Modeling intramolecular energy transfer in lanthanide chelates: A critical review and recent advances

Role of Vibronic Coupling for the Dynamics of Intersystem Crossing in Eu3+ Complexes: an Avenue for Brighter Compounds

Understanding the dynamics of photophysical processes in Ln3+ complexes remains challenging due to the intricate nature involving the metallic center, where sensitization (antenna effect) plays a pivotal role. Current studies have often overlooked the vibronic coupling within the antenna effect, leading to incomplete insights into excited-state dynamics. To address these shortcomings, we introduce a novel theoretical and computational approach that leverages the impact of the vibrational modes of the S1 and T1 states in this effect through the correlation function formalism, offering a comprehensive view of intersystem crossing (ISC). Our approach achieves a desirable alignment between empirical and theoretical rates, outperforming previously employed semiclassical methods. A groundbreaking finding is that vibronic coupling with vibrations in the 700-1600 cm-1 energy range is crucial for higher ISC, and local vibrational mode analysis identified that this process is driven by delocalized vibrations across the molecule. These results shed light on the key molecular fragments responsible for vibronic coupling, opening an avenue for harnessing faster ISC by tailoring the ligand scaffold. Overall, it also demonstrates how ISC dynamics can serve as a bridge between theory and experiment, furnishing detailed mechanistic insights and a roadmap for the development of brighter compounds.

Bifunctional Lanthanide MOFs with Phosphorus Ligands: Selective Luminescent Detection of Borides and CO2 Conversion

There is an increasing demand for the development of lanthanide metal-organic frameworks and derived multifunctional materials. The functional properties of such compounds are influenced by the arrangement of various Lewis basic sites or structural configurations of the ligands. In this study, a series of isostructural Ln-MOFs containing a phosphine-dicarboxylate ligand, [Ln(HL)(L)(DMF)]·DMF (where H2L = 5-(diphenylphosphanyl)isophthalic acid, Ln = Tb3+ (1), Eu3+ (2), Gd3+ (3), Ce3+ (4), and Nd3+ (5)), was synthesized under solvothermal conditions and characterized in detail. Among the obtained compounds, Tb-MOF 1 demonstrated excellent luminescent properties with a high quantum yield (90.45%) and considerable lifetime (1266 μs). Furthermore, 1 acts as a unique luminescent Ln sensor for 4-formylphenylboronic acid and 9-[3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl]carbazole, exhibiting low detection limits of 1.38 and 3.62 mM, respectively. Additionally, Nd-MOF 5 acts as an efficient catalyst for coupling carbon dioxide to epoxy compounds, resulting in high conversion rates (up to 96%). This study further extends the growing family of Ln-MOFs and provides insights for preparing multifunctional materials through the modification of organic ligands with specific functional groups.

Dual Visible and NIR Emission, Mechanoluminescence, and Magnetic Properties of PPh4[LnL4] Chelates with Diphenyl-N-Benzoylamidophosphate

The design, synthesis, and study of lanthanide coordination compounds with luminescent and magnetic properties attractive in modern technologies is still a pressing and challenging task. In the present work, a series of coordination compounds of tetrakis-carbacylamidophosphate PPh4[LnL4] (where HL = diphenyl-N-benzoylamidophosphate) with several lanthanide ions such as NdIII, SmIII, DyIII, and TmIII was prepared and studied by X-ray analysis and luminescence spectroscopy at 293 and 77 K, as well as by magnetic measurements. Coordination compounds are not isostructural, but the type of coordination is the same. All of them have intense sensitized emission. PPh4[SmL4], PPh4[DyL4], and PPh4[TmL4] chelates are characterized by dual visible and infrared emission and mechanoluminescence. In addition, PPh4[DyL4] has multifunctional properties such as Vis and NIR emissions, brilliant mechanoluminescence and single-ion molecular magnet (SIM) properties. This type of compound holds great promise in multifunctional magnetic radiation converters.

