Lanthanide Β-diketonate Complexes Research Articles

Turning 3D Covalent Organic Frameworks into Luminescent Ratiometric Temperature Sensors.

In this study, we report hybrid crystalline lanthanide-containing 3D covalent organic framework (Ln@3D COF) materials that are suitable for temperature sensing applications. Different routes to obtain these hybrid materials were tested and compared for material quality and thermometric properties. In the first approach, a bipyridine-containing 3D COF (Bipy COF) was grafted with a range of visible emitting lanthanide (Eu3+, Tb3+, Dy3+, and Eu3+/Tb3+) β-diketonate complexes. In the second approach, a novel nanocomposite material was prepared by embedding NaYF4:Er,Yb nanoparticles on the surface of a nonfunctionalized 3D COF (COF-300). To the best of our knowledge, the luminescent materials developed here are the first 3D COFs to be tested as ratiometric temperature sensors. In fact, for the Bipy COF, two different types of thermometers were tested (the Eu3+/Tb3+ system and a rare Dy3+ system), with both showing excellent temperature sensing properties. The reported NaYF4:Er,Yb/COF-300 nanocomposite material combines upconverting nanoparticles with 3D COFs, similar to previously reported metal organic framework (MOF) nanocomposite materials; however, this type of hybrid material has not yet been explored for COFs. As such, our findings open a new pathway toward potential multifunctional materials that can combine thermometry with other modalities, such as catalysis or drug delivery, in just one nanocomposite material.

Thermally induced structural transitions and temperature memory effect in a luminescent vitrified film of an anisometric europium(III) β-diketonate complex

• Vitrified Eu(DK12-14)3phen film (sandwiched between quartz plates) has been fabricated. • Performed weeklong monitoring revealed temperature memory effect. • Proposed model indicates thermally-induced transitions between two local states of film. • These two states are most likely close to crystalline and amorphous local structures. • Effect could be utilized in designing photonic materials equipped with memory function. We report on a temperature memory effect, which is highly unusual for known film-type materials based on lanthanide(III) β-diketonate complexes. It was observed in a 20 µm thick vitrified film fabricated by cooling the melt of a powder of an anisometric europium(III) β-diketonate complex between two quartz plates. The excitation of the film with light in the wavelength range of 280-425 nm causes a bright orange-red emission of Eu 3+ ions (due to efficient ligand-to-metal energy transfer). The memory effect shows itself as ability of the film to be in different states with their own luminescence decay time at the same temperature. In order to study the effect we carried out a weeklong experiment to monitor the evolution of the luminescence decay time in series of temperature treatments (alternating keeping the film at 370 K and storage in the dark at room temperature). We proposed a simple two-state model to simulate the results of the spectroscopic measurements. This made it possible to reveal the memory effect mechanism, which is thermally induced transitions between two different local states of the film. They are most likely in their nature close to crystalline and amorphous local structures. The obtained and analyzed data clearly indicate that the memory effect observed in the film opens broad perspectives in designing of new type luminescent photonic materials having memory function.

A reusable and self-recoverable vitrified film of an anisometric europium(III) β-diketonate complex with UV light-responsive Eu3+ emission

We report on photophysical properties of a 20 µm thick vitrified film (fabricated from a powder of an anisometric europium(III) β-diketonate complex through a melt-processing technique and sandwiched between two quartz plates), which are highly unusual for known film-type materials based on non-mesogenic lanthanide(III) β-diketonate complexes. The film excited by light in the wavelength range of 280–425 nm exhibits a bright orange-red narrow-band emission of Eu3+ ions (due to efficient ligand-to-metal energy transfer) with 530 µs decay time. The prolonged irradiation of the film with a 337 nm nitrogen laser decreases the luminescence brightness of the Eu3+ ions without decomposition of the anisometric complexes. Then the luminescence gradually recovers during storage of the film in the dark for at least 19 h at room temperature. Interestingly, the film is capable of repeatedly reversible changing the luminescence brightness upon alternating UV laser irradiation and dark storage. The observed self-recovery phenomenon can be presented as an elastic photodeformation of local structures of the film with following changes in the Eu3+ ions emission parameters. We believe that the found film properties are promising for the design and development of novel reusable luminescent sensors or dosimeters of UV radiation.

