Ga3+/Ln3+ Metallacrowns: A Promising Family of Highly Luminescent Lanthanide Complexes That Covers Visible and Near-Infrared Domains

Tuning white light emission using single-component tetrachroic Dy3+ metallacrowns: the role of chromophoric building blocks

White light production is of major importance for ambient lighting and technological displays. White light can be obtained by several types of materials and their combinations, but single component emitters remain rare and desirable towards thinner devices that are, therefore, easier to control and that require fewer manufacturing steps. We have designed a series of dysprosium(iii)-based luminescent metallacrowns (MCs) to achieve this goal. The synthesized MCs possess three main structural types LnGa4(L')4(L'')4 (type A), Ln2Ga8(L')8(L''')4 (type B) and LnGa8(L')8(OH)4 (type C) (H3L', HL'' and H2L''' derivatives of salicylhydroxamic, benzoic and isophthalic acids, respectively). The advantage of these MCs is that, within each structural type, the nature of the organic building blocks does not affect the symmetry around Dy3+. By detailed studies of the photophysical properties of these Dy3+-based MCs, we have demonstrated that CIE coordinates can be tuned from warm to neutral to cold white by (i) defining the symmetry about Dy3+, and (ii) choosing appropriate chromophoric building blocks. These organic building blocks, without altering the coordination geometry around Dy3+, influence the total emission profile through changing the probability of different energy transfer processes including the 3T1 ← Dy3+* energy back transfer and/or by generating ligand-centered fluorescence in the blue range. This work opens new perspectives for the creation of white light emitting devices using single component tetrachroic molecular compounds.

Ytterbium 10-carboxyperylene-3,4,9-tricarboxylates for targeted NIR luminescent bioimaging

Two new ytterbium coordination compounds Yb(HPTC)(H2O)2 (Yb1) and Yb(HPTC)(Phen) (Yb2) were obtained using 10-carboxyperylene-3,4,9-tricarboxylate ion (HPTC3-) as a sensitizer. Both coordination compounds exhibited intense NIR-II luminescence upon excitation in the visible range and formed stable suspensions with nanoparticles of 50-70 nm in size in an aqueous solution of sodium alginate. Both complexes demonstrated non-toxicity up to at least 25 mg L-1 in two cell cultures: cancer cells MCF7 and embryonic cells HEK293T - making them suitable for bioimaging. For both complexes, the accumulation in cells was directly measured and it was shown that the accumulation of Yb2 was the same for both cell types (0.51-0.52 πg per cell), while Yb1 demonstrated selective accumulation in cancer cells (0.04 πg per cell for HEK293T and 7.00 πg per cell for MCF7). Thus, Yb1 can also be proposed as a selective vis-excited NIR emitting bioprobe.

3–(2–Pyridyl)pyrazole Based Luminescent 1D-Coordination Polymers and Polymorphic Complexes of Various Lanthanide Chlorides Including Orange-Emitting Cerium(III)

A series of 18 lanthanide-containing 1D-coordination polymers 1∞[Ln2(2–PyPzH)4Cl6], Ln = La, Nd, Sm, dinuclear polymorphic complexes α–, β–[Ln2(2–PyPzH)4Cl6], Ln = Sm, Eu, Gd, α–[Tb2(2–PyPzH)4Cl6], and [Gd2(2–PyPzH)3(2–PyPz)Cl5], mononuclear complexes [Ce(2–PyPzH)3Cl3], [Ln(2–PyPzH)2Cl3], Ln = Tb, Dy, Ho, and Er, and salt-like complexes [Gd3(2–PyPzH)8Cl8]Cl and [PyH][Tb(2–PyPzH)2Cl4] were obtained from the reaction of the respective lanthanide chloride with the 3–(2–pyridyl)pyrazole (2–PyPzH) ligand at different temperatures. An antenna effect through ligand-to-metal energy transfer was observed for several products, leading to the highest luminescence efficiency displayed by a quantum yield of 92% in [Tb(2–PyPzH)2Cl3]. The Ce3+ ion in the complex [Ce(2–PyPzH)3Cl3] exhibits a bright and orange 5d-based broadband emission with a maximum at around 600 nm, marking an example of a strong reduction of the 5d-excited states of Ce(III). The absorption spectroscopy shows ion-specific 4f–4f transitions, which can be assigned to Nd3+, Sm3+, Eu3+, Dy3+, Ho3+, and Er3+ in a wide spectral range from UV–VIS to the NIR region.