Spectroscopic and Theoretical Tools Unravel the Thermally-Stabilized Behavior of Eu3+-Based Complex Incorporated in Sustainable Urethanesil Film

Despite significant ongoing efforts to develop luminescent rare-earth β-diketonate complexes, achieving thermal stability remains a persistent challenge. In this study, we present a thermally stable organic-inorganic hybrid (OIH) compound, SiCO-[Eu(tta)3(H2O)2], where tta=thenoyltrifluoroacetonate, Si=3-(triethoxysilyl)propyl isocyanate, and CO=castor oil. Spectroscopic analysis reveals that while [Eu(tta)3(H2O)2] in its powder form undergoes irreversible photoluminescence quenching at 60 °C and embedding it in the SiCO polymer preserves its luminescence even after being annealed up to 180 °C. Notably, the hybrid film maintains stable emission properties after multiple heating-cooling cycles (29-70 °C) and exhibits reversible emission behavior. This finding is attributed to polymer-complex interactions and/or the replacement of water molecules with polymer coordination, resulting in a more rigid environment in the Eu3+ coordination sphere. The thermal dependence of intramolecular energy transfer (IET) indicates that the decrease in Eu3+ luminescence is linked to faster depopulation of the emitting level, driven by non-radiative relaxation and back-energy transfer. This outcome enabled us to strategize approaches to mitigate luminescence quenching in OIH while providing valuable insights into the photophysical properties of these compounds, thus offering a guide towards how we can boost the capabilities of these materials.

Intramolecular energy transfer and its influence on the overall quantum yields of Eu3+ and Tb3+ chelates with dimethyl(phenylsulfonyl)amidophosphate ligands

Lanthanide chelates with dimethyl(phenylsulfonyl)amidophosphate (labeled as HSP) and Lewis base ligands (bpy = 2,2;-bipyridine and phen = 1,10-phenanthroline) of formula Na[Ln(SP)4] (1Ln), [Ln(SP)3bpy] (2Ln); [Ln(SP)3phen] (3Ln) (Ln = Eu3+, Gd3+, Tb3+ and Lu3+) were obtained and characterized by the X-ray, photoluminescence spectroscopy at 293 and 77 K as well as by intrinsic (QLnLn) and overall (QLnL) luminescence quantum yields. These phosphors manifest a very strong emission after excitation in the UV range of the molecular singlet states (S1) and two of them have very high QLnL values (Eu3+ and Tb3+ chelates of the type 2Ln and 3Ln). The dynamics of the excited states are discussed based on the intramolecular energy transfer theory, considering the dipole-dipole, the dipole-multipole and the exchange mechanisms. From the calculated energy transfer rates, a rate equation model was constructed and, thus, the theoretical QLnL can be obtained. A good correlation between the experimentally determined and theoretically calculated QLnL values was achieved, with the triplet state (T1) playing a predominant role in the energy transfer process for Eu3+ compounds, while the sensitization for Tb3+ compounds is dominated by the energy transfer rates from the singlet state (S1).

High red luminescence intensity under sunlight exposure of a PMMA polymer doped with a tetrakis Eu3+ β-diketonate complex containing a benzimidazolium counterion

New tetrakis Eu3+ and Gd3+ β-diketonate complexes containing benzimidazolium (Bzim) as the counterion were synthesized by the one-pot method. The Bzim[Eu(tta)4]·H2O complex was further incorporated into a poly(methyl methacrylate) matrix (PMMA) at 1, 5, and 10% (w/w), which revealed highly desirable photonic features. The Eu3+ and Gd3+ complexes were characterized by elemental and thermal analyses, in addition to ESI-MS spectrometry, FTIR, and Raman spectroscopy. Single-crystal X-ray diffraction studies of the tetrakis Bzim[Eu(tta)4]·EtOH complex revealed that the Bzim+ counteraction and EtOH molecules exhibited several intermolecular interactions with very short hydrogen bond distances between two [Eu(tta)4]- anion units. The PMMA:(1%) Bzim[Eu(tta)4]-doped material was thermally stable up to 120 °C, which was close to the values found for the Eu3+-complex. Regarding the photoluminescence properties, either the Bzim[Eu(tta)4]·H2O or the doped films showed intense emission arising from the metal ion over a wide range of excitation wavelengths comprising UVA, UVB, and UVC regions. In addition, when the polymer films were exposed to sunlight radiation in an open external environment, the materials revealed a high Eu3+-centered red emission arising from the 5D0 → 7F J transition. The Bzim[Eu(tta)4]·H2O and Bzim[Eu(tta)4]·EtOH complexes showed high absolute quantum yields (Q L Eu) of 56% and 70%, respectively, whereas the doped polymer films displayed only ∼38%. All materials exhibited a highly red monochromatic emission characteristic. We believe that such luminescent systems could be promising photonic materials with a wide excitation range, including UVA, UVB, UVC, and sunlight, acting as efficient light-converting molecular devices (LCMDs).