Elastic changes in luminescence intensity of vitrified film of europium(III) β-diketonate complex

We have observed an elastic change in luminescent properties of a 20 µm thick vitrified film (prepared from an amorphous powder of an anisometric europium(III) β-diketonate complex through a melt-processing technique and sandwiched between two quartz plates) caused by the prolonged UV irradiation. The film under UV light irradiation exhibits a typical Eu3+ ion emission peaked at 611 nm. The irradiation of the film by a 337 nm pulsed nitrogen laser with 0.2 mW average output power for 80 min leads to 1.5-fold decrease in the luminescence intensity at 611 nm. Despite the fact that in the case of traditional film-type materials based on lanthanide(III) β-diketonate complexes such effect is attributed to irreversible ligand photofragmentation, we have observed no photodecomposition of the anisometric complexes in the vitrified film. Moreover, we detected full reversibility of the luminescence intensity by keeping the film in the dark for 21 h. It is worth noting that this feature can be used for the design of new reusable luminescent dosimeters for UV light.

New insights into UV laser irradiation effect on luminescent behavior of vitrified films based on mesogenic lanthanide(III) β-diketonate complexes

We revealed the effect of the prolonged UV laser irradiation on luminescent behavior of photonic materials based on mesogenic lanthanide(III) β-diketonate complexes by using a novel 3 μm vitrified film of an europium(III) β-diketonate complex. We used the Eu3+ ions to monitor the UV laser-induced photophysical processes in the film. Despite the fact that usually prolonged UV irradiation leads to irreversible photofragmentation of ligands, we had observed no photodecomposition of the complexes after the irradiation of the film with the 337 nm laser for 5 h. The spectroscopic study revealed (i) a 16.5-fold luminescence intensity decrease due to laser-induced changes in inner coordination spheres of the Eu3+ ions and (ii) the main channel of non-radiative energy losses from the Eu3+ ions through the multiphonon relaxation. These features open up new possibilities for designing advanced photonics materials with excellent photostability and luminescent properties controlled during operation.

Novel tetrakis lanthanide β-diketonate complexes: Structural study, luminescence properties and temperature sensing

In this study, we report the synthesis and some new crystal structures of tetrakis lanthanide β-diketonate complexes throughout the lanthanide series for homonuclear (Pr3+, Nd3+, Sm3+, Eu3+, Tb3+, Dy3+, Er3+ and Yb3+) as well as for heteronuclear (Eu3+-Tb3+ and Tb3+-Sm3+) complexes. The well-known 1,1,1-trifluoro-2,4-pentadione (Htfac) ligand has been used in the synthesis. Here, we show that when employing the same synthesis conditions, we can obtain complexes with different coordination environments of the lanthanide ions. This is strongly linked to the ionic radius of the lanthanide ion. The luminescence properties of the visible emitting complexes (Ln3+ = Eu3+, Tb3+, Sm3+ and Dy3+) were investigated in solution as well as solid state. The near-infrared emitting complexes (Ln3+ = Pr3+, Nd3+, Er3+ and Yb3+) were recorded in the solid state. The heteronuclear complexes ([Eu1-xTbx(tfac)8]2-Na2+ (x: 0.59 (1) and 0.47 (2), Tb1-ySmy(tfac)8]2-Na2+ (y: 0.1 (3) and 0.2 (4)) and homonuclear complex [Dy(tfac)4)]−Na+ (DyL4) exhibited temperature-dependent luminescence properties in the physiological range, with complex DyL4 showing the highest relative sensitivity Sr = 3.45% K−1 (280 K). Complex 2 also showed a high Sr = 2.70% K−1 (353 K), which makes them promising for application as physiological luminescence thermometers. To the best of our knowledge until now no lanthanide β-diketonate complexes have been reported for use as good luminescence thermometers operating in the physiological range.

