Mono- and Mixed Metal Complexes of Eu3+, Gd3+, and Tb3+ with a Diketone,Bearing Pyrazole Moiety and CHF2-Group: Structure,Color Tuning,and Kinetics of Energy Transfer between Lanthanide Ions

Three novel lanthanide complexes with the ligand 4,4-difluoro-1-(1,5-dimethyl-1H-pyrazol-4-yl)butane-1,3-dione (HL), namely [LnL₃(H₂O)₂], Ln = Eu, Gd and Tb, were synthesized, and, according to single-crystal X-ray diffraction, are isostructural. The photoluminescent properties of these compounds, as well as of three series of mixed metal complexes [Eu_xTb_1-xL₃(H₂O)₂] (Eu_xTb_1-xL₃), [EuxGd_1-xL₃(H₂O)₂] (Eu_xGd_1-xL₃), and [Gd_xTb_1-xL₃(H₂O)₂] (Gd_xTb_1-xL₃), were studied. The Eu_xTb_1-xL₃ complexes exhibit the simultaneous emission of both Eu³⁺ and Tb³⁺ ions, and the luminescence color rapidly changes from green to red upon introducing even a small fraction of Eu³⁺. A detailed analysis of the luminescence decay made it possible to determine the observed radiative lifetimes of Tb³⁺ and Eu³⁺ and estimate the rate of excitation energy transfer between these ions. For this task, a simple approximation function was proposed. The values of the energy transfer rates determined independently from the luminescence decays of terbium(III) and europium(III) ions show a good correlation.     

Highly Luminescent,Water‐Soluble Lanthanide Fluorobenzoates: Syntheses,Structures and Photophysics,Part I: Lanthanide Pentafluorobenzoates

Highly luminescent, photostable, and soluble lanthanide pentafluorobenzoates have been synthesized and thoroughly characterized, with a focus on Eu^III and Tb^III complexes as visible emitters and Nd^III, Er^III, and Yb^III complexes as infrared emitters. Investigation of the crystal structures of the complexes in powder form and as single crystals by using X‐ray diffraction revealed five different structural types, including monomeric, dimeric, and polymeric. The local structure in different solutions was studied by using X‐ray absorption spectroscopy. The photoluminescence quantum yields (PLQYs) of terbium and europium complexes were 39 and 15 %, respectively; the latter value was increased almost twice by using the heterometallic complex [Tb_0.5Eu_0.5(pfb)₃(H₂O)] (Hpfb=pentafluorobenzoic acid). Due to the effectively utilized sensitization strategy (pfb)^?→Tb→Eu, a pure europium luminescence with a PLQY of 29 % was achieved.     

Luminescence of Eu3+ and Tb3+ Complexes of Two Macrobicyclic Ligands Derived from a Tetralactam Ring and a Chromophoric Antenna

Two macrobicyclic ligands derived from an 18‐membered tetralactam ring and 2,2′‐bipyridine or 2,6‐bis(pyrazol‐1‐yl)pyridine moieties, 1 and 2 , respectively, form stable complexes with Gd^III, Eu^III, and Tb^III ions in aqueous solution. The ligand‐based luminescence is retained in the Gd^III cryptates, whereas this radiative deactivation is quenched in the Eu^III and Tb^III cryptates by ligand‐to‐metal energy transfer, resulting in the usual metal‐centered emission spectra. Singlet‐ and triplet‐state energies, emission‐decay lifetimes, and luminescence yields were measured. [Tb⊂ 1 ]³⁺ cryptate shows a long luminescence lifetime (τ=1.12 ms) and a very high metal luminescence quantum yield (Φ=0.25) in comparison with those reported in the literature for Tb³⁺ complexes sensitized by a bipyridine chromophore. By comparison to [Ln⊂ 1 ]³⁺, [Ln⊂ 2 ]³⁺ presents markedly lower luminescence properties, due to worse interaction between the 2,6‐bis(pyrazol‐1‐yl)pyridine unit and the metal ion. Moreover, the luminescent metal and the triplet ligand energy levels of [Eu⊂ 2 ]³⁺ do not match. The effects of H₂O molecules coordinated to the metal centre and of thermally activated decay processes on nonradiative deactivation to the ground‐state are also reported.     