Influence of Hydroxycoumarin Substituents on the Photophysical Properties of Chiroptical Tb(III) and Eu(III) Complexes

In this article, the synthesis, density functional theory (DFT) structural characterization, and spectroscopic investigation of chiral and heteroleptic Tb(III) and Eu(III) complexes are presented. These molecules are characterized by two different ligands: the enantiopure N,N'-bis(2-pyridylmethyl)-trans-1,2-diaminocyclohexane-N,N'-diacetic acid (H2bpcd) and a hydroxycoumarin-based ligand bearing different substituents in C(3) position (i.e., acetyl group in Coum, ethyl ester in CoumA, secondary and tertiary amides in CoumB and CoumC, respectively). The coumarin ligands exhibited different luminescence sensitization efficiency toward Tb(III) and Eu(III) ions in the related complexes of chemical formula [Ln(bpcd)(Coum)], [Ln(bpcd)(CoumA)], [Ln(bpcd)(CoumB)], [Ln(bpcd)(CoumC)]. Through theoretical calculations of intramolecular energy transfer (IET) processes (ligand-to-metal) in Eu(III) and Tb(III) complexes, along with quantum yield calculations, we provide a reasonable explanation for the observed differences in their luminescence properties. The nature of the coumarin ligand also affects the chiroptical properties of the Tb(III) complexes [i.e., circularly polarized luminescence (CPL) and electronic circular dichroism (ECD)].

Unusually Large Ligand Field Splitting in Anionic Europium(III) Complexes Induced by a Small Imidazolic Counterion

Luminescent trivalent lanthanide (Ln3+) complexes are compounds of technological interest due to their unique photophysical properties, particularly anionic tetrakis complexes, given their higher stability and emission quantum yields. However, structural studies on the cation-anion interaction in these complexes and the relation of such to luminescence are still lacking. Herein, the cation-anion interactions in two luminescent anionic tetrakis(2-thenoyltrifluoroacetonato)europate(III) complexes with alkylimidazolium cations, specifically 1-ethyl-3-methylimidazolium and 1-butyl-3-methylimidazolium are investigated. The Eu3+ complexes were synthesized and characterized by elemental analysis, mass spectrometry, and single-crystal X-ray crystallography, and their luminescence spectra were recorded at 77 K. Quantum chemical calculations were also performed. X-ray crystallography revealed hydrogen bonds between the enolate ligands and imidazolium ring hydrogens. The 1-butyl-3-methylimidazolium complex had two crystallographic Eu3+ sites, also confirmed by luminescence spectroscopy. The 1-ethyl-3-methylimidazolium complex exhibited an unusual 300 cm-1 splitting in the 5D0 → 7F1 transition, as reproduced by ligand field calculations, suggesting a stronger hydrogen bonding due to the smaller substituent. We hypothesize that this strong bonding likely causes angular distortions, resulting in high ligand field splittings.

Triple-Decker Hexaazamacrocyclic Lanthanide(III) Complexes: Structure, Magnetic Properties, and Temperature-Dependent Luminescence

The reaction of fluoride anions with mononuclear rare-earth(III) complexes of the hexaazamacrocycle derived from 2,6-diformylpyridine and ethylenediamine affords trinuclear coordination compounds [Ln3L3(μ2-F)4(NO3)2](NO3)3. The X-ray crystal structures of these complexes show triplex cationic complexes where the three roughly parallel macrocyclic lanthanide(III) units are linked by bis-μ2-F bridges. The detailed analysis of the photophysical properties of the [Eu3L3(μ2-F)4(NO3)2](NO3)3·2H2O and [Tb3L3(μ2-F)4(NO3)2](NO3)3·3H2O complexes reveals different temperature dependence of luminescence intensity and luminescence decay time of the Eu(III) and Tb(III) derivatives. The spectra of mixed species of average composition [Eu1.5Tb1.5L3(μ2-F)4(NO3)2](NO3)3·3H2O are in accordance with the ratiometric luminescent thermometer behavior. Measurements of the direct-current (dc) magnetic susceptibility of the [Dy3L3(μ2-F)4(NO3)2](NO3)3·2H2O complex indicate possible ferromagnetic interactions between the Dy(III) ions. Alternating current (ac) susceptibility measurements of this complex indicate single-molecule magnet behavior in zero dc field with magnetic relaxation dominated by Orbach mechanism and an effective energy barrier Ueff = 12.3 cm-1 (17.7 K) with a pre-exponential relaxation time, τ0 of 7.3 × 10-6 s. A similar reaction of mononuclear macrocyclic complexes with a higher number of fluoride equivalents results in polymeric {[Ln3L3(μ2-F)5](NO3)4}n complexes. The X-ray crystal structure of the Nd(III) derivative of this type shows trinuclear units that are additionally linked by single fluoride bridges to form a linear coordination polymer.