Triboluminescence of Centrosymmetric Lanthanide β-Diketonate Complexes with Aggregation-Induced Emission.

Triboluminescence (TL) is a form of light emission induced upon mechanical forces on the material. However, our understanding of this phenomenon is still unclear and more examples are therefore needed in order to elucidate its mechanism. In this work, two types of TL complexes, [Eu(pp-dbm-Cl2)3phen] and [Eu(mm-dbm-Cl2)3phen], which also displays aggregation-induced emission (AIE) were synthesized and investigated for its photo-physical and crystal structural properties. These complexes were crystallized in a centro-symmetric space group P21/n, and remarkably, displayed TL upon grinding that may be due to the presence of extensive π···π, C-H···π and C-H···Cl-C interactions in the close molecular packing of its structure. This rare example deviates from the widely accepted mechanism of TL, hence widening the scope of our understanding in the area.

A New Strategy to Design Photostable Luminescent Materials Based on Lanthanide(III) β-Diketonate Complexes for Advanced Photonic Applications

An analysis of the prospects for creating new luminescent molecular photonics materials based on mesogenic lanthanide(III) β-diketonate complexes is presented. We consider vitrified films that combine intense monochromatic luminescence, high optical quality, and complete resistance to UV radiation with the ability to change their photophysical properties (for example, the absorption band width and luminescence intensity) depending on the local structure and under external actions, such as UV radiation and temperature. The problems of control of the photophysical properties of these films and the possibility of their application as working media of high-tech luminescent materials and devices (for example, luminescent sensors of temperature, oxygen, and UV radiation, as well as light-conversion materials) are discussed.

Стратегия создания фотостабильных люминесцентных материалов молекулярной фотоники на основе beta-дикетонатных комплексов лантаноидов (III)-=SUP=-*-=/SUP=-

AbstractAn analysis of the prospects for creating new luminescent molecular photonics materials based on mesogenic lanthanide(III) β-diketonate complexes is presented. We consider vitrified films that combine intense monochromatic luminescence, high optical quality, and complete resistance to UV radiation with the ability to change their photophysical properties (for example, the absorption band width and luminescence intensity) depending on the local structure and under external actions, such as UV radiation and temperature. The problems of control of the photophysical properties of these films and the possibility of their application as working media of high-tech luminescent materials and devices (for example, luminescent sensors of temperature, oxygen, and UV radiation, as well as light-conversion materials) are discussed.

Synthesis, structures and spectroscopy of three new lanthanide β-diketonate complexes with 4,4′-dimethyl-2,2′-bipyridine. Near-infrared electroluminescence of ytterbium(III) complex in OLED

Three new lanthanide(III) β-diketonate complexes [Ln(btfa)3(4,4′-dmbpy)] (Ln = Yb(III), Gd(III) and Nd(III); btfa = anionic 4,4,4-trifluoro-1-phenyl-1,3-butanedione and 4,4′-dmbpy = 4,4′-dimethyl-2,2′-bipyridine) were synthesized and their characterization, including elemental analysis, FTIR spectroscopy and thermal analysis (TG/DTA), is reported. The crystal description based on powder X-ray diffraction reveals that Gd(III) and Yb(III) compounds are isostructural and that lanthanide ion is eight-coordinated by oxygen and nitrogen atoms to give new tris- β-diketonates [Ln(btfa)3(4,4′-dmbpy)]. The cell parameters of Nd(III) complex are close to Gd(III) and Yb(III) ones. Phosphorescence data of Gd(III) complex shows that the triplet states (T1) of the ligands have higher energy than the emitting states of Yb(III) and Nd(III), indicating the possibility of intramolecular energy transfer to these metal ions, which exhibit near-infrared (NIR) emission. We have used the Yb(III) complex as an emitting layer (EML) in a near infrared organic light emitting diode (NIR-OLED) with the structure: CuPc(15 nm)/XD-03(40 nm)/Yb(60 nm)/BCP(15 nm)/Alq3(10 nm)/Al(120 nm). OLEDs exhibit both visible electroluminescence and a NIR electroluminescence at 980 nm from the 2F5/2 → 2F7/2 transition of the Yb(III).