The Series of Rare Earth Complexes [Ln2Cl6(μ‐4,4′‐bipy)(py)6], Ln=Y,Pr, Nd,Sm‐Yb: A Molecular Model System for Luminescence Properties in MOFs Based on LnCl3 and 4,4′‐Bipyridine

A series of 12 dinuclear complexes [Ln₂Cl₆(μ‐4,4′‐bipy)(py)₆], Ln=Y, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, ( 1 – 12 , respectively) was synthesized by an anhydrous solvothermal reaction in pyridine. The complexes contain a 4,4′‐bipyridine bridge and exhibit a coordination sphere closely related to luminescent lanthanide MOFs based on LnCl₃ and 4,4‐bipyridine. The dinuclear complexes therefore function as a molecular model system to provide a better understanding of the luminescence mechanisms in the Ln‐N‐MOFs ${\hbox{}{{\hfill 2\atop \hfill \infty }}}$ [Ln₂Cl₆(4,4′‐bipy)₃] ? 2(4,4′‐bipy). Accordingly, the luminescence properties of the complexes with Ln=Y, Sm, Eu, Gd, Tb, Dy, ( 1 , 4 – 8 ) were determined, showing an antenna effect through a ligand–metal energy transfer. The highest efficiency of luminescence is observed for the terbium‐based compound 7 displaying a high quantum yield (QY of 86 %). Excitation with UV light reveals typical emission colors of lanthanide‐dependent intra 4f–4f‐transition emissions in the visible range (Tb^III: green, Eu^III: red, Sm^III: salmon red, Dy^III: yellow). For the Gd^III‐ and Y^III‐containing compounds 6 and 1 , blue emission based on triplet phosphorescence is observed. Furthermore, ligand‐to‐metal charge‐transfer (LMCT) states, based on the interaction of Cl^? with Eu^III, were observed for the Eu^III compound 5 including energy‐transfer processes to the Eu^III ion. Altogether, the model complexes give further insights into the luminescence of the related MOFs, for example, rationalization of Ln‐independent quantum yields in the related MOFs.     

Hetero‐tri‐spin [2p‐3d‐4f] Chain Compounds Based on Nitronyl Nitroxide Lanthanide Metallo‐Ligands: Synthesis,Structure, and Magnetic Properties

Employing nitronyl nitroxide lanthanide(III) complexes as metallo‐ligands allowed the efficient and highly selective preparation of three series of unprecedented hetero‐tri‐spin (Cu?Ln‐radical) one‐dimensional compounds. These 2p–3d–4f spin systems, namely [Ln₃Cu(hfac)₁₁(NitPhOAll)₄] (Ln^III=Gd 1_Gd , Tb 1_Tb , Dy 1_Dy ; NitPhOAll=2‐(4′‐allyloxyphenyl)‐4,4,5,5‐tetramethylimidazoline‐1‐oxyl‐3‐oxide), [Ln₃Cu(hfac)₁₁(NitPhOPr)₄] (Ln^III=Gd 2_Gd , Tb 2_Tb , Dy 2_Dy , Ho 2_Ho , Yb 2_Yb ; NitPhOPr=2‐(4′‐propoxyphenyl)‐4,4,5,5‐tetramethyl‐imidazoline‐1‐oxyl‐3‐oxide) and [Ln₃Cu(hfac)₁₁(NitPhOBz)₄] (Ln^III=Gd 3_Gd , Tb 3_Tb , Dy 3_Dy ; NitPhOBz=2‐(4′‐benzyloxyphenyl)‐4,4,5,5‐tetramethyl‐imidazoline‐1‐oxyl‐3‐oxide) involve O‐bound nitronyl nitroxide radicals as bridging ligands in chain structures with a [Cu‐Nit‐Ln‐Nit‐Ln‐Nit‐Ln‐Nit] repeating unit. The dc magnetic studies show that ferromagnetic metal–radical interactions take place in these hetero‐tri‐spin chain complexes, these and the next‐neighbor interactions have been quantified for the Gd derivatives. Complexes 1_Tb and 2_Tb exhibit frequency dependence of ac magnetic susceptibilities, indicating single‐chain magnet behavior.     

Lanthanide(III) Complexes of Bis‐semicarbazone and Bis‐imine‐Substituted Phenanthroline Ligands: Solid‐State Structures,Photophysical Properties,and Anion Sensing