3D-Frameworks and 2D-networks of lanthanide coordination polymers with 3-pyridylpyrazole: photophysical and magnetic properties

A series of 15 lanthanide-containing coordination polymers, both 3D- and 2D-networks, as well as complexes of Ln-trichlorides with 3-(3-pyridyl)pyrazole (3-PyPzH), were synthesized. A large structural diversity is observed depending on the ligand content: 3∞[Ln(3-PyPzH)Cl₃], Ln = Eu and Gd, of sra topology, 2∞[Sm(3-PyPzH)Cl₃], 2∞[Ln₂(3-PyPzH)₃Cl₆]·2solv, Ln = Eu³⁺, Tb³⁺, Dy³⁺, Ho³⁺ and Er³⁺, solv = Tol and MeCN, of sql topology and 2∞[Ln(3-PyPzH₂)Cl₄], Ln = La and Nd, of hcb topology with salt like complexes of the formula [(3-PyPzH₂)][Ln(3-PyPzH)₂Cl₄], Ln = Eu, Tb, Dy and Ho. The products were characterized by single-crystal and powder X-ray diffraction, high-temperature X-ray diffraction, differential thermal analysis and thermogravimetry (DTA/TG) combined with mass spectrometry, differential scanning calorimetry (DSC), IR-spectroscopy, UV-visible spectrophotometry, photoluminescence spectroscopy, and magnetic susceptibility. Absorption spectroscopy shows ion-specific 4f-4f transitions that can be assigned to Sm³⁺, Eu³⁺, Dy³⁺, Ho³⁺ and Er³⁺ in a wide range from the UV-VIS to NIR region. An excellent antenna effect through ligand-metal energy transfer was observed in 2∞[Tb₂(3-PyPzH)₃Cl₆]·2solv, leading to high efficiency of the luminescence indicated by a quantum yield up to 76%. Direct current magnetic susceptibility studies reveal the absence of interatomic interaction for Dy³⁺ and Er³⁺ and weak ferromagnetic interaction for Ho³⁺. Thermal analysis shows good stability up to 365 °C for 2∞[Ho₂(3-PyPzH)₃Cl₆]·2MeCN.

Crystal structure of an indium-salicyl-hydroximate complex cation: [In4(H2shi)8(H2O)6](NO3)4·8.57H2O

The synthesis and crystal structure for the title compound, hexa-aqua-hexa-kis(μ-2-hy-droxy-benzene-carbo-hydrox-a-mato)bis-(2-hy-droxy-benzene-carbo-hydrox-a-m-ato)tetra-indium(III) tetra-nitrate 8.57-hydrate + unknown solvent, [In4(H2shi)8(H2O)6](NO3)4·8.57H2O·solvent, where H2shi- is salicylhydrox-imate (C7H5NO3), are reported. The complex cation of the structure, [In4(H2shi)8(H2O)6]4+, is a dimer with a step-like topology and possesses an inversion center that relates each [In2(H2shi)4(H2O)3]2+ side of the complex cation. Each InIII ion is seven-coordinate with a penta-gonal-bipyramidal geometry, and the salicyl-hydroximate ligands have a 1- charge as only the oxime oxygen of the ligand is deprotonated. Four inter-stitial nitrate anions maintain the charge balance of the compound. One of the nitrate anions (and its symmetry equivalent) is disordered over two different orientations with an occupancy ratio of 0.557 (7) to 0.443 (7). The inter-stitial solvent water mol-ecules show substantial disorder. Approximately 8.57 water mol-ecules per formula unit were refined as disordered and partially occupied, while a suitable model could not be devised for the other extensively disordered solvent mol-ecules (water and possibly methanol as this was the synthesis solvent). Thus, these latter solvent mol-ecules were instead treated with the SQUEEZE routine [Spek (2015). Acta Cryst. C71, 9-18.] as implemented in the program PLATON, and the procedure corrected for 151 electrons within solvent-accessible voids of 367 Å3.

Terbium (III)-based Metallacrowns with aggregation-induced emission feature coupled with cu (II) for fluorescence detection of cysteine

The Metallacrowns (MCs) composed of repeated [Metal-N-O] subunits are a type of new material, but the MCs have not been developed and utilized in analytical applications. This essay reports on a new kind of terbium(III)-based Metallacrowns (Tb-MCs) with aggregation-induced emission (AIE) feature to build a sensing platform. It is first time that Tb-MCs are able to aggregate to larger aggregates in water along with a bright green emission, so that the property makes it possible to apply in biosensing. Thereafter, the AIE of Tb-MCs can be quenched effectively by Cu²⁺. Based on the high affinity of thiol to Cu²⁺, cysteine (Cys) recovers the fluorescence of Tb-MCs in the presence of Cu²⁺. There is a good linear range varying from 0.02 to 20 μM with a low limit of detection (LOD) 9.67 nM of Cys. In the end, this novel probe is also successfully applied to the determination of Cys in human serum with satisfactory results.

