bundles that preserves the ground- and excited-state chirality.

Circularly polarized luminescence in molecular recognition systems: Recent achievements

Circularly polarized luminescence (CPL) dyes are recognized to be new generation materials and have been actively developed. Molecular recognition systems provide nice approaches to novel CPL materials, such as stimuli-responsive switches and chemical sensing materials. CPL may be induced simply by mixing chiral or achiral, luminescent or nonluminescent host and guest; there are several combinations. Molecular recognition can potentially save time and effort to construct well-ordered chiral structures with noncovalent attractive interactions as compared with the multi-step synthesis of covalently bonded dyes. It is a challenging subject to engage molecular recognition events with CPL, and it is important and interesting to see how it is achieved. In fact, simple molecular recognition systems can even enable the fine adjustment of CPL performance and detailed conformational/configurational analysis of the excited state. Here we overview the recent achievements of simple host-guest complexes capable of exhibiting CPL, summarizing concisely the host/guest structures, CPL intensities, and characteristics.

Multifunctional Helicene-Based Ytterbium Coordination Polymer Displaying Circularly Polarized Luminescence, Slow Magnetic Relaxation and Room Temperature Magneto-Chiral Dichroism

The combination of physical properties sensitive to molecular chirality in a single system allows the observation of fascinating phenomena such as magneto-chiral dichroism (MChD) and circularly polarized luminescence (CPL) having potential applications for optical data readout and display technology. Homochiral monodimensional coordination polymers of Yb^III were designed from a 2,15-bis-ethynyl-hexahelicenic scaffold decorated with two terminal 4-pyridyl units. Thanks to the coordination of the chiral organic chromophore to Yb(hfac)₃ units (hfac^- =1,1,1,5,5,5-hexafluoroacetylaconate), efficient NIR-CPL activity is observed. Moreover, the specific crystal field around the Yb^III induces a strong magnetic anisotropy which leads to a single-molecule magnet (SMM) behaviour and a remarkable room temperature MChD. The MChD-structural correlation is supported by computational investigations.

Enhanced circularly polarized luminescence of chiral Eu(III) coordination polymers with structural strain

Three types of Eu(III) coordination polymers with different distorted chiral ligands, [Eu(+tfc)₃(p-dpeb)]_n, [Eu(+pfc)₃(p-dpeb)]_n, and [Eu(+hfc)₃(p-dpeb)]_n (+tfc: (+)-3-(trifluoroacetyl)camphorate, +pfc: (+)-3-(pentafluoropropionyl)camphorate, +hfc: (+)-3-(heptafluorobutyryl)camphorate, p-dpeb: 1,4-bis(diphenylphosphorylethynyl)benzene), were prepared for elucidating the relationship between their structural distortions, ligand-to-metal charge transfer (LMCT), and circularly polarized luminescence (CPL) properties. Their strain factors in the ligands were evaluated using crystallographic data obtained by single-crystal X-ray structural analyses. The characteristics of the LMCT excited states were estimated from theoretical calculations. The introduction of a bulky substituent into the chiral ligand afforded a distorted structure of β-diketonates and changed the direction of the transition electric dipole moments, which are related to the magnitude of the CPL intensity. The CPL dissymmetry factor (g_CPL) of [Eu(+hfc)₃(p-dpeb)]_n, with a large distorted structure, was -0.22, while those of [Eu(+tfc)₃(p-dpeb)]_n and [Eu(+pfc)₃(p-dpeb)]_n, with small distorted structures, were -0.05 and -0.10, respectively. The controlled steric hindrance of the chiral ligands in Eu(III) coordination polymers is one of the strain factors enhancing their CPL properties.