Effect of Pyrrolidinedithiocarbamate Ligand on the Luminescence Properties of Heteroligand Samarium and Europium Complexes: Experimental and Theoretical Study

The photophysical properties of two isostructural heteroligand lanthanide complexes of general formula Ln(pdtc)3(phen) (pdtc = pyrrolidinedithiocarbamate anion, phen = 1,10-phenanthroline), Ln = Sm3+ (1), Eu3+ (2)) were studied in solid state and dichloromethane (DCM) solution. The two lanthanide complexes were investigated by experimental techniques for structural (single-crystal X-ray diffraction analysis of 1, powder XRD, TG-DTA) and spectroscopic [electron paramagnetic resonance (EPR), infrared (IR), ultraviolet-visible (UV-vis), photoluminescence (PL)] characterization. DFT/TDDFT/ωB97xD and multireference SA-CASSCF/NEVPT2 calculations with perturbative spin-orbit coupling corrections were applied to construct the Jablonski energy diagrams and to discuss the excited state energy transfer mechanism with competing excited state processes and possible sensitized mechanism of metal-centered emission. The first excited state (S1) involved in the excited state energy transfer L(antenna)-to-Ln was predicted to have interligand (pdtc-to-phen) charge transfer character in contrast to the previously predicted ligand-to-metal charge transfer character. The theoretical consideration showed similar relaxation paths and luminescence quenching channels and appropriate Donor*(phen)-Acceptor*(Ln3+) energy gap for 1 and 2. The experimental measurements in the solid state, however, showed efficient luminescence and good ability to convert UV to visible light only for the Sm(pdtc)3(phen) complex. The minor emission of 2 was explained by partial reduction of Eu3+, confirmed by EPR and calculated electron density distribution data.

[Nd(NTA)2·H2O]3- complex with high-efficiency emission in NIR region

The distinctive photophysical characteristics possessed by lanthanides, including europium, neodymium, and ytterbium, render them adaptable molecular tools for studying biological systems. Specifically, their enduring photoluminescence, precise emission spectra, and significant Stokes shifts allow for experiments not achievable with organic fluorophores or fluorescent proteins. Moreover, the capacity of these metal ions for luminescence resonance energy transfer and photon upconversion extends the potential applications of lanthanide probes even further. In this research, a new [Nd(NTA)2·H2O]3- complex was synthesized and its optical properties were assessed using practical characterization techniques such as UV-Vis absorption, photoluminescence, and FTIR. It was discovered that when the sample was excited by a 357 nm wavelength, it emitted a strong line at 1076 nm with a full-width at half maximum (FWHM) of 10 nm, a phenomenon not previously documented. The Judd-Ofelt theory and its intensity parameters were utilized in a theoretical approach to determine the fluorescence branching ratio and the radiative lifetime of the [Nd(NTA)2·H2O]3- complex. The absorption and luminescence spectra were then analyzed accordingly. Experimental findings validated the potential applications of the prepared sample in bioimaging.

