Chemical Partition of the Radiative Decay Rate of Luminescence of Europium Complexes

Experimental and theoretical photophysical study of new synthesized highly asymmetric nona-coordinated trivalent Sm and Eu complexes

The synthesis and characterizations of two new complexes, [Sm(btfa)3Cl-terp] (SmC45H28F9O6N3Cl) and [Eu(btfa)3Cl-terp] (EuC45H28F9O6N3Cl) were done successfully. The complexes have nona-coordinate Sm and Eu centres coordinated to six oxygen atoms of benzoyltrifluoro acetone (btfa) and three nitrogen atoms of chloro-terpyridine (Cl-terp). The ground state geometry of both complexes was optimized using the sparkle model and the shape analysis was carried out using SHAPE v2.1. The detailed photophysical properties of the complexes were studied in solution, solid and 5 % PMMA thin film. The emission spectra of the complexes are devoid of any ligand fluorescence that marks the utilized organic ligands as efficient sensitizers. The decay time and absolute quantum yield of the Sm complex show 12- and 7-fold increase than its precursor [Sm(btfa)3(H2O)2]. The longer decay time of 93 µs is higher than most of the reported trivalent samarium β-diketonate complexes. The Judd-Ofelt intensity parameters, radiative and non-radiative rate constants, intensity ratio, radiative and experimental lifetime, absolute as well as intrinsic quantum yield and sensitization efficiency were calculated and analysed for Eu complex. The decay time, absolute quantum yield and intrinsic quantum yield of [Eu(btfa)3Cl-terp] are 3-, 15- and 3-times larger than precursor, [Eu(btfa)3(H2O)2]. The energy transfer from organic sensitizers to europium ion and the contribution of individual sensitizers along with the luminescence branching ratio of emitting levels of Eu ion were investigated and analysed.

Microwave Synthesis and Luminescence Efficiencies in Mixed-Ligand Europium Complexes

The microwave-assisted methodology is now extended and fine-tuned for the synthesis of mixed-ligand europium complexes with an average reaction time of 12 min. Overall, 14 different complexes were synthesized to improve luminescence using our previously proposed strategy to boost luminescence through ligand diversification, specifically by applying it to quaternary europium complexes with at least one DBM (1,3-diphenylpropane-1,3-dionate) ligand. DBM is a strong absorbant of UV radiation that can dissipate energy through nonradiative channels; thus, it is a useful molecular scaffold for sunblockers and cosmetics. Accordingly, the following luminescent tetrakis and quaternary complexes were prepared: K[Eu(DBM)4], K[Eu(β)4], K[Eu(DBM)3(β)], K[Eu(DBM)2(β)2], K[Eu(DBM)2(β)(β')], and the fully mixed complex K[Eu(DBM)(BTFA)(TTA)(HFAC)], where β can be either BTFA (4,4,4-trifluoro-1-phenylbutane-1,3-dionate), TTA (4,4,4-trifluoro-1-(2-thienyl)butane-1,3-dionate), or HFAC (1,1,1,5,5,5-hexafluoropentane-2,4-dionate). For all the complexes, luminescence experiments were performed in chloroform and acetone solutions. Our findings confirm that mixed-ligand complexes exhibit superior quantum efficiencies compared to the average of their homoleptic counterparts. The presence of DBM in the complexes tends to dramatically increase the nonradiative decay rates of the solutions. Finally, we present formulae that provide a detailed understanding of the distinctive roles of each ligand and their relevant interactions in luminescence.

Synthesis, X-ray crystal structure and photophysics of butterfly shape orange and red emanating polynuclear complexes of tris(dibenzoylmethanato)Ln(III) (Ln = Sm/Eu) and exo-bidentate 4,4'-bipyridine

