Efficient Layers of Emitting Ternary Lanthanide Complexes for Fabricating Red, Green, and Yellow OLEDs

Terpyridyl-Imidazole Based Ligand Coordinated to Ln(Hexafluoroacetyl acetonate)3 Core: Synthesis, Structural Characterization, Luminescence Properties, and Thermosensing Behaviors in Solution and PMMA Film

A new array of ternary lanthanide complexes of the form [Ln(hfa)3(tpy-HImzphen)] have been synthesized and thoroughly characterized wherein Ln = LaIII (1), EuIII (2), SmIII (3), and TbIII (4); hfa = hexafluoroacetylacetonate; and tpy-HImzphen = 2-(4-[2,2':6',2″]terpyridin-4'-yl-phenyl)-1H-phenanthro[9,10-d]imidazole. Incorporation of tpy-HImzphen onto the Ln-hfa moiety induced a bathochromic shift of the absorption window of the complexes into the visible region. Extensive investigations of the luminescence characteristics have been conducted both at RT and at 77 K to understand the deactivation pathways of the complexes. Both steady-state and time-resolved emission spectral behaviors indicate four distinctive behaviors upon incorporation of tpy-HImzphen onto the lanthanide core, viz., a huge red-shift of the ligand-centered peak for LaIII; almost complete energy transfer for EuIII; very little energy transfer for SmIII, while reverse energy transfer in the case of TbIII. In addition, the EuIII-complex exhibits its excellence in luminescence thermometry in the solution state as well as in poly(methyl methacrylate) (PMMA) thin films. The thermosensitive luminescence response in solution was further utilized to mimic set-reset flip-flop logic operation. A plausible energy transfer scheme has been devised to explain dissimilar luminescence behaviors in the complexes. The role of LMCT was also considered for the observed thermosensing property of the Eu(III) complex.

Pure red-light emitting europium based complexes as efficient UV light converters: synthesis, crystal structure and photoluminescence properties

This paper reports three new crystallographically characterized europium complexes with composition as follows: [Eu(fod)3(L1)] (1), [Eu(fod)3(L2)] (2) and [Eu(fod)3(L3)] (3) {L1 = benzimidazole (bzi), L2 = 4,7-diphenyl-1,10-phenanthroline (bath), L3 = 2-(2-pyridyl) benzimidazole (py-im) and fod = anion of 6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedione (Hfod)}. The single crystal (SC) XRD analysis shows that complex 1 is seven-coordinated while complexes 2 and 3 are eight-coordinated with the geometrical structures of a mono-capped octahedron and a trigonal dodecahedron, respectively. The NMR spectra of the complexes validate the SC-XRD results in solution. The complexes are stable in solution as no dissociation of any ligand was observed in the NMR spectra of the complexes. The photophysical properties of the complexes in solution, solid state, and PMMA thin films were studied. The hypersensitive transition 5D0 → 7F2 dominates the emission spectra in all phases, showing the highly asymmetric environment around the Eu(III) ion. The bath ligand is found to be the best sensitizer of the Eu ion and hence complex 2 shows the strongest luminescence properties with the highest absolute quantum yield among the three complexes. The CIE coordinate analysis shows that pure red-luminescence is emitted by the Eu complexes in the solid state since the coordinates found in this phase are closer to the standard NTSC 1987 values. The optical band gaps were determined for the complexes and the observed values suggest that the complexes can have possible applications in the field of semiconductor materials.

Preorganized Polyaromatic Soft Terdentate Hosts for the Capture of [Ln(β-diketonate)3 ] Guests in Solution