Insights into NdIII to YbIII Energy Transfer and Its Implications in Luminescence Thermometry

This work challenges the conventional approach of using NdIII 4F3/2 lifetime changes for evaluating the experimental NdIII → YbIII energy transfer rate and efficiency. Using near-infrared (NIR) emitting Nd:Yb mixed-metal coordination polymers (CPs), synthesized via solvent-free thermal grinding, we demonstrate that the NdIII [2H11/2 → 4I15/2] → YbIII [2F7/2 → 2F5/2] pathway, previously overlooked, dominates energy transfer due to superior energy resonance and J-level selection rule compatibility. This finding upends the conventional focus on the NdIII [4F3/2 → 4I11/2] → YbIII [2F7/2 → 2F5/2] transition pathway. We characterized Nd0.890Yb0.110(BTC)(H2O)6 as a promising cryogenic NIR thermometry system and employed our novel energy transfer understanding to perform simulations, yielding theoretical thermometric parameters and sensitivities for diverse Nd:Yb ratios. Strikingly, experimental thermometric data closely matched the theoretical predictions, validating our revised model. This novel perspective on NdIII → YbIII energy transfer holds general applicability for the NdIII/YbIII pair, unveiling an important spectroscopic feature with broad implications for energy transfer-driven materials design.

A combined experimental-molecular modeling study of crown ether europium complexes: Effects of the coordinated anion on structural and luminescence properties

It is reported the synthesis, characterization by elemental analysis, thermogravimetry; electronic absorption, infrared, excitation, and emission spectroscopies of the [Eu(12C4)(phen)2(X)n]X2 complexes, where 12C4 = 12-crown-4, phen = 1,10-phenanthroline, and X  = F-, Cl-, Br-, SCN-, ClO4-, and NO3-. It is verified that the polarizability of the anion X- exerts remarkable effects on the emission process. As a general trend, lower wavenumbers for the 7F0→5L6, 7F0→5D2 and 7F0→5D1 transitions are associated with the anions with higher volumes and, consequently, higher polarizability. The molecular modeling results performed with quantum methods (RHF and DFT) suggest some relationships between the calculated structures, electronic, and luminescence properties with the presence of the LMCT (ligand-to-metal charge transfer) states, which explains the differences in the emission spectra of these complexes due to the coordinated anion.

Synthesis, solid state characterization, theoretical and experimental spectroscopic studies of the new lanthanide complexes

Three new complexes Na[Ln(pic)₄]ּ⋅2.5H₂O (Ln = Tb, Eu or Gd; pic = picolinate) were synthesized and characterized by infrared spectroscopy, powder X-ray diffraction and thermogravimetric analyses. The molecular structures of the complexes have been determined by single-crystal X-ray diffraction. The three isostructural lanthanide complexes crystalize in the hexagonal system with space group P6₁22 to Eu complex and Gd complex and space group P6₅22 to Tb complex. In each of the complexes, the picolinate ligands are bonded to Ln³⁺ and Na⁺ ions by different coordination modes promoting polymeric structures. The photoluminescent properties of complexes were studied and combined with theoretical studies using the density functional theory (DFT: B3LYP, PBE1PBE) and the semiempirical method AM1/Sparkle from the single crystal X-ray diffraction structures to assign a suitable model for describing the system. The B3LYP DFT functional was considered the most adequate for providing structural properties of the compounds and for describing luminescence properties. The excited triplet states (T₁) and excited singlet states (S₁) of the ligand were determined theoretically using Time-dependent DFT calculations (TD-DFT: B3LYP, CAM-B3LYP and LC-wPBE) and INDO/S-CIS, with the best agreement with experimental values obtained from the LC-wPBE DFT functional. The photoluminescent spectra of the complexes and their lifetime measurements were determined indicating that the Eu complex and Tb complex display different intramolecular energy transfer mechanisms with higher efficiency to ligand-to-terbium energy transfer. In addition, the experimental and theorical Judd-Ofelt intensity parameters and quantum yields of the complexes were also determined and discussed besides to a proposed 9-state diagram to describe the luminescence properties of the Eu complex. The low value of emission quantum efficiency of ⁵D₀ emitting level of Eu(III) ion was explained by the presence of the ligand-to-metal charge transfer state (LMCT) evidenced experimentally and theoretically. A good agreement was obtained between the proposed kinetic model and experimental results showing the consistency of the set of rate equations assumed and the intramolecular pathways proposed.