Reaction of two equivalents of [Ln(dbm)3(H2O)] (Ln = Sm/Eu/Gd) with one equivalent of 4,4'-bipyridine (4,4'-bpy) led to the formation of rare polynuclear complexes of the type [Ln(dbm)3(4,4'-bpy)]n (dbm is the anion of 1,3-diphenyl-1,3-propanedione) instead of symmetrically bridged dinuclear complexes. The structure of the complexes has been established by the single crystal X-ray diffraction (SC-XRD) method and shows that the coordination sphere is composed of a LnO6N2 core (octacoordinated). Shape analysis further revealed that the geometry around Ln(III) is distorted square anti-prismatic with SHAPE value 0.738 and 25.719 for [Sm(dbm)3(4,4'-bpy)]n and [Eu(dbm)3(4,4'-bpy)]n, respectively. Photoluminescence (PL) properties of [Sm(dbm)3(4,4'-bpy)]n and [Eu(dbm)3(4,4'-bpy)]n are discussed in the solid-state and PMMA hybrid film (w/w 6%). By employing theoretical modelling in conjunction with the experimental PL data and crystal structure and an energy transfer (ET) mechanism for the sensitized PL of [Eu(dbm)3(4,4'-bpy)]n is proposed and discussed in detail. Finally, the role of each ligand in sensitized PL of [Eu(dbm)3(4,4'-bpy)]n is calculated and discussed by the chemical partitions of the radiative decay.Graphical abstract.

The increase of europium-based OLED luminance through reducing the excited state lifetime by mixed-ligand complex formation

An approach to the luminance increase of the europium-based OLED is proposed through the formation of the mixed-ligand complex. The introduction of two diverse anionic ligands around one europium ion forming a mixed-ligand complex is confirmed by powder X-ray diffraction, ¹H and ¹⁹F NMR spectroscopy, MALDI MS spectroscopy, and luminescence spectroscopy. A decrease in the symmetry of the coordination environment leads to a 50% reduction of the lifetime of the excited state. The obtained OLEDs based on mixed ligand europium complexes are significantly superior in luminance to OLEDs based on individual complexes.

Luminescent Lanthanide Complexes for Effective Oxygen-Sensing and Singlet Oxygen Generation

Oxygen quantification using luminescence has attracted considerable attention in various fields, including environmental monitoring and clinical analysis. Among the reported luminophores, trivalent lanthanide complexes have displayed characteristic narrow emission bands with high brightness. This bright emission is based on photo-sensitized energy transfer via organic triplet states. The organic triplet states in lanthanide complexes effectively react with the triplet oxygen, enabling oxygen quantification by lanthanide luminescence. Some Tb^III and Eu^III complexes with slow deactivation processes have also formed the excited state equilibrium, thus resulting in the emission-lifetime based oxygen sensing property. The combination of Tb^III /Eu^III emission, Eu^III /Sm^III emission, Eu^III /ligand phosphorescence, and ligand fluorescence/ligand phosphorescence provide the ratiometric oxygen-sensing properties. Moreover, the reaction generates singlet oxygen species which exhibit numerous applications in the photo-medical field. The ligands with large π-conjugated aromatic systems, such as porphyrin, phthalocyanine, and polyaromatic compounds, induces highly efficient oxygen generation. The combination of effective luminescence with singlet-oxygen generation by the lanthanide complexes render them suitable for photo-driven theranostics. This review summarizes the research progress of lanthanide complexes with efficient oxygen-sensing and singlet-oxygen generation properties.

Theoretical and experimental spectroscopic investigation of new Eu(III)-FOD complex containing 2-pyrrolidone ligand

The preparation and photoluminescent properties of the new [Eu(FOD)₃(2-Pyr)₂] complex (FOD = 6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octadionate; 2-Pyr = 2-pyrrolidone) are reported. The obtained complex was characterized by elemental analysis, complexometric titration using EDTA, infrared spectroscopy, and single-crystal X-ray diffraction studies. The coordination polyhedron of the complex is described as a distorted square antiprismatic with both 2-Pyr monodentate ligands coordinated to Eu(III) via the oxygen atoms, in neutral form, while the three FOD molecules are coordinated in the anionic form. Structural modeling at the PBE1PBE/SVP/MWB52 level of theory provided a geometry in excellent agreement with the one obtained experimentally. Spectroscopy properties such as intensity parameters (Ω₂ and Ω₄), radiative emission rate (A_rad), and chemical partition of A_rad for [Eu(FOD)₃(2-Pyr)₂] and [Eu(FOD)₃(H₂O)₂] were calculated by using the QDC model with help of the semiempirical wavefunctions. The modeling of the ligand-to-metal energy transfer for both complexes was performed, allowing to obtain the theoretical emission quantum yield and to characterize the most relevant molecular orbitals involved.