Two New Series of Potentially Triboluminescent Lanthanide(III) β‐Diketonate Complexes

Two new series of anionic lanthanide(III) tetrakis(dibenzoylmethanato) complexes of the type [C4H8NEtH][Ln(dbm)4] [Ln = Pr (2a), Nd (2b), Eu (2c), Tb (2d), Lu (2e); dbm = dibenzoylmethanate anion; [C4H8NEtH]+ = N‐ethylpyrrolidinium cation] and [C5H10NEtH][Ln(dbm)4] [Ln = Pr (3a), Nd (3b), Eu (3c), Tb (3d), Lu (3e); [C5H10NEtH]+ = N‐ethylpiperidinium cation]. The results showed that anions of the type [Ln(dbm)4]– are accessible for Ln3+ ions covering a wide range of ionic radii. While neither 2c nor 3c showed the expected triboluminescence (TL) properties, the new TL‐active europium(III) complex salt [iPr2NH2][Eu(dbm)4] (4) could be isolated in a similar manner. Moreover, the unexpected formation and structural characterization of the nonanuclear europium cluster Eu9(μ4‐OH)2(μ3‐OH)8(bnac)16 (5) (bnac = benzoylacetonate anion) is described.

Mechanistic Investigation of Inducing Triboluminescence in Lanthanide(III) β-Diketonate Complexes.

In this work, we synthesized a series of lanthanide(III) β-diketonate complexes to investigate the induction of triboluminescence. Triboluminescence (TL) spectra, solid-state emission spectra, and luminescence lifetimes of the complexes were obtained to prove consistent emitting species for steady-state and triboluminescence measurements. Detailed analyses of the crystal lattice packing were conducted in an attempt to correlate crystal symmetry, gas discharge, and structural arrangements with "triboexcitation", and it is found that either noncentrosymmetric or centrosymmetic compounds can be TL-active. Furthermore, an intensely TL compound, Eu(dbm)4TMP, was achieved, and its light emission can be seen under daylight upon mechanical stress.

Optical Properties of Heavily Fluorinated Lanthanide Tris β-Diketonate Phosphine Oxide Adducts

The construction of lanthanide(III) chelates that exhibit superior photophysical properties holds great importance in biological and materials science. One strategy to increase the luminescence properties of lanthanide(III) chelates is to hinder competitive non-radiative decay processes through perfluorination of the chelating ligands. Here, the synthesis of two families of heavily fluorinated lanthanide(III) β-diketonate complexes bearing monodentate perfluorinated tris phenyl phosphine oxide ligands have been prepared through a facile one pot reaction [Ln(hfac)3{(ArF)3PO}(H2O)] and [Ln(F7-acac)3{(ArF)3PO}2] (where Ln = Sm3+, Eu3+, Tb3+, Er3+ and Yb3+). Single crystal X-ray diffraction analysis in combination with photophysical studies have been performed to investigate the factors responsible for the differences in the luminescence lifetimes and intrinsic quantum yields of the complexes. Replacement of both bound H2O and C–H oscillators in the ligand backbone has a dramatic effect on the photophysical properties of the complexes, particularly for the near infra-red emitting ion Yb3+, where a five fold increase in luminescence lifetime and quantum yield is observed. The complexes [Sm(hfac)3{(ArF)3PO}(H2O)] (1), [Yb(hfac)3{(ArF)3PO}(H2O)] (5), [Sm(F7-acac)3{(ArF)3PO}2] (6) and [Yb(F7-acac)3{(ArF)3PO}2] (10) exhibit unusually long luminescence lifetimes and attractive intrinsic quantum yields of emission in fluid solution (ΦLn = 3.4% (1); 1.4% (10)) and in the solid state (ΦLn = 8.5% (1); 2.0% (5); 26% (6); 11% (10)), which are amongst the largest values for this class of compounds to date.