The concept of preorganization is famous in coordination chemistry for having transformed flexible bidentate 2,2'-bipyridine scaffolds into rigid 1,10-phenanthroline platforms. The resulting boosted affinities for d-block cations has successfully paved the way for the design of a wealth of functional complexes, devices and materials for analysis and optics. Its extension toward terdentate homologues adapted for the selective complexation of f-block cations with larger coordination numbers remains more overlooked. The resulting rigidification of 2,6-bis(1-methyl-1H-benzo[d]imidazol-2-yl)pyridine ligands (L1-L7) produces the highly preorganized and extended polyaromatic benzo[4',5']imidazo[1',2' : 1,2]pyrido[3,4-b]benzo[4,5]imidazo[1,2-h][1,7]naphthyridines (L8-L11) receptors, which offer some novel and rare opportunities for efficiently complexing trivalent lanthanides with polyaromatic soft terimine ligands. The crystal structures of the stable heteroleptic [LkLn(hfac)₃ ] adducts (Lk=L1, L8, L9; Ln=La, Eu, Gd, Er, Yb, Y; H-hfac=1,1,1,5,5,5-hexafluoropentane-2,4-dione) show a drastic decrease in the Ln-N bond valences upon replacement of the flexible ligand L1 with its preorganized counterparts L8 and L9. This points to a limited match between the preorganized cavity and the entering [Ln(hfac)₃ ] lanthanide containers. However, thermodynamic studies conducted in dichloromethane reach the opposite conclusion, with an improved affinity, by up to three orders of magnitude for catching Ln(hfac)₃ when L1 is replaced by the preorganized L8-L9 receptors. The key to the enigma lies in the removal of the energy penalty which accompanies the formation of flexible [L1Ln(hfac)₃ ] complexes in solution. This driving force overcomes the poor match between the preorganized terdentate N^∩ N^∩ N cavity in L8 and L9 and the size of trivalent lanthanides. As planned, the rigid, planar and extended π-conjugated system found in L8 and L9 shifts the ligand-centered absorption bands by about 5000 cm^-1 toward lower energies, a crucial point if these stable [L8Ln(hfac)₃ ] and [L9Ln(hfac)₃ ] platforms have to be considered for the visible sensitization of luminescent lanthanides in metallopolymers.

Fluorinated β-diketone-based Sm(III) complexes: spectroscopic and optoelectronic characteristics

The synthesis and characterization of a series of octa-coordinated Sm(III) complexes with 4,4,4-trifluoro-1-(2-naphthyl)-1,3-butanedione (TFNB) and 2,2'-bipyridine (Bpy) derivatives as ancillary ligand are described here. The complexes were analyzed by elemental, spectroscopic such as infrared spectroscopy, ¹ H NMR, and thermogravimetric analyses. The fluorinated TFNB ligand absorbs in the range from 200 to 400 nm. The complexes show the sharp and structured Sm-based emissions in visible region upon irradiation in UV range. Excitation spectra of complexes show similarity to the absorption spectra of ligands suggesting that excitation energy is transferred from ligands to Sm(III) centre by the antenna effect. Photoluminescence emission spectra and colour parameters affirmed that the complexes show luminescence in orange-red region. These luminous Sm(III) complexes might be applied as emissive layer in organic electroluminescent devices.

Red-emitting β-diketonate Eu(III) Complexes With Substituted 1,10-phenanthroline Derivatives: Optoelectronic and Spectroscopic Analysis

A series of europium diketonate complexes with 1-phenyl-1,3-butanedione (PBD) and 1,10-phenanthroline derivatives were synthesized and explored spectroscopically. Photophysical characteristics of synthesized complexes have been investigated experimentally as well as theoretically. Photoluminescence emission spectra of complexes do not contain any peak of ligand revealing efficient transferal of energy from ligand to Eu³⁺ ion. Presence of peak at 611 nm corresponding to ⁵D₀ → ⁷F₂ transition is responsible for red emanation of ternary europium complexes. Photophysical parameters viz., Judd-Ofelt, quantum efficiency, radiative and non radiative decay rates were also estimated theoretically from LUMPAC software. Geometry optimization of complexes was done via Avogadro software. All synthesized trivalent complexes exhibit red emission which was further sustained by the position of chromaticity coordinates in CIE triangle. These red emanating materials could be utilized in designing electroluminescent display devices.

