Enhancement of near-infrared luminescence of ytterbium in triple-stranded binuclear helicates

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.

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.

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.

Coordination-Directed Self-Assembly of Functional Polynuclear Lanthanide Supramolecular Architectures

Lanthanide supramolecular chemistry is a fast growing and intriguing research field due to the unique photophysical, magnetic, and coordination properties of lanthanide ions (Ln^III). Compared with the intensively investigated mononuclear Ln-complexes, polymetallic lanthanide supramolecular assemblies offer more structural superiority and functional advantages. In recent decades, significant progress has been made in polynuclear lanthanide supramolecules, varying from structural evolution to luminescent and magnetic functional materials. This review summarizes the design principles in ligand-induced coordination-driven self-assembly of polynuclear Ln-structures and intends to offer guidance for the construction of more elegant Ln-based architectures and optimization of their functional performances. Design principles concerning the water solubility and chirality of the lanthanide-organic assemblies that are vital in extending their applications are emphasized. The strategies for improving the luminescent properties and the applications in up-conversion, host-guest chemistry, luminescent sensing, and catalysis have been summarized. Magnetic materials based on supramolecular assembled lanthanide architectures are given in an individual section and are classified based on their structural features. Challenges remaining and perspective directions in this field are also briefly discussed.

Cyclized Polyacrylonitrile as a Promising Support for Single Atom Metal Catalyst with Synergistic Active Site

Metal single atom catalysts (SAC) have been successfully used in heterogeneous catalysis but developing a scalable and economic support for SAC is still a great challenge. Here, cyclized polyacrylonitrile (CPAN) is proposed as a promising support for single atom metal catalysts. CPAN can be easily prepared from cheap industrial product polyacrylonitrile (PAN), which has excellent processability. A series of SAC on CPAN (M/CPAN, M = Ag, Cu, Ru) are designed and the catalytic activities of the as synthesized M/CPAN are investigated by the model reduction reaction of p-nitrophenol (4-NP). M/CPAN presents excellent catalytic performance with high stability and theoretical calculations elucidate that Ag/CPAN synergistically catalyze 4-NP reduction following the Langmuir-Hinshelwood (L-H) mechanism with 4-NP preferentially adsorbing at the Ag sites and H adsorbing at the bridge C sites. These results, for the first time, reveal that the single atom on CPAN can catalyze 4-NP reduction efficiently. This methodology provides a convenient route for the preparation of a variety of SAC, and this strategy is readily scalable and holds great potential in catalytic applications.

Designing water-quenching resistant highly luminescent europium complexes by regulating the orthogonal arrangement of bis-β-diketone ligands

Luminescent β-diketone-based lanthanide complexes have been well explored as chemical sensor materials for biomedicine applications. Herein, three mononuclear Eu3+ complexes based on bis-β-diketone ligands (L1, L2 and L3) that can reduce luminescence quenching caused by water were developed. The ligands feature two β-diketone units covalently bound at the 1,8-position of the derivatized anthracene (modified with tetracyanoethylene, TCNE). X-ray crystallographic analysis reveals that their self-assemblies with Ln3+ ions in a 2 : 1 stoichiometric ratio form mononuclear anion complexes, [EuL2]-, in which two ligands coordinate to the metal center in a mutually orthogonal manner. This kind of arrangement together with the bulge of TCNE from the anthracene plane well protected the complexes from the quenching effects of water molecules in the second coordination. The photophysical measurements showed that the complexes not only had high luminescence quantum yields (QYs, up to 50-67%) but also presented excellent water-quenching resistant capability.