Luminescent Metal Complexes for Bioassays in the Near-Infrared (NIR) Region

Near-infrared (NIR, 700-1700 nm) luminescent imaging is an emerging bioimaging technology with low photon scattering, minimal autofluorescence, deep tissue penetration, and high spatiotemporal resolution that has shown fascinating promise for NIR imaging-guided theranostics. In recent progress, NIR luminescent metal complexes have attracted substantially increased research attention owing to their intrinsic merits, including small size, anti-photobleaching, long lifetime, and metal-centered NIR emission. In the past decade, scientists have contributed to the advancement of NIR metal complexes involving efforts to improve photophysical properties, biocompatibility, specificity, pharmacokinetics, in vivo visualization, and attempts to exploit new ligand platforms. Herein, we summarize recent progress and provide future perspectives for NIR metal complexes, including d-block transition metals and f-block lanthanides (Ln) as NIR optical molecular probes for bioassays.

Identification of seven-coordinate LnIII ions in a LnIII[15-MCFe III N(shi)-5](OAc)2Cl species crystallized from methanol and pyridine

The title metallacrown (MC) complexes LnIII[15-MCFeIIIN(shi)-5](OAc)2CI(C5H5N)6 (Ln1), where OAc- is acetate, shi3- is salicylhydroximate, and Ln = Gd and Dy, were synthesized via a self-assembly reaction in methanol and pyridine. Single crystals were grown using slow evaporation and characterized using X-ray diffraction. Seven-coordinate capped octahedron geometries were observed for the lanthanide ion in both complexes, which is uncommon for trivalent lanthanide species. The 15-MC-5 is a ruffled metallacrown archetype similar to previously reported mixed-valent manganese metallacrowns.

Sensitization of Nd3+ Luminescence by Simultaneous Two-Photon Excitation through a Coordinating Polymethinic Antenna

We have designed and synthesized two new cyaninic Nd³⁺ complexes where the lanthanide emission can be induced from simultaneous two-photon absorption followed by energy migration. These complexes correspond to a molecular design that uses an antenna ligand formed by the functionalization of a heptamethine dye with 5-ol-phenanthroline or 4-phenyl-terpyridine derivatives. These complexes employ the important nonlinear optical properties of symmetric polymethines to sensitize the lanthanide ion. We verified that simultaneous biphotonic excitation indirectly induces the ⁴F_3/2 → ⁴I_11/2 Nd³⁺ emission using femtosecond laser pulses tuned below the first electronic transition of the antenna. The simultaneous two-photon excitation events initially form the nonlinear-active second excited singlet of the polymethine antenna, which rapidly evolves into its first excited singlet. This state in turn induces the formation of the emissive Nd³⁺ states through energy transfer. The role of the first excited singlet of the antenna as the donor state in this process was verified through time resolution of the antenna's fluorescence. These measurements also provided the rates for antenna-lanthanide energy transfer, which indicate that the phenanthroline-type ligand is approximately five times more efficient for energy transfer than the phenyl-terpyridine derivative due to their relative donor-acceptor distances. The simultaneous two-photon excitation of this polymethine antenna allows for high spatial localization of the Nd³⁺excitation events.

Visible and near-infrared emitting heterotrimetallic lanthanide-aluminum-sodium 12-metallacrown-4 compounds: discrete monomers and dimers

The luminescence properties of two types of heterotrimetallic aluminum-lanthanide-sodium 12-metallacrown-4 compounds are presented here, LnNa(ben)₄[12-MC_{Al(III)N(shi)}-4] (LnAl4Na) and {LnNa[12-MC_{Al(III)N(shi)}-4]}₂(iph)₄ (Ln2Al8Na2), where Ln = Gd^III, Tb^III, Er^III, and Yb^III, MC is metallacrown, ben^- is benzoate, shi^3- is salicylhydroximate, and iph^2- is isophthalate. The aluminum-lanthanide-sodium metallacrowns formed with benzoate are discrete monomers while, upon replacement of the benzoate with the dicarboxylate isophthalate, two individual metallacrowns can be joined to form a dimer. In the solid state, the terbium version of each structure type displays emission in the visible region, and the erbium and ytterbium complexes emit in the near-infrared. The luminescence lifetimes (τ_obs) and quantum yields have been collected under ligand excitation (QLLn) for both LnAl4Na monomers and Ln2Al8Na2 dimers. Several of these values tend to be shorter (luminescence lifetimes) and smaller (quantum yields) than the corresponding values recorded for the structurally similar gallium-lanthanide monomer and dimer 12-MC-4 molecules. However, the quantum yield value recorded for the visible emitting Tb2Al8Na2 dimer, 43.9%, is the highest value observed in the solid state to date for a Tb^III based metallacrown.