Theoretical Modeling (Sparkle RM1 and PM7) and Crystal Structures of the Luminescent Dinuclear Sm(III) and Eu(III) Complexes of 6,6,7,7,8,8,8- Heptafluoro-2,2-dimethyl-3,5-octanedione and 2,3-Bis(2-pyridyl)pyrazine: Determination of Individual Spectroscopic Parameters for Two Unique Eu3+ Sites

Heteroleptic homo dinuclear complexes [Sm(fod)₃(μ-bpp)Sm(fod)₃] and [Eu(fod)₃(μ-bpp)Eu(fod)₃] and their diamagnetic analogue [Lu(fod)₃(μ-bpp)Lu(fod)₃] (fod is the anion of 6,6,7,7,8,8,8- heptafluoro-2,2-dimethyl-3,5-octanedione (Hfod) and bpp is 2,3-bis(2-pyridyl)pyrazine) are synthesized and thoroughly characterized. The lanthanum gave a 1:1 adduct of La(fod)₃ and bpp with the molecular formula of [La(fod)₃bpp]. The ¹H NMR and ¹H-¹H COSY spectra of the complexes were used to assign the proton resonances. In the case of paramagnetic Sm³⁺ and Eu³⁺ complexes, the methine (of the fod moiety) and the bpp resonances are shifted in the opposite direction and the paramagnetic shifts are dipolar in nature, which decrease with increasing distance of the proton from the metal ion. The single-crystal X-ray analyses reveal that the complexes (Sm³⁺ and Eu³⁺) are dinuclear and crystallize in the triclinic P1 space group. Each metal in a given complex is eight coordinate by coordinating with six oxygen atoms of three fod moieties and two nitrogen atoms of the bpp. Of the two metal centers, in a given complex, one has a distorted square antiprism arrangement and the other acquires a distorted dodecahedron geometry. The Sparkle RM1 and PM7 optimized structures of the complexes are also presented and compared with the crystal structure. Theoretically observed bond distances are in excellent agreement with the experimental values, and the RMS deviations for the optimized structures are 2.878, 2.217, 2.564, and 2.675 Å. The photophysical properties of Sm³⁺ and Eu³⁺ complexes are investigated in different solvents, solid, and PMMA-doped thin hybrid films. The spectroscopic parameters (the Judd-Ofelt intensity parameters, radiative parameters, and intrinsic quantum yield) of each Eu³⁺ sites are calculated using the overlap polyhedra method. The theoretically obtained parameters are close to the experimental results. The lifetime of the excited state is 38.74 μs for Sm³⁺ and 713.62 μs for the Eu³⁺ complex in the solid state.

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.

Optical Properties of New Terbium(III) Ternary Complexes Containing Anionic 6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedione and Neutral Sensitizers in Solution, Solid and PMMA Thin Films: Intra and Interphase Colour Tuning

Four new terbium(III) ternary complexes, [Tb(fod)₃ (indazole)] (1), [Tb(fod)₃ (tptz)] (2), [Tb(fod)₃ (impy)] (3) and [Tb(fod)₃ (tppo)₂ ] (4) with 6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedione, (Hfod) and N and O donors are synthesized and structurally characterized. The photoluminescence of the complexes are studied in solid, chloroform solution and thin PMMA hybrid films (PMMA is a poly(methyl methacrylate)). The ligand-based emission is dominant in solution of 3 and 4. The effect of ancillary ligands on the photoluminescence of terbium(III) ion is investigated. The mechanism of energy transfer is discussed and correlates the luminescence of acceptor terbium ion with the triplet state energy level of donor ligands. Among the four complexes studied, the luminescence from 1 is most intense followed by 2, 4 and 3. The Commission International de I'Eclairage (CIE) color coordinates of these complexes are calculated and presented. The complexes 3 and 4 show intraphase color tuning when excited under different UV wavelengths. The emission color of [Tb(fod)₃ (tppo)₂ ] (4) changes from pure blue to turquoise green via almost pure white under 360, 328 and 280 nm UV excitation wavelengths respectively. The [Tb(fod)₃ (impy)] (3) show similar results. The complex [Tb(fod)₃ (tptz)] (2) displays interphase color tuning from yellow (solution) to turquoise green (solid) and fascinatingly incorporation of complex in PMMA polymer generates white light under same excitation wavelength (360 nm).

Structures and pure near-infrared photophysics of erbium and ytterbium(iii) complexes incorporating fluorinated β-diketone and neutral unidentate ligands

This paper reports on the synthesis, structures and pure near-infrared photophysical properties of low symmetry Er³⁺ and Yb³⁺ β-diketonate ternary complexes.

All-perylene-derivative for white light emitting diodes

Tb(H3PTC)3PTCDAAn organic-based bright white light emitting compound, namely Tb(H3PTC)3, able to be used as part of a WOLED and as a part of a RGB system that can withstand high temperatures (∼700 K), is developed using PTCDA and Tb(NO₃)₃·5H₂O as precursors by hydrothermal synthesis.