Zirconia-based luminescent organic-inorganic hybrid materials with ternary europium (III) complexes bonded

In this work, a novel red-emitting organic-inorganic hybrid material with europium (III) lanthanide β-diketonate complexes linked to a zirconia was reported, which was realized by adduct formation with zirconia-tethered terpyridine moieties. Luminescence enhancement of the hybrid material has been observed compared with pure Eu(tta)3·2H2O. Transparent and strongly luminescent thin films based on PMMA were also prepared at room temperature, which are highly luminescent under UV-light irradiation and possess a promising prospect in the area of optics.

Enhanced NIR Luminescence of Nanozeolite L Loading Lanthanide β‐Diketonate Complexes

AbstractHerein, we report the preparation of zeolite NIR luminescence materials with a remarkable increase of luminescence intensity by attaching stopper molecule (an imidazolium salt) to the channel entrances of zeolite L loading with NIR lanthanide (Er3+ or Nd3+) β‐diketonate complexes. This results from the formation of Ln3+‐β‐diketonate complexes (Ln=Er or Nd) with high coordination numbers through the decreasing of the proton strength in the zeolite channels. The obtained materials were characterized with SEM and photoluminescence spectroscopy. We believe that this hybrid material will be an appealing candidate for the applications of optical fiber, telecommunications and bio‐imaging.

Interaction of TbCl3 and lanthanide complexes with poly(N-vinylpyrrolidone)

The interaction of TbCl3 and lanthanide(III) β-diketonate complexes with poly(N-vinylpyrrolidone) (PVP) polymer was studied using fluorescence spectroscopy. In the presence of PVP the emission of Tb3+ in ethanolic solution of TbCl3 significantly increased owing to the excitation energy transfer from PVP to Tb3+ ions, which was more efficient as compared to a direct excitation of the Tb3+ 4f75d1 levels. In contrast, emission of NEt4Tb(hfa)4 and NEt4Eu(hfa)4 complexes was almost completely quenched in solution containing PVP. The PVP bound both Ln3+ ions and hfa ligands which caused decomposition of Ln-hfa complexes and switched off the excitation energy transfer from the ligand to the emitting Ln3+ ion. The important role of hydrogen bonds in stabilization of interaction of hfa ligands with PVP was indicated.

Interaction of Lanthanide β-Diketonate Complexes with Polyvinylpyrrolidone: Proton-Controlled Switching of Tb3+ Luminescence

In the present study the interaction of lanthanide β-diketonate complexes with polyvinylpyrrolidone (PVP) has been systematically investigated using fluorescence spectroscopy. The emission of terbium complexes with hexafluoroacetylacetone (hfa) ligands is almost completely quenched by PVP in the absence of H(+) ions, while it is enhanced by more than 30 times in the presence of H(+) ions. Conversely, the H(+) ions increase the quenching efficiency of terbium complex with acetylacetone ligands. The strikingly different behavior of complexes with fluorinated and nonfluorinated β-diketonates is accounted for by the interaction mechanism, in which the important role of hydrogen bonds is indicated. The proposed mechanism includes ion-dipol Coulombic interactions between negative charge localized on ligands O-C-CH2-C-O group and positive charge induced in N atom of PVP pyrrolidone group, as well as interactions through hydrogen bonds formed between ligand C-O groups and PVP, occurring directly or through solvent molecules. Additional stabilizing effect, significantly influencing the binding strength of ligands with PVP, results from hydrogen bonds formed by terminal (CF3 or CH3) substituents of ligands with C-O group of PVP. Importantly, the quenching and enhancement of luminescence of terbium complexes with hfa ligands in PVP solution is a reversible process. This enables one to obtain the emission switch-OFF-ON system triggered externally by H(+) ions, which can find possible application in the development of molecule-based devices.