NIR luminescence thermometers based on Yb-Nd coordination compounds for the 83-393 K temperature range

Multimetallic neodymium-ytterbium-gadolinium compounds with 9-anthracenate and 9-acridinate anions were tested in order to create the first luminescent thermometer for elevated temperatures. High luminescence intensity and high signal resolution were reached thanks to the concentration quenching elimination due to the partial substitution of the emitting ions with Gd³⁺. As a result, NIR emitting materials for luminescence thermometry in the wide temperature range (83-393 K) based on lanthanide coordination compounds (CCs) were obtained. The best thermometric properties among the studied systems were demonstrated by Yb_0.02Nd_0.12Gd_0.86(ant)₃, and its temperature sensitivity reached 1.8% K^-1 in the temperature range of 293-393 K.

Near-Infrared Lanthanide-Based Emission from Fused Bis[Ln(III)/Zn(II) 14-metallacrown-5] Coordination Compounds

A set of (Ln[14-MC_{Zn(II)N(quinHA)}-5])₂Ln₂Zn₂(quinHA)₂(ph)₂(Hph)₄(OH)₈(H₂O)₄ metallacrowns (Ln-1, Ln = Tb, Gd, or Yb; H₂quinHA = quinaldic hydroxamic acid, H₂ph = phthalic acid) have been synthesized via solution-state self-assembly. The metallacrowns possess an uncommon topology within the metallacrown family where two rarely seen 14-metallacrown-5 moieties are fused by a Yb₂Zn₂(quinHA)₂ bridge. Moreover, Yb-1 analyzed in the solid state exhibits a characteristic near-infrared luminescence signal arising from Yb^{3+ 2}F_5/2→²F_7/2 transition despite the proximity of high energy O-H oscillators.

Gadolinium(III) Porphyrinoid Phototheranostics

Molecular phototheranostics as the emerging field of modern precision medicine recently has attracts increasing research attentions owing to non-invasiveness, high precision, and controllable nature of light. In this work, we reported alluring gadolinium (Gd3+) porphyrinoids phototheranostic agents for magnetic resonance imaging (MRI) and photodynamic therapy (PDT). The synthesized Gd-1-4-Glu featured with meso-glycosylation and β-lactonization to endow good biocompatibility and improved photophysical properties. In particular, β-lactonization of glycosylated Gd3+ porphyrinoids substantially red-shifted its absorption band to near-infrared (NIR) region and boosted generation of reactive oxygen species including 1O2, and some radical species that engaged both type II and type I PDT pathways. In addition, the number and regioisomerism of β-oxazolone moieties was observed to play an essential role in improving longitude relaxivity (r1) of Gd-1-4-Glu up to 4.6 mM-1s-1 for the first time by affecting environmental water exchange. Taking Gd-4-Glu as a promising complex, we further achieved real-time T1-weighted MRI and PDT on HeLa tumour mice in vivo, revealing the appealing potential of Gd3+ porphyrinoids in phototheranostics.

Tuning the photophysical properties of lanthanide(iii)/zinc(ii) 'encapsulated sandwich' metallacrowns emitting in the near-infrared range

A family of Zn16Ln(HA)16 metallacrowns (MCs; Ln = YbIII, ErIII, and NdIII; HA = picoline- (picHA2-), pyrazine- (pyzHA2-), and quinaldine- (quinHA2-) hydroximates) with an 'encapsulated sandwich' structure possesses outstanding luminescence properties in the near-infrared (NIR) and suitability for cell imaging. Here, to decipher which parameters affect their functional and photophysical properties and how the nature of the hydroximate ligands can allow their fine tuning, we have completed this Zn16Ln(HA)16 family by synthesizing MCs with two new ligands, naphthyridine- (napHA2-) and quinoxaline- (quinoHA2-) hydroximates. Zn16Ln(napHA)16 and Zn16Ln(quinoHA)16 exhibit absorption bands extended into the visible range and efficiently sensitize the NIR emissions of YbIII, ErIII, and NdIII upon excitation up to 630 nm. The energies of the lowest singlet (S1), triplet (T1) and intra-ligand charge transfer (ILCT) states have been determined. LnIII-centered total (Q L Ln) and intrinsic (Q Ln Ln) quantum yields, sensitization efficiencies (η sens), observed (τ obs) and radiative (τ rad) luminescence lifetimes have been recorded and analyzed in the solid state and in CH3OH and CD3OD solutions for all Zn16Ln(HA)16. We found that, within the Zn16Ln(HA)16 family, τ rad values are not constant for a particular LnIII. The close in energy positions of T1 and ILCT states in Zn16Ln(picHA)16 and Zn16Ln(quinHA)16 are preferred for the sensitization of LnIII NIR emission and η sens values reach 100% for NdIII. Finally, the highest values of Q L Ln are observed for Zn16Ln(quinHA)16 in the solid state or in CD3OD solutions. With these data at hand, we are now capable of creating MCs with desired properties suitable for NIR optical imaging.