Bifunctional Temperature and Oxygen Dual Probe Based on Anthracene and Europium Complex Luminescence

In this work, we synthesized a polydimethylsiloxane membrane containing two emitter groups chemically attached to the membrane structure. For this, we attached the anthracene group and the [Eu(bzac)₃] complex as blue and red emitters, respectively, in the matrix via hydrosilylation reactions. The synthesized membrane can be used as a bifunctional temperature and oxygen ratiometric optical probe by analyzing the effects that temperature changes and oxygen levels produce on the ratio of anthracene and europium(III) emission components. As a temperature probe, the system is operational in the 203-323 K range, with an observed maximum relative sensitivity of 2.06% K^-1 at 290 K and temperature uncertainties below 0.1 K over all the operational range. As an oxygen probe, we evaluated the ratiometric response at 25, 30, 35, and 40 °C. These results show an interesting approach to obtaining bifunctional ratiometric optical probes and also suggest the presence of an anthracene → europium(III) energy transfer, even though there is no chemical bonding between species.

Dynamics of the Energy Transfer Process in Eu(III) Complexes Containing Polydentate Ligands Based on Pyridine, Quinoline, and Isoquinoline as Chromophoric Antennae

In this work, we investigated from a theoretical point of view the dynamics of the energy transfer process from the ligand to Eu(III) ion for 12 isomeric species originating from six different complexes differing by nature of the ligand and the total charge. The cationic complexes present the general formula [Eu(L)(H₂O)₂]⁺ (where L = bpcd^2– = N,N′-bis(2-pyridylmethyl)-trans-1,2-diaminocyclohexane N,N′-diacetate; bQcd^2– = N,N′-bis(2-quinolinmethyl)-trans-1,2-diaminocyclohexane N,N′-diacetate; and bisoQcd^2– = N,N′-bis(2-isoquinolinmethyl)-trans-1,2-diaminocyclohexane N,N′-diacetate), while the neutral complexes present the Eu(L)(H₂O)₂ formula (where L = PyC3A^3– = N-picolyl-N,N′,N′-trans-1,2-cyclohexylenediaminetriacetate; QC3A^3– = N-quinolyl-N,N′,N′-trans-1,2-cyclohexylenediaminetriacetate; and isoQC3A^3– = N-isoquinolyl-N,N′,N′-trans-1,2-cyclohexylenediaminetriacetate). Time-dependent density functional theory (TD-DFT) calculations provided the energy of the ligand excited donor states, distances between donor and acceptor orbitals involved in the energy transfer mechanism (R_L), spin-orbit coupling matrix elements, and excited-state reorganization energies. The intramolecular energy transfer (IET) rates for both singlet-triplet intersystem crossing and ligand-to-metal (and vice versa) involving a multitude of ligand and Eu(III) levels and the theoretical overall quantum yields (ϕ_ovl) were calculated (the latter for the first time without the introduction of experimental parameters). This was achieved using a blend of DFT, Judd–Ofelt theory, IET theory, and rate equation modeling. Thanks to this study, for each isomeric species, the most efficient IET process feeding the Eu(III) excited state, its related physical mechanism (exchange interaction), and the reasons for a better or worse overall energy transfer efficiency (η_sens) in the different complexes were determined. The spectroscopically measured ϕ_ovl values are in good agreement with the ones obtained theoretically in this work.

Photophysical properties of 12 Eu(III) complexes with pyridine, quinoline, and isoquinoline ligands in aqueous solutions were elucidated and predicted through a theoretical protocol using a blend of DFT, Judd−Ofelt theory, intramolecular energy transfer theory, and coupled rate equation modeling calculations. The theoretical procedure is general and can be extended to any lanthanide-based complexes.