Helix-mediated over 1 nm-range chirality recognition by ligand-to-ligand interactions of dinuclear helicates

Long-range chirality recognition between the two chiral guest ligands can be tuned based on the helix distances (d_Ln–Ln = 11.5 and 14.0 Å) of bis-diketonate bridged dinuclear lanthanide complexes (2Th and 3Th, respectively) used as mediators. Both 2Th and 3Th form one-dimensional (1D) helical structures upon terminal binding of two chiral guest co-ligands (L^R or L^S). Long-range chiral self-recognition is achieved in self-assembly of 2Th with L^R and L^S to preferentially form homochiral assemblies, 2Th-L^R·L^R and 2Th-L^S·L^S, whereas there is no direct molecular interaction between the two guest ligands at the terminal edges. X-ray crystal structure analysis and density functional theory studies reveal that long-range chiral recognition is achieved by terminal ligand-to-ligand interactions between the bis-diketonate ligands and chiral guest co-ligands. Conversely, in self-assembly of 3Th with a longer helix length, statistical binding of L^R and L^S occurs, forming heterochiral (3Th-L^R·L^S) and homochiral (3Th-L^R·L^R and 3Th-L^S·L^S) assemblies in an almost 1 : 1 ratio. When phenyl side arms of the chiral guest co-ligands are replaced by isopropyl groups (L′^R and L′^S), chiral self-recognition is also achieved in the self-assembly process of 3Th with the longer helix length to generate homochiral (3Th-L′^R·L′^R and 3Th-L′^S·L′^S) assemblies as the favored products. Thus, subtle modification of the chiral guests is capable of achieving over 1.4 nm-range chirality recognition.

Long-range chirality recognition between the two chiral guest ligands can be tuned based on the helix distances (d_Ln–Ln = 11.5 and 14.0 Å) of bis-diketonate bridged dinuclear lanthanide complexes (2Th and 3Th, respectively).

Dinuclear triple stranded phenyl-spaced 1,3-bis-β-diketonato lanthanide(iii) complexes: synthesis, structures and spectroscopy

The synthesis and crystal structures of a series of Ln(iii) (Ln = Gd, Nd, Yb as well as Y) complexes of a bis-β-diketone ligand incorporating a 1,3-substituted phenyl ring between its two β-diketone domains (1,1'-(1,3-phenylene)bis(4,4-dimethylpentane-1,3-dione), H₂L¹) is reported. The crystal structures show the complexes are seven coordinate in the solid state with the general formula [Ln₂L¹₃(solvent)₂]. The photophysical properties of the complexes were explored in both solution and the solid state, which show an increase in coordination number in solution.

Novel quadruple-stranded heterometallic Ln2Na complexes hosting sodium ions inside the cryptand-like cavity

The first examples of lanthanide tetrakis-complexes based on bis-chelating carbacylamidophosphate were synthesized and characterized.

Enhanced E/Z-photoisomerization and luminescence of stilbene derivative co-coordinated in di-β-diketonate lanthanide complexes

A new tetradentate chelating ligand appending a stilbene derivative, E-N',N'-bis(pyridin-2-ylmethyl)-4-styrylbenzohydrazide (HL) was synthesized, together with two β-diketonates (4,4,4-trifluoro-1-phenylbutane-1,3-dionate, tfd), with or without the trifluoroacetate anion present as a ligand for coordination with lanthanide(iii) ions to form [Ln(tfd)2(HL)(CF3CO2)] (LnC49H36F9N4O7, Ln = La (1), Nd (2), Eu (3), Gd (4)) and [Yb(tfd)2(L)] (YbC47H35F6N4O5 (5), L = deprotonated HL). All five complexes were structurally characterized, and five crystals were obtained and analyzed by single-crystal X-ray diffraction. The quantum yield of trans-to-cis photoisomerization of the stilbene group in gadolinium complex 4 was enhanced about five-fold compared with that of HL itself. Other complexes showed slightly enhanced or depressed photoisomerization. The total luminescence quantum yield/sensitization efficiency of europium complex 3 in the solid state and acetonitrile solution were 22.1%/96.7% and 19.3%/97.9%, respectively. The transfer of ligand energy to the Eu3+ ion was highly efficient. This enhanced photoisomerization and luminescence of the stilbene group within complexes was found to be related to the energy level of lanthanide ions and whether a ligand-to-metal center or ligand-to-ligand charge transfer process was present. The interpretation of experimental results is rationally supported by time-dependent density-functional theory calculations. In complex 4, except for the intramolecular absorption transition of HL ligand itself (IL, πHL-π*HL), the presence of the ligand-to-ligand charge transfer transition from tfd to HL (LLCT, πtfd-π*HL) and the triplet state energy of HL being unable to transfer to the higher 6P7/2 excited energy level of the Gd3+ ion would facilitate HL photoisomerization. For complex 3, this was due to reversed ligand-to-ligand charge transfer transition from HL to tfd (LLCT, πHL-π*tfd) and its energy transfer to the metal center. Although the observed radiative lifetimes of NIR luminescent complexes 2 and 5 were around 10 μs, these systems contained only two diketone ligands, indicating that HL still had a certain promoting effect compared with tris(diketonate) lanthanide complexes. These results offer an important route for the design of new lanthanide-based molecular switching materials.