Phenanthroline‐based hexadentate ligands L¹ and L² bearing two achiral semicarbazone or two chiral imine moieties as well as the respective mononuclear complexes incorporating various lanthanide ions, such as La^III, Eu^III, Tb^III, Lu^III, and Y^III metal ions, were synthesized, and the crystal structures of [ML¹Cl₃] (M=La^III, Eu^III, Tb^III, Lu^III, or Y^III) complexes were determined. Solvent or water molecules act as coligands for the rare‐earth metals in addition to halide anions. The big Ln^III ion exhibits a coordination number (CN) of 10, whereas the corresponding Eu^III, Tb^III, Lu^III, and Y^III centers with smaller ionic radii show CN=9. Complexes of L², namely [ML²Cl₃] (M=Eu^III, Tb^III, Lu^III, or Y^III) ions could also be prepared. Only the complex of Eu^III showed red luminescence, whereas all the others were nonluminescent. The emission properties of the Eu derivative can be applied as a photophysical signal for sensing various anions. The addition of phosphate anions leads to a unique change in the luminescence behavior. As a case study, the quenching behavior of adenosine‐5′‐triphosphate (ATP) was investigated at physiological pH value in an aqueous solvent. A specificity of the sensor for ATP relative to adenosine‐5′‐diphosphate (ADP) and adenosine‐5′‐monophosphate (AMP) was found. ³¹P NMR spectroscopic studies revealed the formation of a [EuL²(ATP)] coordination species.     

Excited State Properties of Lanthanide(III) Complexes with a Nonadentate Bispidine Ligand

Eu^III, Tb^III, Gd^III and Yb^III complexes of the nonadentate bispidine derivative L² (bispidine=3,7-diazabicyclo[3.3.1]nonane) were successfully synthesized and their emission properties studied. The X-ray crystallography reveals full encapsulation by the nonadentate ligand L² that enforces to all Ln^III cations a common highly symmetrical capped square antiprismatic (CSAPR) coordination geometry (pseudo C_4v symmetry). The well-resolved identical emission spectra in solid state and in solution confirm equal structures in both media. As therefore expected, this results in long-lived excited states and high emission quantum yields ([Eu^IIIL²]⁺, H₂O, 298 K, τ=1.51 ms, ϕ=0.35; [Tb^IIIL²]⁺, H₂O, 298 K, τ=1.95 ms, ϕ=0.68). Together with the very high kinetic and thermodynamic stabilities, these complexes are a possible basis for interesting biological probes.     

Luminescence Tuning and White‐Light Emission of Co‐doped Ln–Cd–Organic Frameworks

Four new three‐dimensional isostructural lanthanide–cadmium metal–organic frameworks (Ln–Cd MOFs), [LnCd₂(imdc)₂(Ac)(H₂O)₂]?H₂O (Ln=Pr ( 1 ), Eu ( 2 ), Gd ( 3 ), and Tb ( 4 ); H₃imdc=4,5‐imidazoledicarboxylic acid; Ac=acetate), have been synthesized under hydrothermal conditions and characterized by IR, elemental analyses, inductively coupled plasma (ICP) analysis, and X‐ray diffraction. Single‐crystal X‐ray diffraction shows that two Ln^III ions are surrounded by four Cd^II ions to form a heteronuclear building block. The blocks are further linked to form 3D Ln–Cd MOFs by the bridging imdc^3? ligand. Furthermore, the left‐ and right‐handed helices array alternatively in the lattice. Eu–Cd and Tb–Cd MOFs can emit characteristic red light with the Eu^III ion and green light with the Tb^III ion, respectively, while both Gd–Cd and Pr–Cd MOFs generate blue emission when they are excited. Different concentrations of Eu³⁺ and Tb³⁺ ions were co‐doped into Gd–Cd/Pr–Cd MOFs, and tunable luminescence from yellow to white was achieved. White‐light emission was obtained successfully by adjusting the excitation wavelength or the co‐doping ratio of the co‐doped Gd–Cd and Pr–Cd MOFs. These results show that the relative emission intensity of white light for Gd–Cd:Eu³⁺,Tb³⁺ MOFs is stronger than that of Pr–Cd:Eu³⁺,Tb³⁺ MOFs, which implies that the Gd complex is a better matrix than the Pr complex to obtain white‐light emission materials.     

Syntheses,Structures, Photoluminescence,and Magnetism of a Series of Discrete Lanthanide Complexes based on 2,5‐Dichlorobenzoate and 1,10‐Phenanthroline

The syntheses and crystal structures of eight lanthanide complexes with formula [Ln(2,5‐DCB)_x(phen)_y] are reported, which are characterized via single‐crystal, powder X‐ray diffraction, elemental analysis, IR spectroscopy, thermogravimetric analysis, photoluminescence measurement, and DC/AC magnetic measurement. These eight complexes are isostructural, and possess a discrete dinuclear structure. The adjacent dinuclear molecules are linked by the hydrogen bonding interactions into a one‐dimensional (1D) supramolecular chain. The neighboring 1D chains are further extended into a two‐dimensional (2D) supramolecular layer by the π–π stacking interactions. The photoluminescent properties of complexes 1 (Nd^III), 2 (Sm^III), 3 (Eu^III), 5 (Tb^III), 6 (Dy^III), and 8 (Yb^III) were investigated. Magnetic investigations also reveal the presence of ferromagnetic interactions in complexes 4 (Gd^III), 6 (Dy^III), and 7 (Er^III). Additionally, complex 6 (Dy^III) demonstrates field‐induced slow magnetic relaxation behavior.     