Spinolate Lanthanide Complexes for High Circularly Polarized Luminescence Metrics in the Visible and Near-Infrared

Analogues of Shibasaki's complexes supported by enantiopure Spinol are synthesized and characterized. The tris(Spinol) Ln^III complexes are generated either by ligand deprotonation followed by complexation with lanthanide triflate salts or by in situ deprotonation by Ln(N(SiMe₃)₂)₃ salts in the presence of additional base. The resulting complexes are found to be luminescent and chiroptically active for both circular dichroism and circularly polarized luminescence (CPL), notably producing strong CPL with dissymmetry factors (g_lum) of up to 0.50, 0.53, and 0.53 for Sm, Tb, and Dy, respectively. The Sm complex is found to be CPL-active in the near-infrared (NIR) region at 980 nm, representing the first report of NIR CPL from Sm. Additionally, the Tb complex, due to efficient sensitization (Φ = 0.846 in tetrahydrofuran) coupled with strong dissymmetry factors, achieves a CPL brightness (B_CPL) of 3760 M^-1 cm^-1, the highest reported for any CPL-active compound to date. These are rare examples of compounds that show simultaneous improvement of both CPL metrics (g_lum and B_CPL). Solid-state structural analysis of the Spinolate complexes and comparisons to other CPL-active analogues of Shibasaki's complexes also suggest that nondistorted geometries should generate even stronger metrics.

Effect of Oxidation on the Chiroptical Properties of Sulfur-Bridged Binaphthyl Dimers

A series of axially chiral sulfur-bridged dimers were prepared from 1,1'-binaphthyl-2,2'-diol and subsequently oxidized to the respective sulfones. The chiroptical properties of the chiral chromophores were studied as a function of the oxidation state. Upon oxidation, an increase in quantum yields was observed for directly linked sulfur bridged binaphthyls (0.04 to 0.32), and a modest increase in dissymmetry factor was observed for diphenylsulfide-bridged binaphthyls (-8.9 × 10^-4 to -1.4 × 10^-3). Computational calculations were used to elucidate the changes in photophysical properties.

A Raman optical activity spectrometer can sensitively detect lanthanide circularly polarized luminescence

Recently, many studies have appeared in which the Raman optical activity (ROA) instrument was found to be convenient for measuring circularly polarized luminescence (CPL). Typically, weak lanthanide luminescence including circular polarization could be detected. The new detection scheme is referred to as ROA-CPL spectroscopy. It is particularly useful when also the vibrational (ROA) itself is detectable as the molecule structure can be examined more reliably. In this review, development of this chiroptical approach and its applications in structural studies of biomolecules are summarized.

Circularly Polarized Luminescence from Uranyl Improves Resolution of Electronic Transitions

The first reported example of circularly polarized luminescence from a chiral, molecular uranyl (UO₂²⁺) complex in solution is presented. This uranyl chiroptical activity is enabled by complexation with ibuprofen, an enantiopure chiral carboxylate ligand. Salt metathesis between [UO₂Cl₂(thf)₂]₂ and the sodium ibuprofenate salts results in the formation of the anionic tris complexes; these complexes are found to be luminescent in solution, both under visible excitation, directly targeting the metal, and through sensitization by UV absorption and energy transfer from the ligand. Each enantiomer displays both circular dichroism and circularly polarized luminescence (CPL) with |g_abs| ≤ 8.1 × 10^-2 and |g_lum| ≤ 8.0 × 10^-3 under UV excitation, comparable to chiral transition metal complexes or purely organic emitters. The strength of the CPL emission is found to be comparable following excitation of either the ligand or metal directly. Further, use of CPL allows for resolution of subcomponents of the emission spectrum not previously possible at room temperature using standard fluorescence techniques. Observation of CPL following direct uranyl excitation presents a new tool for probing speciation of uranyl complexes when chiral ligands are used, without the need for synthetic modification to incorporate a suitable chromophore, and could enable the design of improved ligands for uranyl extraction from wastewater.

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.

Strong Circularly Polarized Luminescence at 1550 nm from Enantiopure Molecular Erbium Complexes

Circularly polarized luminescence (CPL) in two subregions of the near-infrared (NIR) has been achieved. By leveraging the rigidity and diminishing detrimental vibrations of the heterobimetallic binolate complexes of erbium [(Binol)₃ErNa₃], species exhibiting an exceptionally high dissymmetry factor (|g_lum |) of 0.47 at 1550 nm were obtained. These erbium complexes are the first reported examples of CPL observed beyond 1200 nm. Analogous complexes of ytterbium and neodymium also exhibited strong CPL (|g_lum| = 0.17, 0.05, respectively) in a higher energy NIR window (800-1200 nm). All complexes exhibit high quantum yields (Er: 0.58%, Yb: 17%, Nd: 9.3%) and high B_CPL values (Er: 57 M^-1 cm^-1, Yb: 379 M^-1 cm^-1, Nd: 29 M^-1 cm^-1). Because of their strong CPL emission in the telecom band (1550 nm), biologically relevant NIR emission window (800-1100 nm), and synthetic versatility, the complexes reported here could permit further promising developments in quantum communication technologies and biologically relevant sensors.