Highly symmetric Ln(iii) boron-containing macrocycles as bright fluorophores for living cell imaging

Boron-assisted highly symmetric rigid Ln macrocycles were designed and synthesized, showing high brightness and promising potential applications in bioimaging.

Complexation Behavior of Pinene–Bipyridine Ligands towards Lanthanides: The Influence of the Carboxylic Arm

The complexation behavior of two novel, chiral pinene–bipyridine-type ligands ((–)-HL1 and (–)-HL2) containing a carboxylic arm towards lanthanide Ln(III) (Ln = La, Eu, Lu) ions was investigated through spectroscopic methods. The association constants of the mononuclear complexes determined from the UV-Vis titrations indicated that the ligand (–)-HL1 possessing a shorter carboxylic arm formed more stable complexes compared with (–)-HL2, whose carboxylic arm had one more methylene unit. This is due to the formation of more stable seven-member metal chelate rings in the first case as compared with the eight-member metal chelate rings in the second. IR and fluorescence spectroscopy provided additional information about the structure of these complexes.

Reduced quenching effect of pyridine ligands in highly luminescent Ln(III) complexes: the role of tertiary amide linkers

Luminescent Eu(III) and Tb(III) complexes were synthesised from octadentate ligands carrying various carbostyril sensitizing antennae and two bidentate picolinate donors. Antennae were connected to the metal binding site via tertiary amide linkers. Antennae and donors were assembled on a 1,4,7-triazacyclononane (tacn) platform. Solution- and solid-state structures were comparable to those of previously reported complexes with tacn architectures, with nine-coordinate distorted tricapped trigonal prismatic Ln(III) centres, and distinct from those based on 1,4,7,10-tetraazacyclododecane (cyclen) macrocycles. In contrast, the photophysical properties of these tertiary amide tacn-based complexes were more comparable to those of previously reported systems with cyclen ligands, showing efficient Eu(III) and Tb(III) luminescence. This represents an improvement over secondary amide-linked analogues, and is due to a greatly increased sensitization efficiency in the tertiary amide-linked complexes. Tertiary amide-linked Eu(III) and Tb(III) emitters were more photostable than their secondary amide-linked analogues due to the suppression of photoinduced electron transfer and back energy transfer.

A hybrid erbium(III)-bacteriochlorin near-infrared probe for multiplexed biomedical imaging

Spectrally distinct fluorophores are desired for multiplexed bioimaging. In particular, monitoring biological processes in living mammals needs fluorophores that operate in the 'tissue-transparent' near-infrared (NIR) window, that is, between 700 and 1,700 nm. Here we report a fluorophore system based on molecular erbium(III)-bacteriochlorin complexes with large Stokes shift (>750 nm) and narrowband NIR-to-NIR downconversion spectra (full-width at half-maximum ≤ 32 nm). We have found that the fast (2 × 109 s-¹) and near-unity energy transfer from bacteriochlorin triplets to the erbium(III) 4I13/2 level overcomes the notorious vibrational overtones quenching, resulting in bright and long-lived (1.73 μs) 1,530 nm luminescence in water. We demonstrate the excitation/emission-multiplexed capability of the complexes in the visualization of dynamic circulatory and metabolic processes in living mice, and through skull tracking of cancer cell metastases in mouse brain. This hybrid probe system facilitates robust multiplexed NIR imaging with high contrast and spatial resolution for applications ranging from fluorescence-guided surgery, diagnostics and intravital microscopy.

Radical-Doped Crystalline Lanthanide-Based Photochromic Complexes: Self-Assembly Driven by Multiple Interactions and Photoswitchable Luminescence

Stimuli-responsive functional materials, especially the light stimulation color change and tunable fluorescent materials, have received considerable attention because of their broad applications in smart materials. Herein, a series of lanthanide-based [Ln = Nd(III) (1), Sm(III) (2), Eu(III) (3), Gd(III) (4), Tb(III) (5), Yb(III) (6), and Lu(III) (7)] crystalline complexes were attained by simply adding the aqueous lanthanide nitrate solution to the water-soluble naphthalenediimide derivative. The obtained lanthanide-based crystalline materials not only show significant photochromism but also possess reactive organic radicals under ambient conditions. Intriguingly, photoswitchable near-infrared (NIR) fluorescence was realized in the crystalline complex 1. The structures of these crystalline materials were systematically studied to clarify the weak interaction-assisted charge-transfer process. The underlying multiple-interaction-assisted supramolecular self-assembly, the radical-doped nature, and the corresponding photochromic mechanism were thoroughly unearthed by single-crystal X-ray diffraction, in situ solid-state UV-vis diffuse reflectance, and electron paramagnetic resonance spectrometric analysis.