Tunable Optical Molecular Thermometers Based on Metallacrowns

The effect of ligands' energy levels on thermal dependence of lanthanide emission was examined to create new molecular nanothermometers. A series of Ln₂Ga₈L₈^'L₈^″ metallacrowns (shorthand Ln₂L₈^'), where Ln = Gd³⁺, Tb³⁺, or Sm³⁺ (H₃L' = salicylhydroxamic acid (H₃shi), 5-methylsalicylhydroxamic acid (H₃mshi), 5-methoxysalicylhydroxamic acid (H₃moshi), and 3-hydroxy-2-naphthohydroxamic acid (H₃nha)) and H₂L″ = isophthalic acid (H₂iph), was synthesized and characterized. Within the series, ligand-centered singlet state (S₁) energy levels ranged from 23,300 to 27,800 cm^-1, while triplet (T₁) energy levels ranged from 18,150 to 21,980 cm^-1. We demonstrated that the difference between T₁ levels and relevant energies of the excited ⁴G_5/2 level of Sm³⁺ (17,800 cm^-1) and ⁵D₄ level of Tb³⁺ (20,400 cm^-1) is the major parameter controlling thermal dependence of the emission intensity via the back energy transfer mechanism. However, when the energy difference between S₁ and T₁ levels is small (below 3760 cm^-1), the S₁ → T₁ intersystem crossing (and its reverse, S₁ ← T₁) mechanism contributes to the thermal behavior of metallacrowns. Both mechanisms affect Ln³⁺-centered room-temperature quantum yields with values ranging from 2.07(6)% to 31.2(2)% for Tb₂L₈^' and from 0.0267(7)% to 2.27(5)% for Sm₂L₈^'. The maximal thermal dependence varies over a wide thermal range (ca. 150-350 K) based on energy gaps between relevant ligand-based and lanthanide-based electronic states. By mixing Tb₂moshi₈^' with Sm₂moshi₈^' in a 1:1 ratio, an optical thermometer with a relative thermal sensitivity larger than 3%/K at 225 K was created. Other temperature ranges are also accessible with this approach.

Magneto-Induced Hyperthermia and Temperature Detection in Single Iron Oxide Core-Silica/Tb3+/Eu3+(Acac) Shell Nano-Objects

Multifunctional nano-objects containing a magnetic heater and a temperature emissive sensor in the same nanoparticle have recently emerged as promising tools towards personalized nanomedicine permitting hyperthermia-assisted treatment under local temperature control. However, a fine control of nano-systems' morphology permitting the synthesis of a single magnetic core with controlled position of the sensor presents a main challenge. We report here the design of new iron oxide core-silica shell nano-objects containing luminescent Tb³⁺/Eu³⁺-(acetylacetonate) moieties covalently anchored to the silica surface, which act as a promising heater/thermometer system. They present a single magnetic core and a controlled thickness of the silica shell, permitting a uniform spatial distribution of the emissive nanothermometer relative to the heat source. These nanoparticles exhibit the Tb³⁺ and Eu³⁺ characteristic emissions and suitable magnetic properties that make them efficient as a nanoheater with a Ln³⁺-based emissive self-referencing temperature sensor covalently coupled to it. Heating capacity under an alternating current magnetic field was demonstrated by thermal imaging. This system offers a new strategy permitting a rapid heating of a solution under an applied magnetic field and a local self-referencing temperature sensing with excellent thermal sensitivity (1.64%·K^-1 (at 40 °C)) in the range 25-70 °C, good photostability, and reproducibility after several heating cycles.

Spectroscopic aspects for the Yb3+ coordination compound with a large energy gap between the ligand and Yb3+ excited states

We present the experimental and theoretical results that made it possible to propose the energy transfer mechanism for a Yb complex with a large energy gap between the ligand and Yb excited states using a theoretical model and experimental data. Absorption and emission spectroscopy in the 300-4 K range is used for the study of the Yb³⁺ compound with N-phosphorylated sulfonamide (Na[YbL₄]), which, despite the large energy gap, is characterized by high emission sensitization efficiency (η_sens = 40%) and relatively long Yb³⁺ emission lifetime (27 μs). The crystal structure of Na[YbL₄], radiative lifetime (930 μs), refractive index (1.46), intrinsic (3.0%), and overall (1.3%) emission quantum yield were determined. To obtain the electronic properties of the Na[YbL₄], a time-dependent density functional theory (TD-DFT) was performed. The intramolecular energy transfer (IET) rates from the excited states S₁ and T₁ to the Yb³⁺ ion as well as between the ligand and the ligand-to-metal charge transfer (LMCT) states were calculated. Once the intersystem crossing S₁ → T₁ is not so effective due to a large energy gap between S₁ and T₁ (≈10000 cm^-1), it has been shown that the LMCT state acts as an additional channel to feed the T₁ state. Then, the T₁ can transfer energy to the Yb^{3+ 2}F_5/2 energy level (W_T), where W_T is dominated by the exchange mechanism. Based on IET and a rate equation model, the overall emission quantum yield Q^L_Ln was simulated with and without the LMCT, this also confirmed that the pathway S₁ → LMCT → T₁ → Yb³⁺ is more likely than the S₁ → T₁ → Yb³⁺ one.