Near Infrared (NIR) imaging: Exploring biologically relevant chemical space for lanthanide complexes

Near Infrared (NIR) imaging agents are extensively used in the biological or preclinical treatment and diagnosis of a wide range of diseases including cancers and tumors. The current arsenal of NIR compounds are most constituted by organic dyes, polymers, inorganic nanomaterials, whereas Ln molecular complexes explore an alternative approach to design NIR probes that are potentially bring new molecular toolkits into the biomedicine. In this review, NIR imaging agents are categorized according to their molecular sizes, constitution and the key properties and features of each class of compounds are briefly defined wherever possible. To better elucidate the features of Ln complexes, we provide a succinct understanding of sensitization process and molecular Ln luminescence at a mechanistic level, which may help to deliver new insights to design NIR imaging probes. Finally, we used our work on NIR ytterbium (Yb³⁺) probes as an example to raise awareness of exploring biologically relevant chemical space for lanthanide complexes as chemical entities for biological activity.

Crystal structure of (E)-4-hydroxy-3-((5-phenyl-1,3,4-oxadiazol-2-yl)thio)pent-3-en-2-one, C13H12N2O3S

C13H12N2O3S, monoclinic, P21/c (no. 14), a = 12.3541(8) Å, b = 14.3818(10) Å, c = 7.3757(5) Å, β = 96.471(2)°, V = 1302.12(15) Å3, Z = 4, R gt(F) = 0.0417, wR ref(F 2) = 0.1080, T = 293(2) K.

Eu(III) Tetrahedron Cage as a Luminescent Chemosensor for Rapidly Reversible and Turn-On Detection of Volatile Amine/NH3

Because of the involvement of the gas-solid diffusion, device fabrication, and the relatively complex photophysical process, the lanthanide complexes are rarely exploited as fluorescence sensors for volatile compound (VC) detection. Herein, we report the first example of a discrete 3D Ln-based architecture as a sensor for VCs. The designed Eu₄L₄ tetrahedral cage shows highly selective, rapidly reversible, and turn-on emissive responses toward volatile amines/NH₃ in a spin-coated film. Through the comprehensive spectral characteristic and density functional theory calculation, an intermolecular weak nucleophilic interaction is proposed for this response mechanism. Combining this weak interactions with the permeability of the cage, the film presents subsecond to second timescales rapid response; combining the fitting electrophilic capability of the β-diketonate units to amine nitrogen with the tunable intramolecular charge-transfer feature, the cage shows excellent selectivity and turn-on emissive response. This work provides a new clue to develop the lanthanide complexes as luminescence probes for VCs.