3d/4f Sandwich Complex Based on Metallacrowns

A novel lanthanide double-decker complex with nickel metallacrowns (MCs) as coordinating ligands has been synthesized. In the 3d/4f metallacrown complex Tb^III[12-MC_Ni^IIN(shi)-4]₂, the central lanthanide ion is sandwiched between two [12-MC-4] units, forming an almost ideal square-antiprismatic coordination sphere. The resulting zenithal angles at the central lanthanide ion are smaller than those for previously reported sandwich compounds. Magnetic measurements reveal an energy barrier of 346 K under zero field and up to 585 K under 3200 Oe, the highest reported for metallacrowns with D_4d symmetry.

Stepwise Increase of NdIII -Based Phosphorescence by AIE-Active Sensitizer: Broadening the AIPE Family from Transition Metals to Discrete Near-Infrared Lanthanide Complexes*

We designed two near-infrared (NIR) lanthanide complexes [(L)₂ -Nd(NO₃ )₃ ] (L=TPE₂ -BPY for 1, TPE-BPY for 2) by employing aggregation-induced emission (AIE)-active tetraphenylethylene (TPE) derivatives as sensitizers, which possessed matched energy to Nd^III , prevented competitive deactivation under aggregation, even shifted the excitation window toward 600 nm by twisted intramolecular charge transfer. Furthermore, benefiting from the 4 f electron shielding effect and antenna effect, the enhanced excitation energies of the AIE-active sensitizers by structural rigidification transferred into the inert Nd^III excited state through ³ LMCT, affording the first aggregation-induced phosphorescence enhancement (AIPE)-active discrete NIR-emitting lanthanide complexes. As 1 equipped with more AIE-active TPE than 2, L→Nd energy transfer efficiency in the former was higher than that in the latter under the same conditions. Consequently, the crystal of 1 exhibited one of the longest lifetimes (9.69 μs) among Nd^III -based complexes containing C-H bonds.

Magnetic properties of two GdIIIFeIII4 metallacrowns and strategies for optimizing the magnetocaloric effect of this topology

Two Gd³⁺ [12-MC_Fe^III_(N)shi-4] metallacrowns are analyzed for magnetic properties, and calculations concerning the magnetic exchange parameters of this topology are considered.

Silicon-hybrid carbon dots derived from rice husk: promising fluorescent probes for trivalent rare earth element ions in aqueous media

As a novel fluorescence probe, Si–CDs could not only be applied to distinguish four groups of different rare earth element ions (REEs) but also exhibit a rapid and sensitive response towards individual Tb3+,Eu3+ and Dy3+.

Effect of the aromatic substituent on the para-position of pyridine-bis(oxazoline) sensitizers on the emission efficiency of their EuIII and TbIII complexes

Two efficient lanthanide ion sensitizers 2,6-bis(oxazoline)-4-phenyl-pyridine (PyboxPh, 1) and 2,6-bis(oxazoline)-4-thiophen-2-yl-pyridine (Pybox2Th, 2) were synthesized. 1 crystallizes in the monoclinic space group P21/c with cell parameters a = 16.3794(4) Å, b = 7.2856(2) Å, c = 11.7073(3) Å, β = 97.229(1)° and V = 1385.97(6) Å3. 2 crystallizes in the monoclinic space group P21/n with cell parameters a = 5.9472(2), b = 16.0747(6), c = 14.3716(5) Å, β = 93.503(1)° and V = 1371.35(8) Å3. Photophysical characterization of 1 shows that its triplet state energy is located at 22 250 cm-1 and efficient energy transfer is observed for EuIII and TbIII. Solutions of [Ln(PyboxPh)3]3+ in dichloromethane display an emission efficiency of 37.2% for Ln[double bond, length as m-dash]Eu and 24.0% for Ln[double bond, length as m-dash]Tb. The excited state lifetimes for EuIII and TbIII are 2.227 ms and 723 μs, respectively. The triplet state energy of 2 is located at 19 280 cm-1 and is therefore too low to efficiently sensitize TbIII emission. However, the sensitization of EuIII is effective, with an emission quantum yield of 14.5% and an excited state lifetime of 714 μs. This shows that the derivatization of the chelator is strongly influenced by the aromatic substituents on the para-position of the pyridine ring. New isostructural 1 : 1 complexes of PyboxPh with EuIII (3) and TbIII (4) were also isolated and crystallize in the triclinic space group P1[combining macron] with cell parameters a = 9.1845(2) Å, b = 10.3327(2) Å, c = 11.9654(2) Å, α = 98.419(1)°, β = 108.109(1)°, γ = 91.791(1)°, V = 1064.08(4) Å3 and a = 7.8052(1) Å, b = 11.8910(1) Å, c = 14.2668(2) Å, α = 72.557(1)°, β = 86.355(1)°, γ = 77.223(1)°, V = 1231.95(3) Å3, respectively.