Structure and Stereochemistry of Adducts of Tris(dipivaloylmethanato)europium(III), Eu(dpm)3, with Some Dipolar Aprotic Unidentate O-Donors

Single crystal X-ray structural characterisations are reported for adducts of the form [(L-O)Eu(O,O′-dpm)3] obtained by the crystallisation of tris(dipivaloylmethanato)europium(iii) (dpm=[HC(C(tBu).CO)2]−) from an array of dipolar aprotic oxygen-donor solvents L (L=N-methylpyrrolidinone (nmp), trimethylphosphate, (MeO)3PO, (tmp), hexamethylphosphoramide (hmpa), dimethylacetamide (dma), dimethyl sulfoxide (dmso), and the bidentate octamethylpyrophosphoramide (ompa). In all adducts, the resulting arrays contain seven-coordinate metal atoms, which adopt two different isomeric forms of the mono-capped trigonal prismatic stereochemistry, the L=dma and dmso adducts corresponding to one type, nmp and tmp the other. The adduct formed with ompa behaves as a pair of discrete metal environments bridged by the O-ompa-O′ ligand, thus; [(dpm-O,O′)3Eu(O-ompa-O′)Eu(O,O′-dpm)3], and is found in two forms, one in which both Eu environments is of the tmp type, the other of the dmso/dma type. In the hmpa adduct, the asymmetric unit of the structure is a disordered composite of both types. In none of the adducts is there any further solvation beyond coordination of a single L.

Lanthanide complexes based on a conjugated pyridine carboxylate ligand: structures, luminescence and magnetic properties

Three lanthanide compounds have been synthesized, namely, {[Dy₂(bpda)₃(H₂O)₃]₄·2H₂O}(Dy-1), {[Sm(bpda)₂·(H₂O)]·H₂O}_n (Sm-2) and {[Tb₂(bpda)₃(H₂O)₃]₄·2H₂O} (Tb-3) (H₂bpda = 2,2′-bipyridine-6,6′-dicarboxylic acid). Their structures were determined by single crystal X-ray diffraction and characterized by elemental analysis, infrared spectroscopy and thermogravimetric analysis. Dy-1 and Tb-3 are isostructural with a conjugate bimolecular four-nuclear cluster structure constructed with intramolecular hydrogen bonds and they form a 3D supramolecular structure with intermolecular hydrogen bonding. Sm-2 is a one-dimensional chain structure and is further connected by intricate hydrogen bonds into a three-dimensional supramolecular structure. These three compounds exhibit significant characteristic luminescence from the ligand to the central Ln(iii) ion, which is found by solid-state photoluminescence measurement. Sm-2 exhibits a long luminescence lifetime and high fluorescence quantum yield. A slow relaxation phenomenon is observed for the dysprosium compound by measuring the alternating-current susceptibility at low temperature and the underlying mechanism was further confirmed by theoretical calculations.

The structures, fluorescence and magnetic properties of series three lanthanide complexes have been investigated. Dysprosium complex exhibit the single-molecule behavior.

A Suitable Functionalization of Nitroindazoles with Triazolyl and Pyrazolyl Moieties via Cycloaddition Reactions

The alkylation of a series of nitroindazole derivatives with 1,2-dibromoethane afforded the corresponding N-(2-bromoethyl)- and N-vinyl-nitro-1H-indazoles. The Cu(I)-catalysed azide- alkyne 1,3-dipolar cycloaddition was selected to substitute the nitroindazole core with 1,4-disubstituted triazole units after converting one of the N-(2-bromoethyl)nitroindazoles into the corresponding azide. The reactivity in 1,3-dipolar cycloaddition reactions with nitrile imines generated in situ from ethyl hydrazono-α-bromoglyoxylates was studied with nitroindazoles bearing a vinyl unit. The corresponding nitroindazole-pyrazoline derivatives were obtained in good to excellent yields.