Dinuclear Lanthanide(III) Complexes Showing Single-Molecule Magnet Behaviour and Optical Properties

Three dinuclear lanthanide(iii) complexes with an acetate bridge, which are synthesised by a Salen-type Schiff base and β-diketonate ligands, i.e. [Ln2L2(TTA)4(OAc)2]·CH2Cl2 (Ln=Eu (1), Gd (2), Dy (3); TTA=thenoyltrifluoroacetonate, H2L=N,N′-ethylenebis(salicylideneimine), are investigated. We obtained the structures of complexes 1–3 by X-ray crystallography. Notably, in terms of the structure of these complexes, what is intriguing is that the acetate groups link two Lniii ions, whereas the cadmium ions do not coordinate. Lanthanide-based luminescence is exhibited by complex 1, which exists in both the solid state and a methanol solution. Through magnetic analysis, it is found that a field-induced single-molecule magnet behaviour is exhibited by complex 3, and the energy barrier is shown to be Ueff=45.97K.

Enhanced near-infrared luminescence in Ln2Cd2 (Ln = Nd, Yb) heterotetranuclear complexes

Due to the effect of the combination of the rigid structure and synergistic interactions of two ligands on energy transfer, the Cd–Ln complexes show excellent NIR luminescence properties with the QYs reaching 0.34% for Nd³⁺ and 2.13% for Yb³⁺.

Monometallic lanthanide salicylhydrazone complexes exhibiting strong near-infrared luminescence

We report the synthesis of monometallic lanthanide complexes supported by one salicylhydrazone ligand. Complexes of neodymium, erbium, and ytterbium have been synthesized, and exhibit near infrared luminescence with modest to high quantum yields. The luminescence of ytterbium is increased by a factor of five when comparing our complex to other salicylhydrazone-supported complexes.

Visible light sensitized near-infrared luminescence of ytterbium via ILCT states in quadruple-stranded helicates

Quadruple-stranded helicates show visible light sensitized near-infrared luminescence of ytterbium via ILCT states.

Near-infrared circularly polarized luminescence from chiral Yb(iii)-diketonates

We report a rare example of near infrared circularly polarized luminescence (NIR-CPL) in the 920–1050 nm region associated with the f–f transitions of Yb diketonate complexes bearing chiral ancillary ligands.

Chiral BINAPO-Controlled Diastereoselective Self-Assembly and Circularly Polarized Luminescence in Triple-Stranded Europium(III) Podates

Chiral lanthanide helical architectures have received intense attentions in recent years because of their potential applications as chiral probes and sensors and as circularly polarized luminescence (CPL) materials. However, stereoselectivity control in the self-assembly of lanthanide helicate is challenging due to the poor stereochemical preference and variable coordination numbers of Ln(III) ions. Herein, we reported the employing chiral ancillary ligand R/S-BINAPO to induce achiral tripodal ligand to form a pair of homochiral lanthanide triple-helical podates [Eu(TTEA)((R/S)-BINAPO); R/S-1] {(R/S)-BINAPO = ( R/ S)-2,2'-bis(diphenylphosphoryl)-1,1'-binaphthyl; TTEA = tris[(4-(4,4,4-trifluoro-1,3-dioxobutyl)-benzamido)ethyl]amine}. X-ray crystallographic analysis for rac-1 reveals that the chirality of BINAPO is transferred during the self-assembly process to give either P or M helical architectures in podates. The ¹H and ³¹P NMR and circular dichroism measurements confirm the diastereopurity of the assemblies in solution. A detailed optical and chiroptical characterization reveals that the luminescent enantiopure podates not only exhibit intense CPL with | g_lum| values reaching 0.072 but also show high luminescence quantum yields of 32.8%. Our results provide a feasible strategy for designing homochiral helical lanthanide supramolecular architecture and synthesizing excellent CPL materials.

Near-Infrared Photoluminescence and Reversible Trans-to-Cis Photoisomerization of Mononuclear and Binuclear Ytterbium(III) Complexes Functionalized by Azobenzene Groups