[Ga3+ 8 Sm3+ 2 , Ga3+ 8 Tb3+ 2 ] Metallacrowns are Highly Promising Ratiometric Luminescent Molecular Nanothermometers Operating at Physiologically Relevant Temperatures

Nanothermometry is the study of temperature at the submicron scale with a broad range of potential applications, such as cellular studies or electronics. Molecular luminescent-based nanothermometers offer a non-contact means to record these temperatures with high spatial resolution and thermal sensitivity. A luminescent-based molecular thermometer comprised of visible-emitting Ga³⁺ /Tb³⁺ and Ga³⁺ /Sm³⁺ metallacrowns (MCs) achieved remarkable relative thermal sensitivity associated with very low temperature uncertainty of S_r =1.9 % K^-1 and δT<0.045 K, respectively, at 328 K, as an aqueous suspension of polystyrene nanobeads loaded with the corresponding MCs. To date, they are the ratiometric molecular nanothermometers offering the highest level of sensitivity in the physiologically relevant temperature range.

Crystal structures of three anionic lanthanide-aluminium [3.3.1] metallacryptate complexes

The three isomorphous [3.3.1] metallacryptate complexes bis-(pyridinium) di-aqua-dipyridine-hexa-kis-[μ3-salicyl-hydroximato(3-)]bis-[μ2-salicyl-hydroxim-ato(1-)]hexa-aluminiumgadolinium-pyridine-water (1/7.396/1), (C5H6N)2[GdAl6(C7H6NO3)2(C7H4NO3)7(C5H5N)1.855(H2O)2]·7.396C5H5N·H2O or [Hpy]2[GdAl6(H2shi)2(shi)7(py)1.855(H2O)2]·7.396py·H2O, 1, bis-(pyridinium) di-aqua-dipyridine-hexa-kis-[μ3-salicyl-hydroximato(3-)]bis-[μ2-salicyl-hydroxim-ato(1-)]hexa-aluminiumdysprosium-pyridine-water (1/7.429/1), (C5H6N)2[DyAl6(C7H6NO3)2(C7H4NO3)7(C5H5N)1.855(H2O)2]·7.429C5H5N·H2O or [Hpy]2[DyAl6(H2shi)2(shi)7(py)1.891(H2O)2]·7.429py·H2O, 2, and bis-(pyrid-in-ium) di-aqua-dipyridine-hexa-kis-[μ3-salicyl-hydroximato(3-)]bis-[μ2-salicyl-hydrox-imato(1-)]hexa-aluminiumytterbium-pyridine-water (1/7.386/1), (C5H6N)2[YbAl6(C7H6NO3)2(C7H4NO3)7(C5H5N)1.855(H2O)2]·7.429C5H5N·H2O or [Hpy]2[YbAl6(H2shi)2(shi)7(py)1.818(H2O)2]·7.386py·H2O, 3, where Hpy+ is pyridinium, shi3- is salicyl-hydroximate, and py is pyridine, consist of an aluminium-based metallacryptand that captures an Ln III ion in the central cavity. The metallacryptand portions are comprised of an Al-N-O repeat unit; thus, they can be considered three-dimensional metallacrowns. The encapsulated Ln III ions are nine-coordinate with a spherical capped-square-anti-prism geometry, while the six AlIII ions are all octa-hedral. Four of the AlIII ions are chiral centers with 2 Δ and 2 Λ stereoconfigurations. The remaining two AlIII ions have trans chelate rings from two different shi3- ligands. For 1-3, a section of the main mol-ecule is disordered induced by the presence or absence of a pyridine ligand coordinated to one of the AlIII ions. In the absence of the pyridine moiety, an H2shi- ligand moves into the space otherwise occupied by the pyridine and the phenol oxygen atom coordinates to the AlIII ion. The movement of the H2shi- ligand induces movement for the Ln III ion, for another AlIII ion that also binds the same H2shi- ligand, and for one of the shi3- ligands coordinated to the latter AlIII ion. For 1-3 the occupancy ratio of the metallacryptand portions refined to 0.8550 (13):0.1450 (13), to 0.8909 (13):0.1091 (13), and to 0.8181 (14):0.1819 (14), respectively.