Synthesis, crystal structures and near-infrared luminescent properties of three lanthanide-based enantiomeric pairs

Three lanthanide-based enantiomeric pairs formulated as Ln(dbm)3LR/S (Ln=Nd3+ (R-1 and S-1), Er3+ (R-2 and S-2) and Yb3+ (R-3 and S-3), R- and S-configuration complexes containing the LR and LS ligands, respectively, and dbm=dibenzoylmethanate) have been obtained successfully upon the reaction of corresponding lanthanide precursors Ln(dbm)3·H2O with enantiopure chiral ligands (−)/(+)-4,5-pinene bipyridine (LR/S), respectively. Their structures were characterized by X-ray single-crystal analyses, elementary analyses and IR spectra. The circular dichroic (CD) spectra verified their chiroptical activities and enantiomeric natures. The photoluminescence investigations demonstrated that they display characteristic near-infrared (NIR) emissions of corresponding Nd3+, Er3+ and Yb3+ ions with notable lifetime values, representing the first example of chiral lanthanide β-diketonate complexes with NIR luminescent property.

Colour-tuneable double-layer polymeric films doped with lanthanide β-diketonate complexes.

Double-layer polymeric films doped with lanthanide β-diketonate complexes were obtained. To eliminate the ligand exchange during the film processing the layers are composed of poly(methyl-methacrylate) (PMMA) and poly(N-vinylpyrrolidone) (PVP) polymers, which have different solubilities in organic solvents. Under selective excitation the pure green emission of Tb3+ and red emission of Eu3+ is observed with maxima at 543–544 and 612–613nm, respectively. The emission colour tuneability may be achieved not only by tailoring the films composition but also by changing the excitation wavelength. Films can be easily fabricated by sequential casting from solution, using e.g. the doctor blade technique. Moreover, due to the thermal stability of β-diketonate complexes the double-layer films can be also obtained by hot-press moulding of PMMA. The photophysical properties of the obtained films are presented and discussed. The emission intensity of Tb3+ tetrakis-complex with hexafluoroacetylacetonate ligand is significantly enhanced in PVP matrix as compared to that in PMMA.

Photo–Click Chemistry to Design Highly Efficient Lanthanide β-Diketonate Complexes Stable under UV Irradiation

Europium (t-Eu) and gadolinium (t-Gd) β-diketonate complexes with photoactive t-bpete ligand, [Ln(btfa)3(t-bpete)(MeOH)] (Ln = Eu, Gd), where btfa– and t-bpete are 4,4,4-trifluoro-1-phenyl-1,3-butanedionate and trans-1,2-bis(4-pyridyl)ethylene, respectively, were synthesized, characterized by vibrational, absorption (reflectance) and photoluminescence spectroscopies and their crystal structure was determined using single-crystal X-ray diffraction. B3LYP calculations were performed to support the interpretation and rationalization of the experimental results. The complexes, under UV irradiation, do not display the typical photodegradation of the β-diketonate ligands exhibiting, in turn, an unprecedented photostability during, at least, 10 h. During UV-A exposure (>330 nm), the emission intensities of both complexes increase drastically (∼20 times), whereas for t-Eu the emission quantum yield is enhanced at least 30-fold. A mechanism based on a photoclick trans-to-cis isomerization of both t- and c-bpete moieties was proposed to explain the abnormal photostability of these compounds, either in solid state or in solution. The experimental and computational results are consistent with a photostationary state involving the trans-to-cis isomerization of the bpete ligand under continuous UV-A exposure, which thus diverts the incident radiation from other deleterious photochemical or photophysical processes that cause the typical photobleaching behavior of chelate lanthanide complexes. This shielding mechanism could be extended to other ligands permitting the design of new lanthanide-based photostable systems under UV exposure for applications in lighting, sensing, and displays.