Two mononuclear and one binuclear ytterbium complexes with dual near-infrared (NIR) photoluminescence and reversible trans-to-cis photoisomerization functions were synthesized and characterized. The central ytterbium(III) ion coordinates with two β-diketonate (4,4,4-trifluoro-1-phenylbutane-1,3-dionate (tfd)) ligands and one deprotonated azobenzene-containing tetradentate ligand [(E)-4-(phenyldiazenyl)-N,N-bis(pyridin-2-ylmethyl) benzohydrazide (HL), (E)-4-((4-(dimethylamino)phenyl)diazenyl)-N,N-bis(pyridin-2-ylmethyl)benzohydrazide (HNL), or (E)-4,4'-N',N'-bis(pyridin-2-ylmethyl)benzohydrazide azobenzene (H2DL)] to form a neutral ternary complex ([Yb(tfd)2L], [Yb(tfd)2(NL)], or [Yb2(tfd)4(DL)], respectively), where the ytterbium(III) ion is eight-coordinated to N3O5 donor sets. X-ray crystallographic analysis shows that all three complexes form a trigonal dodecahedron geometry with similar -N=N- distances that are slightly longer than those of the pure azobenzene-containing ligands. The NIR luminescence properties of the Yb(III) complexes were determined at a wavelength of about 980 nm with quantum yields in the range of 0.4-0.6% in ethanol and acetonitrile solutions at room temperature, and trans-to-cis photoisomerization was determined with the quantum yields (Φt→c = 10-2) at the same level as their pure ligands. The trans-to-cis photoisomerization rates of the complexes (10-4 s-1) are slightly higher than those of the pure ligands and similar to azobenzene (10-5 to 10-4 s-1). From time-dependent density functional theory calculations of the energy levels of the first excited triplet states of the ligands, the energies of the lowest excited triplet states of all of the ligands are higher than the resonance level of Yb3+ (2F5/2, 1.2722 eV). We suggest that these azo-containing ligands may participate in energy transfer to the ytterbium ion, in addition to the main "antenna effect" ligand tfd. This is the first report of azobenzene group-functionalized ytterbium complexes with dual NIR luminescence and photoisomerization properties, indicating that azobenzene-containing lanthanide(III) complexes have potential applications as dual function materials in biological systems.

Insight into the roles of structures and energy levels of mono- and bis-β-diketones on sensitizing Nd(iii) NIR-luminescence

Three neodymium complexes Nd(TTA)3(DMSO)2 (1, TTA = 2-thenoyltrifluoroacetone), Nd2(BDT)3(DMSO)6 (2, BDT = bis(4,4,4-trifluoro-1,3-dioxobutyl)thiophene) and Nd2(BTT)3(DMSO)4 (3, BTT = bis(4,4,4-trifluoro-1,3-dioxobutyl)(2,2'-bithiophene)) constructed from three thiophene-based β-diketonate ligands, were prepared for the purpose of building the relationships between the structures, energy levels of the complexes and NIR luminescence properties of Nd(iii) ions. X-ray crystallographical analysis reveals that complex 1 is a mononuclear structure, the central Nd(iii) ion is coordinated by eight oxygen atoms from three mono-β-diketones (TTA) and two DMSO, whereas, complexes 2 and 3 adopt triple-stranded dinuclear structures, in which the two Nd(iii) ions are wrapped by three bis-β-diketones, the central Nd(iii) ions are nine and eight coordinated by oxygen atoms from ligands and the coordinated DMSO molecules. The photophysical properties related to the electronic transition are characterized by the absorbance spectra, the excitation spectra, the phosphorescence spectra, the emission spectra, the emission quantum yields, and the emission lifetimes. The luminescence quantum yields experiment reveals that the dinuclear complexes (0.49% and 0.33% for 2 and 3) show higher luminescence efficiencies compared to the mononuclear complex 1 (0.22%). This enhancement is mainly attributed to their binuclear structures, which effectively represses the nonradiative transition caused by high-energy oscillators in ligands and/or solvents. On the other hand, the energy level matching also plays an important role in this enhancement.

Near-infrared luminescence of Nd3+ and Yb3+ complexes using a polyfluorinated pyrene-based β-diketonate ligand

Highly efficient near-infrared emitting Nd³⁺ and Yb³⁺ complexes have been developed based on a new polyfluorinated pyrene-appended β-diketonate ligand.