Metal‐Ion Sensing Europium(III) Complexes with Bidentate Phosphine Oxide Ligands Containing a 2,2′‐Bipyridine Framework

Novel Eu^III complexes with bidentate phosphine oxide ligands containing a bipyridine framework, i.e., [3,3′‐bis(diphenylphosphoryl)‐2,2′‐bipyridine]tris(hexafluoroacetylacetonato)europium(III) ([Eu(hfa)₃(BIPYPO)]) and [3,3′‐bis(diphenylphosphoryl)‐6,6′‐dimethyl‐2,2′‐bipyridine]tris(hexafluoroacetylacetonato)europium(III) ([Eu(hfa)₃(Me‐BIPYPO)]), were synthesized for lanthanide‐based sensor materials having high emission quantum yields and effective chemosensing properties. The emission quantum yields of [Eu(hfa)₃(BIPYPO)] and [Eu(hfa)₃(Me‐BIPYPO)] were 71 and 73%, respectively. Metal‐ion sensing properties of the Eu^III complexes were also studied by measuring the emission spectra of Eu^III complexes in the presence of Zn^II or Cu^II ions. The metal‐ion sensing and the photophysical properties of luminescent Eu^III complexes with a bidentate phosphine oxide containing 2,2′‐bipyridine framework are demonstrated for the first time.     

Synthesis,Structure, and Magnetic Properties of a Series of Dinuclear Lanthanide Complexes Assembled by Acetate and a Schiff Base Ligand

Five dinuclear lanthanide complexes [Ln₂L₂(NO₃)₂(OAc)₄] · 2CH₃CN [Ln = Gd ( 1 ), Tb ( 2 ), Dy ( 3 ), Ho ( 4 ), and Er ( 5 )] [L = 2‐((2‐pyridinylmethylene)hydrazine)ethanol] were synthesized from the reactions of Ln(NO₃)₃ · 6H₂O with L and CH₃COOH in the presence of triethylamine. Their crystal structures were determined. They show similar dinuclear cores with the two lanthanide ions bridged by four acetate ligands in the μ₂‐η¹:η² and μ₂‐η¹:η¹ bridging modes. Each Ln^III ion in complexes 1 – 5 is further chelated by one L ligand and one nitrate ion, leading to the formation of a nine‐coordinated mono‐capped square antiprism arrangement. The dinuclear molecules in 1 – 5 are consolidated by hydrogen bonds and π ··· π stacking interactions to build a two‐dimensional sheet. Their magnetic properties were investigated. It revealed antiferromagnetic interactions between the Gd^III ions in 1 and ferromagnetic interactions between the Tb^III ions in 2 . The profiles of χ_mT vs. T curves of 3 – 5 reveal that the magnetic properties of 3 – 5 are probably dominated by the thermal depopulation of the Stark sublevels of Ln^III ions.     

Triboluminescence of Lanthanide Coordination Polymers with Face‐to‐Face Arranged Substituents

Luminescence upon the grinding of solid materials (triboluminescence, TL) has long been a puzzling phenomenon in natural science and has also attracted attention because of its broad application in optics. It has been generally considered that the TL spectra exhibit similar profiles as those of photoluminescence (PL), although they occur from distinct stimuli. Herein, we describe for the first time a large spectral difference between these two physical phenomena using lanthanide^III coordination polymers with efficient TL and PL properties. They are composed of emission centers (Tb^III and Eu^III ions), antenna (hexafluoroacetylacetonate=hfa), and bridging ligands (2,5‐bis(diphenylphosphoryl)furan=dpf). The emission color upon grinding (yellow TL) is clearly different from that upon UV irradiation (reddish‐orange PL) in Tb^III/Eu^III‐mixed coordination polymers [Tb,Eu(hfa)₃(dpf)]_n (Tb/Eu=1). The results directly indicate the discrete excitation processes of PL and TL.     