Non-salen coumarin Schiff base chiral fluorescent probe turn on circularly polarized luminescence by Mg2+ and Zn2+

L1 can be used as highly sensitive dual detection ion probe for Mg2+ and Zn2+. Mg2+ and Zn2+ can turn on the CPL emission of L1. L1 showed reversible fluorescence “on/off” response with Zn2+ and EDTA.

Synthesis of bright water-soluble circularly polarized luminescence emitters as potential sensors

Complexes exhibiting strong circularly polarized luminescence in water were synthesized and used as CPL probes sensing pH and metal ions.

Chiral Dinuclear Eu(III), Tb(III), and Y(III) Complexes Supported by P-Stereogenic Linear Tetraphosphine Tetraoxide

A P -stereogenic linear tetraphosphine tetraoxide, ( R,R )- or ( S,S )-dpmppm(=O) 4 , was synthesized to prepare C 2 dinuclear M(hfa) 3 complexes (M = Eu, Tb, Y) as the first example of lanthanide(III) complexes with P -chiral multidentate phosphine oxides. The mononuclear M(hfa) 3 complexes (M = Eu, Y) with a P -chiral diphosphine oxide, tpdpb(=O) 2 , were also prepared, and comparison of their photophysical properties for the Eu(III) complexes revealed that significant chiral induction from the P -chiral centers arises on the achiral M(hfa) 3 units through intramolecular π-π stacking constraint in the dinuclear system.

Anion-Dependent Outstanding Luminescence Enhancement of Eu(D-facam)3 Upon Coexistence With the Tetramethylammonium Cation

The effect of a series of tetramethylammonium salts with different counter anions on the photophysical properties of a chiral Eu(III) complex (Eu(D-facam)₃ ) was investigated. Anion-dependent luminescence of the Eu(III) complex was observed, and particularly in the presence of acetate ions, an outstanding luminescence enhancement (>300 times) and induced circularly polarized luminescence (g_lum =-0.63) were obtained. The energy transfer process was then evaluated using key photophysical parameters, and it was found that the sensitisation efficiency of the Eu(III) complex significantly increased in the presence of tetramethylammonium acetate (TMAOAc). The interactions between Eu(D-facam)₃ and TMAOAc were confirmed by luminescence analysis, circular dichroism spectroscopy, Fourier transform infrared spectroscopy and mass spectral measurements.

Ruthenium Complexes Bearing Axially Chiral Bipyridyls: The Mismatched Diastereomer Showed Red Circularly Polarized Phosphorescence

Ruthenium^II complexes bearing three axially chiral bipyridyl ligands were synthesized as a new family of chiral complex dyes, and Δ-(S)- and Λ-(S)-diastereomers were obtained. The X-ray crystal structure analyses, spectroscopy, and DFT calculations suggested that all the bipyridyls maintained chirality in both the ground and excited states, and the Δ-(S)- and Λ-(S)-isomers are the matched (more relaxed) and mismatched (more constrained) pairs, respectively. The mismatched Λ-(S)-isomer exhibited red circularly polarized phosphorescence (CPP) both in solution and in the solid state. The solution state CPP is the most intense of ruthenium complexes, while the solid state CPP is the first example of them. It is supposed that, for the Λ-(S)-isomer, the six cumulative CH/π interactions suppress further distortion in the T₁ state.