Bis[3-(anthracen-9-yl)pentane-2,4-dionato-κ2 O,O'](N,N-di-methyl-formamide-κO)[tris-(pyrazol-1-yl-κN 2)hydroborato]europium(III)

The title compound, [Eu(C9H10BN6)(C19H15O2)2(C3H7NO)] or [TpEu(Anthracac)2(DMF)], was synthesized by reaction of a tris-(pyrazol-yl)borate (Tp-) Eu3+ complex with 3-(anthracen-9-yl)pentane-2,4-dione (HAnthracac) in the presence of tri-ethyl-amine. In the title compound, Eu3+ is located in an octa-vertex square-pyramidal coordination environment. In the two Anthracac- ligands, the anthracene and nearly planar acetyl-acetonate fragments are almost orthogonal. Neighboring mol-ecules of TpEu(Anthracac)2(DMF) are connected in the crystal by inter-molecular van der Waals inter-actions, while π-stacking inter-actions are limited to the edges of two anthracene rings.

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.

Surface Plasmon-Photon Coupling in Lanthanide-Doped Nanoparticles

Lanthanide-doped nanoparticles have great potential for energy conversion applications, as their optical properties can be precisely controlled by varying the doping composition, concentration, and surface structures, as well as through plasmonic coupling. In this Perspective we highlight recent advances in upconversion emission modulation enabled by coupling upconversion nanoparticles with well-defined plasmonic nanostructures. We emphasize fundamental understanding of luminescence enhancement, monochromatic emission amplification, lifetime tuning, and polarization control at nanoscale. The interplay between localized surface plasmons and absorbed photons at the plasmonic metal-lanthanide interface substantially enriches the interpretation of plasmon-coupled nonlinear photophysical processes. These studies will enable novel functional nanomaterials or nanostructures to be designed for a multitude of technological applications, including biomedicine, lasing, optogenetics, super-resolution imaging, photovoltaics, and photocatalysis.

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.

Synthesis, Structure Study, First-Principles Investigations and Luminescence Properties of Europium and Terbium Complexes

The synthesis of 1-benzyl-2-((2-Aminoethyl) amino)-5-oxopyrrolidine-3,4-diyl diacetate (boad), an oxopyrrolidine type ligand; designed to coordinate lanthanides (Eu³⁺ and Tb³⁺) to get luminescent material. The target complexes showed good photoluminescence properties, which indicate that this type of compound can be used as sensitizers having luminescence for the green (Tb³⁺) and red (Eu³⁺) emission. The obtained results revealed that sensitizer efficiency can be improved by adding ligands like acac (Eu(acac)₃, which has also enhanced the luminescence quantum output and period for Eu³⁺ ions. The ground state geometries were developed by using density functional theory at B3LYP/6-31G** level. The charge transfer analysis and electronic properties were performed. The Europium and Terbium complexes formation with boad ligand was explored based on molecular electrostatic potential, MDC-q charges, and frontier molecular orbitals (FMOs) analysis.

Diarylphosphate as a New Route for Design of Highly Luminescent Ln Complexes

Organophosphate-chloride complexes [{(2,6-ⁱPr₂C₆H₃-O)₂POO}₂LnCl(CH₃OH)₄]·2CH₃OH, Ln = Nd (1), Eu (2), Gd (3), and Tb (4) have been obtained and structurally characterized. Their reaction with 2,2':6',2″-terpyridine leads to the formation of 1:1 adducts ([{(2,6-ⁱPr₂C₆H₃-O)₂POO}₂LnCl(terpy)(H₂O)₂(CH₃OH)], Ln = Eu (5), Gd (6), Tb (7) with exception of Nd, where tris-diisopropylphenylphosphate complex [{(2,6-ⁱPr₂C₆H₃-O)₂POO}₃Nd) (terpy)(H₂O)(CH₃OH)] (8) was obtained due to the ligand metathesis. A bright luminescence observed for the Eu and Tb organophosphate complexes is the first example of an application of organophosphate ligands for 4f-ions luminescence sensitization. Photophysical properties of all complexes were analyzed by optical spectroscopy and an energy transfer scheme was discussed. A combination of two types of ligands into the coordination sphere (phosphate and phenanthroline) allows designing the Eu surrounding with very high intrinsic quantum yield QEuEu (0.92) and highly luminescent Ln complexes for both visible and near-infrared (NIR) regions.