Symmetry correlations between crystallographic and photoluminescence study of ternary β-diketone europium(iii) based complexes using 1,10-phenanthroline as the ancillary ligand

This work entails deep red emitting Eu^III based complexes with a variety of ternary β-diketonate ligands and 1,10-phenanthroline as the ancillary ligand in the system. The solid state structure and spectroscopic analysis has been outlaid in terms of photoluminescence and crystallography. A luminescence quantum efficiency of 50% was obtained for the [tris-(4,4,4-trifluoro-1-chlorophenyl-butanedione)mono-(1,10-phenanthroline)europium(iii)] complexes. Moreover, complexes [tris-(2,2,6,6-tetramethyl-heptanedione)mono-(1,10-phenanthroline) europium(iii)] and {[hexa-(benzyl carboxylic acid) bis-(1,10-phenanthroline)di-europium(iii)]-μ-[κ²-O,O'-(benzyl carboxylic acid)]₂} were found to also have quantum yields of 9% and 28% with respective sensitization efficiencies of 85%, 15% and 58%. These results were articulated with crystallographic details pertaining to the nature of coordination and the effect of steric and electronic properties thereof which somewhat impacts the Eu-N bond distances. A symmetry correlation was drawn between the crystallographic data and the photoluminescence data.

Recent Advances on Metal-Based Near-Infrared and Infrared Emitting OLEDs

During the past decades, the development of emissive materials for organic light-emitting diodes (OLEDs) in infrared region has focused the interest of numerous research groups as these devices can find interest in applications ranging from optical communication to defense. To date, metal complexes have been most widely studied to elaborate near-infrared (NIR) emitters due to their low energy emissive triplet states and their facile access. In this review, an overview of the different metal complexes used in OLEDs and enabling to get an infrared emission is provided.

Syntheses, crystal structure and photophysical properties of [Sm(dbm)3(impy)] and [Tb(dbm)3(impy)] and their hybrid films

This paper reports the syntheses, crystal structure and photophysical properties of eight-coordinate complexes [Sm(dbm)₃(impy)] and [Tb(dbm)₃(impy)] (dbm is the anion of 1,3-diphenyl-1,3-propanedione and impy is 2-(1H-imidazol-2-yl)pyridine).

Synthesis, characterization, photoluminescence and intensity parameters of high quantum efficiency pure-red emitting Eu(iii) fluorinated β-diketone complexes

The high quantum efficiency mononuclear 7-, 8- and 9-coordinate complexes of tris(heptafluoro-dimethyl-octanedionato)Eu(iii) and 1H-indazole, 2-(1H-imidazol-2-yl)pyridine and 2,4,6-tris(2-pyridyl)-s-triazine are synthesized and thoroughly characterized.

Synthesis, crystal structure and photoluminescence studies of [Eu(dbm)3(impy)] and its polymer-based hybrid film

An eight-coordinate Eu(iii) complex with 1,3-dibenzoylmethane (Hdbm) and 2-(1H-imidazol-2-yl)pyridine (impy), [Eu(dbm)₃(impy)], was synthesized and characterized via elemental analysis, FTIR spectroscopy, ESI-MS⁺ studies and thermal analysis (TGA/DTA).

Red, orange-red and near-infrared light emitting ternary lanthanide tris β-diketonate complexes with distortedC4vgeometrical structures

This paper presents the photophysical and optoelectronic properties (in the visible to NIR range) of isostructural nona-coordinate Ln complexes with distortedC_4vgeometrical structures.

Syntheses and crystal structures of new dinuclear lanthanide complexes based on 3-(4-hydroxyphenyl)propanoic acid: Hirshfeld surface analyses and photoluminescence sensing

A series of ten new lanthanide complexes, [Ln₂(HPA)₆(bipy)₂·4H₂O·2bipy], were synthesized and their photoluminescence sensing and Hirshfeld surface analyses performed.

Remarkable high efficiency of red emitters using Eu(iii) ternary complexes

We have synthesized Eu(iii) ternary complexes possessing record photoluminescence yields up to 90%. This high luminescence performance resulted from the absence of quenching moieties in the Eu coordination environment and an efficient energy transfer between ligands, combined with a particular symmetry of the coordination environment.

Teaching an old molecule new tricks: evidence and rationalisation of the slow magnetisation dynamics in [DyTp2Acac]

Combined analyses of magnetic and luminescence measurements, with the support of theoretical calculations, help elucidate the origin of [DyTp₂Acac]'s SMM behaviour.