Cyclodextrins with Multiple Pyrenyl Groups: An Approach to Organic Molecules Exhibiting Bright Excimer Circularly Polarized Luminescence

Circularly polarized luminescence (CPL) has attracted much attention because of its potential for electronic and biological applications. Herein, we report a simple and effective approach to organic molecules exhibiting bright CPL by combining a chiral cyclic molecular scaffold and multiple excimer-enabling moieties. An a-cyclodextrin ( CyD ) scaffold was modified with six pyrenyl groups to obtain pyrene-cyclodextrins ( PCD s) in a one-step synthesis from commercially available compounds. The PCDs exhibited high molar extinction coefficients (ε ~ 10 5 M -1 cm -1 ), polarized emission with a good dissymmetry factor (| g lum | ~ 10 -2 ), and quantum yield ( Φ f ~ 0.5). Owing to the excellent photophysical properties of the PCDs, the circularly polarized luminescence brightness ( B CPL ) reached 340 M -1 cm -1 . Comprehensive photophysical and chiroptical studies of the PCD s with only five pyrene units and with linkers of various lengths connecting the CyD with the pyrene units revealed that the formation of a pyrene excimer in a spatially crowded environment is crucial for CPL anisotropy. This study paves the way for the development of bright CPL organic molecules.

Toward Large Dissymmetry Factor of Circularly Polarized Luminescence in Donor-Acceptor Hybrid Systems

Chiroptical materials with circularly polarized luminescence (CPL) activity have aroused a lot of interest. One essential factor for evaluating the features of CPL-active materials is the dissymmetry factor (g_lum), which represents the circular polarization of emitted light. Essentially, for the practical application of CPL, the most important issue is to greatly improve the g_lum value. Recently, benefiting from the flexible and efficient design in hybrid donor-acceptor systems, more and more examples involving g_lum value amplification have been reported. In this Perspective, we highlight the proposed mechanism for the generation and amplification of CPL in these hybrid systems. We also present the corresponding design principles and potential pitfalls in experimental processes. We hope that this Perspective can shed light on the development of highly efficient CPL-active materials.

A Family of Superhelicenes: Easily Tunable, Chiral Nanographenes by Merging Helicity with Planar π Systems

We designed a straightforward synthetic route towards a full-fledged family of π-extended helicenes: superhelicenes. They have two hexa-peri-hexabenzocoronenes (HBCs) in common that are connected via a central five-membered ring. By means of structurally altering this 5-membered ring, we realized a versatile library of molecular building blocks. Not only the superhelicene structure, but also their features are tuned with ease. In-depth physico-chemical characterizations served as a proof of concept thereof. The superhelicene enantiomers were separated, their circular dichroism was measured in preliminary studies and concluded with an enantiomeric assignment. Our work was rounded-off by crystal structure analyses. Mixed stacks of M- and P-isomers led to twisted molecular wires. Using such stacks, charge-carrier mobilities were calculated, giving reason to expect outstanding hole transporting properties.

Structure Determination of Europium Complexes in Solution Using Crystal-Field Splitting of the Narrow f-f Emission Lines

Nine nona-coordinated Eu(III) complexes (1-9) studied here have three unsymmetric β-diketonate ligands and one chiral Ph-Pybox ligand, which can produce eight possible coordination isomers, depending on the position of the three unsymmetric β-diketonate ligands. Substituents on the β-diketonate ligands cause a rational structural rearrangement upon crystallization. Substituents with higher polarity, including -CN, -F, -Cl, -Br, -OMe, and -OEt, employ intercomplex hydrogen bonding to generate an association complex through structural rearrangement upon crystallization. Substituents with lower polarity, including -CF3, -SMe, and -Me, cause the most energetically favorable isomer to crystallize directly from solution. These two crystal structures exhibit well-resolved f-f emission lines with characteristic Stark splitting structures. This work revealed that the configuration of the Eu(III) complexes in solution can be determined by systematic comparison of their Stark splitting structures to those obtained from the solid phase using density functional theory (DFT)-based predictions combined with circular dichroism data.

Excited State Properties of Lanthanide(III) Complexes with a Nonadentate Bispidine Ligand

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 ([EuIII L2 ]+ , H2 O, 298 K, τ=1.51 ms, ϕ=0.35; [TbIII L2 ]+ , H2 O, 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.