Synthesis of cool white light emitting novel dysprosium (Dy3+ ) complexes with tetradentate β-ketoamide and heterocyclic auxiliary ligands

To improve current multiphase white light emitting diodes (WLEDs), a novel series of five complexes consisting of one binary and four ternary complexes that emitted cool white light was successfully synthesized using a chelating tetradentate ligand and auxiliary ligands, i.e. 5,6-dimethyl-1,10-phenanthroline, 1,10-phenanthroline, 4,4'-dimethyl-2,2'-bipyridyl, and 2,2'-bipyridyl. The series was examined structurally using elemental analysis, Fourier transform infrared spectroscopy, energy dispersive X-ray analysis, ultraviolet-visible spectroscopy, and proton nuclear magnetic resonance spectroscopy. These complexes had the appropriate thermal stability required for the generation of white organic LEDs (WOLEDs). Dysprosium (III) (Dy³⁺ ) ion complexes demonstrated the characteristic emission peaks of blue colour at 482 nm and yellow colour at 572 nm, respectively, when excited using near ultraviolet light. Band gap, refractive index, and decay lifetime of the optimized samples were recorded as 2.68 eV, 2.12, and 1.601 ms, respectively. Correlated colour temperature value (7875 K), Commission International de l'Eclairage coordinates (0.300, 0.294), and colour purity (21.04 × 10^-2 ) of the optimized complex were near to those of white illuminants as defined by the National Television System Committee. These complexes had promise as commercial LEDs for the advanced optoelectronics devices, especially as WOLEDs for illumination applications.

Quantum Yield and Photoluminescence Intensity Enhancement Effects of a Diphosphine Dioxide Ligand on a 6-Coordinate Eu(III)-β-Diketonate Complex with Low Luminescence

Tris{6,6,7,7,8,8,8-heptafluoro-1-[2-(9,9-dimethylfluorenyl)]-1,3-octanedionate} europium(III) (Eu(III)(hfod)₃ 1) was synthesized, which was designed to have low luminescence and a large absorption coefficient in order to elucidate the coordination effects of phosphine oxide ligands. The quantum yield (Φ_TOT) and photoluminescence intensity of complex 1 were dramatically enhanced by coordinating a diphenyl-4-(dibutylphosphinyl)butyl phosphine oxide (DPDB) ligand, thanks to the increased intrinsic photoluminescence quantum yield of the lanthanide (Φ_Ln) and the increased energy transfer efficiency (Φ_ET) in the solution and solid states. In the solid state, there was no energy dissipation by solvent molecules. This excluded the steric shielding effects of the DPDB ligand and allowed the effects of the ligand field environment to be extracted. Φ_Ln and Φ_ET of complex 2 were much larger in the solid state than those in the solution state, resulting in larger Φ_TOT (solution state: Φ_Ln 0.50, Φ_ET 0.42, and Φ_TOT 0.21 and solid state: Φ_Ln 0.74, Φ_ET 0.47, and Φ_TOT 0.35). Larger asymmetry ratios (ratio R) of Eu(III)(hfod)₃(DPDB) 2 than those of complex 1 in the solution and solid states indicate that the ligand field of the Eu(III) ion becomes more asymmetric by coordination of the DPDB ligand. Density functional theory calculations showed that Φ_Ln and Φ_ET increased when the ligand field around the Eu(III) ion became more asymmetric. Based on these results, we propose a hypothesis on the enhancement of the photoluminescence intensity of 6-coordinated Eu(III)-β-diketonate by a DPDB ligand. When a DPDB ligand coordinates to a Eu(III) ion, the positions of the nearest oxygen atoms around the Eu(III) ion are shifted by steric repulsion and the relative positions of the nearest oxygen atoms are distorted. The distorted coordination environment induces asymmetry in the ligand field, increasing Φ_Ln and Φ_ET. Φ_TOT is enhanced by the DPDB ligand because it is the product of Φ_Ln and Φ_ET. Photoluminescence intensity increases because of the enhanced Φ_TOT.