Crystal structures of two dysprosium-aluminium-sodium [3.3.1] metallacryptates that form two-dimensional sheets

The two [3.3.1] metallacryptate complexes, namely, poly[[μ3-acetato-hexa-kis-(μ-N,N-di-methyl-formamide)-bis-(N,N-di-methyl-formamide)bis-[salicyl-hydroxi-mato(2-)]hepta-kis-[salicyl-hydrox-im-ato(3-)]hexa-aluminium(III)dysprosium(III)penta-sodium(I)] N,N-di-methyl-formamide tetra-solvate monohydrate], [DyAl6Na5(OAc)(Hshi)2(shi)7(DMF)8]·4DMF·H2O or {[DyAl6Na5(C7H5NO3)2(C7H4NO3)7(C2H3O2)(C3H7NO)8]·4C3H7NO·H2O} n , 1, and poly[[di-μ4-acetato-nona-kis-(μ-N,N-di-methyl-form-amide)-octa-kis-(N,N-di-methyl-formamide)tetra-kis-[sali-cyl-hydroximato(2-)]tetra-deca-kis-[salicyl-hydroximato(3-)]dodeca-aluminium(III)didysprosium(III)deca-sodium(I)] N,N-di-methyl-form-amide 6.335-solvate], [DyAl6Na5(OAc)(Hshi)2(shi)7(DMF)8.5]2·6.335DMF or {[Dy2Al12Na10(C7H5NO3)4(C7H4NO3)14(C2H3O2)2(C3H7NO)17]·6.335C3H7NO} n , 2, where shi3- is salicyl-hydroximate and DMF is N,N-di-methyl-formamide, both consist of an aluminium-based metallacryptand. In 1 and 2, the metallacryptand encapsulates a dysprosium(III) ion in the central cavity, and the resulting metallacryptates are connected to each other via sodium-DMF linkages to generate a two-dimensional sheet. The metallacryptates of 1 and 2 are the three-dimensional analogues of metallacrowns as the metallacryptates contain a metal-nitro-gen-oxygen cyclic repeat unit throughout the complexes. For 1 the building block of the two-dimensional sheet is comprised of only one type of metallacryptate, which is connected to four neighboring metallacryptates via four sodium-DMF linkages. In 2, the building block is a dimeric unit of two metallacryptates. Each dimeric metallacryptate unit is connected to four other dimeric units via six sodium-DMF linkages. The two metallacryptates of each dimeric unit can be considered enanti-omers of each other. In both 1 and 2, chirality is imparted to the metallacryptate due to the Λ and Δ propeller configurations of the four octa-hedral aluminium ions of the metallacryptand shell.

Aspects of lanthanide complexes for selectivity, intensity and sharpness in luminescence bands from twenty-four praseodymium, europium and gadolinium complexes with differently distorted-hexadentate ligands

We structurally and spectroscopically investigated a series of praseodymium (Pr) complexes with eight ligands that form helicate molecular structures. The mother ligand skeleton (L) has two bipyridine moieties bridged with ethylenediamine. The bridged skeleton of PrL was changed to diamines 1-methyl-ethylenediamine, trimethylenediamine and 2,2'-dimethyl-trimethylenediamine, and the corresponding ligands were designated as L^me, L^pr and L^dmpr, for each Pr in these complexes upon UV-excitation. The luminescence quantum yields of PrL and PrL^pr in the visible and near infrared (NIR) regions indicate that PrL is excited by both the electronic state of the ligand and the ff absorption band, whereas PrL^pr is excited through the ligand. The addition of a methyl group to PrL and PrL^pr has a different effect on the Pr emission intensity with the intensity of PrL^me decreasing more than that of PrL and PrL^dmpr and increasing more than that of PrL^pr. Thus, the coordination of Pr and the increased rigidity of the ligand upon methylation enhance luminescence. The azomethine moieties on L^me, L^pr and L^dmpr were reduced and formed the corresponding PrLH, PrL^meH, PrL^prH and PrL^dmprH complexes. The luminescence of the non-methylated series is due to transitions related to the ¹D₂ level and thus the methylated series luminesces due to high energy levels such as ³P_J arising from the shortened π electronic systems. We also discuss the strong red emission of a series of Eu complexes with eight ligands from the viewpoint of their molecular structures and luminescence efficiencies and evaluate the Judd-Ofelt parameters from the luminescence spectra of Eu complexes.

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.

Ship-in-a-Bottle Preparation of Long Wavelength Molecular Antennae in Lanthanide Metal-Organic Frameworks for Biological Imaging

While metal-organic frameworks (MOFs) have been identified as promising materials for sensitizing near-infrared emitting lanthanide ions (Ln³⁺) for biological imaging, long-wavelength excitation of such materials requires large, highly delocalized organic linkers or guest-chromophores. Incorporation of such species generally coincides with fewer Ln³⁺ emitters per unit volume. Herein, the excitation bands of ytterbium-based MOFs are extended to 800 nm via the postsynthetic coupling of acetylene units to form a high density of conjugated π-systems throughout MOF pores. The resulting long wavelength excitation/absorption bands are a synergistic property of the composite material as they are not observed in the individual organic components after disassociation of the MOFs, thus circumventing the need for large organic chromophores. We demonstrate that the long wavelength excitation and emission properties of these modified MOFs are maintained in the biological conditions of cell culture (aqueous environment, salts, heating), pointing toward their promising use for biological imaging applications.