Energy transfer mechanism in luminescence Eu(III) and Tb(III) complexes of coumarin-3-carboxylic acid: A theoretical study

Excited state energy level diagrams of coumarin-3-carboxylic acid (HCCA) chromophore, Eu(CCA)Cl₂(H₂O)₂ (1), Eu(CCA)₂Cl(H₂O)₂ (2)_, Eu(CCA)₃(H₂O)₃ (3), Tb(CCA)₂Cl(H₂O) (4) and Tb(CCA)₂(NO₃)(H₂O) (5) in gas phase and polar solution have been calculated by means of DFT/TDDFT/ωB97XD methods. Based on these results, the ability of CCA to sensitize Eu(III) and Tb(III) luminescence has been examined. The competitive excited state processes in the complexes - fluorescence, intersystem crossing (ISC) and phosphorescence, were analyzed depending on the environment, number of the ligands, Ln(III) ion type (Eu and Tb) and counteranion (Cl^- and NO₃^-). It has been found that the environment altered the S₁ state energy, oscillator strength, fluorescence lifetime as well as the S₁ character - polar solution stabilized the S₁(ππ*) state, whereas non-polar solution (gas phase, solid state) stabilized the S₁(nπ*) state. The S₁(nπ*) state was decisive for the efficient energy transfer as it suppressed the S₁ emission of CCA and favored ISC or direct transfer to the emitting levels of Eu(III). The HCCA triplet (T₁) state minimum energy (~2.7, ~2.6^ZPE eV) and (ππ*) character were retained in Eu/Tb-CCA complexes regardless of the environment. The energy gap between the higher energy T₁ donor state and the acceptor levels ⁵D₁ of Eu(III) (~0.5 eV) and ⁵D₄ of Tb(III) (~0.1 eV) provided optimal resonance conditions for effective energy transfer for Eu(III), but less probability for Tb(III). The nonradiative energy (CCA → Eu(III)) transfer rates and quantum luminescence yield for 2 and 3 were calculated by a strategy combining DFT geometries, INDO/S excitation energies and calculated Judd-Ofelt parameters. The excitation channel T₁ → ⁵D₀ through an exchange mechanism was predicted as the most probable one to populate the main emissive Eu-centered state in complexes 2 and 3. The more efficient luminescence of 3 than that of 2 was discussed and explained.

Misconceptions in electronic energy transfer: bridging the gap between chemistry and physics

Many treatments of energy transfer (ET) phenomena in current literature employ incorrect arguments and formulae and are not quantitative enough. This is unfortunate because we witness important breakthroughs from ET experiments in nanoscience. This review aims to clarify basic principles by focusing upon Förster-Dexter electric dipole-electric dipole (ED-ED) ET. The roles of ET in upconversion, downconversion and the antenna effect are described and the clichés and simple formulae to be avoided in ET studies are highlighted with alternative treatments provided.

Europium Complexes: Luminescence Boost by a Single Efficient Antenna Ligand

We advance the concept that a single efficient antenna ligand substituted in or added to an otherwise weakly luminescent europium complex is enough to significantly boost its luminescence. Our results, on the basis of photophysical measurements on 5 novel europium complexes and 15 known ones, point in the direction that ligand dissimilarity and ligand diversity are all concepts that clearly play a fundamental role in the luminescence of europium complexes. We show that it is important that a symmetry breaker ligand exists in the complex to enhance ligand dissimilarity and ligand diversity, all mainly affecting the nonradiative decay rate by reducing it. Because the presence of at least one antenna ligand is also obviously necessary, the optimal and the most cost-effective situation can be achieved by adding a single coordination symmetry breaker that is also an efficient antenna, such as 1-(2-thenoyl)-3,3,3-trifluoroacetone or 4,4,4-trifluoro-1-phenyl-1,3-butanedione. In such cases the quantum efficiency, η, is decidedly boosted, as can be verified by going from complex [EuCl₂(TPPO)₄]Cl·3H₂O with η = 0% to the novel complex [EuCl₂(BTFA)(TPPO)₃], where TPPO stands for triphenylphosphine oxide, with η = 62%.

Substantial luminescence enhancement in ternary europium complexes by coordination of different ionic ligands

We demonstrate in a general and comprehensive manner that a substantial enhancement of luminescence in europium complexes can be achieved by increasing ionic ligand diversity. The measured boosts in quantum efficiency ranged from 100% to 543%.