Three Phenoxo‐Bridged Dinuclear Lanthanide Complexes: Syntheses,Crystal Structures,and Magnetic Properties

Three dinuclear lanthanide complexes [Ln₂(H₂L)₂(NO₃)₄] [Ln = Dy ( 1 ), Tb ( 2 ), and Gd ( 3 )] [H₃L = 2‐hydroxyimino‐N′‐[(2‐hydroxy‐3‐methoxyphenyl)methylidene]‐propanohydrazone] were solvothermally synthesized by varying differently anisotropic rare earth ions. Single‐crystal structural analyses demonstrate that all the three complexes are crystallographically isostructural with two centrosymmetric Ln^III ions aggregated by a pair of monodeprotonated H₂L^– anions. Weak intramolecular antiferromagnetic interactions with different strength were mediated by a pair of phenoxo bridges due to superexchange and/or single‐ion anisotropy. Additionally, the Dy^III‐based entity shows the strongest anisotropy exhibits field‐induced single‐molecule magnetic behavior with two thermally activated relaxation processes. In contrast, 3 with isotropic Gd^III ion has a significant cryogenic magnetocaloric effect with the maximum entropy change of 25.7 J · kg^–1 · K^–1 at 2.0 K and 70.0 kOe.     

Crystal Structures of the Lanthanum(III), Europium(III), and Terbium(III) Cryptates of Tris(bipyridine) Macrobicyclic Ligands

The crystal structures of the La^III, Eu^III, and Tb^III complexes of macrobicyclic [bpy.bpy.bpy] ligands, [La³⁺ ? 1 ]3 Cl^? ( = 3- La), [Tb³⁺ ? 1 ]3 Cl^? ( = 3- Tb), and [Eu³⁺ ? 2 ]3 C1^? ( = 3- Eu), have been determined. They confirm the cryptate nature of these species, the cations being bound to the eight N-sites of the ligand. The macrobicycle presents two open faces, thus allowing additional coordination of two species, Cl^? ions or H₂O molecules, to the bound cations. These data provide structural support for the photophysical studies of the luminescent properties of the Eu^III and Tb^III cryptates, which indicated residual coordination of H₂O molecules.     

Unprecedented Sensitization of Visible and Near‐Infrared Lanthanide Luminescence by Using a Tetrathiafulvalene‐Based Chromophore

Ligand L was synthesized and then coordinated to [Ln(hfac)₃] ? 2 H₂O (Ln^III=Tb, Dy, Er; hfac^?=1,1,1,5,5,5‐hexafluoroacetylacetonate anion) and [Ln(tta)₃]?2 H₂O (Ln^III=Eu, Gd, Tb, Dy, Er, Yb; tta^?=2‐thenoyltrifluoroacetonate) to give two families of dinuclear complexes [Ln₂(hfac)₆( L )] ? C₆H₁₄ and [Ln₂(tta)₆( L )] ? 2 CH₂Cl₂. Irradiation of the ligand at 37 040 cm^?1 and 29 410 cm^?1 leads to tetrathiafulvalene‐centered and 2,6‐di(pyrazol‐1‐yl)‐4‐pyridine‐centered fluorescence, respectively. The ligand acts as an organic chromophore for the sensitization of the infrared Er^III (6535 cm^?1) and Yb^III (10 200 cm^?1) luminescence. The energies of the singlet and triplet states of L are high enough to guarantee an efficient sensitization of the visible Eu^III luminescence (17 300–14 100 cm^?1). The Eu^III luminescence decay can be nicely fitted by a monoexponential function that allows a lifetime estimation of (0.49±0.01) ms. Finally, the magnetic and luminescence properties of [Yb₂(hfac)₆( L )] ? C₆H₁₄ were correlated, which allowed the determination of the crystal field splitting of the ²F_7/2 multiplet state with M_J=±1/2 as ground states.     

A New Tricarboxylate‐Terbium(III) Compound: Synthesis,Structure, and Luminescent Property

The Tb^III compound [Tb(tci)(H₂O)]_n · n(DMF) ( 1 ) [H₃tci = tri(2‐carboxythyl)isocyanurate, DMF = N,N′‐dimethylformamide] was synthesized by the reaction of terbium oxide, H₃tci, and two drops of concentrated nitric acid in the presence of DMF and H₂O. Single crystal X‐ray analysis reveals that it features a three‐dimensional (3D) framework based on infinite –Tb–COO–Tb– chains. The tci ligand in 1 links six different Tb^III ions with its two carboxylate groups in μ₂‐κ¹O;κ²O,O′ mode and the third in μ₂‐κ¹O;κ¹O′ mode. Thermal analysis reveals that it remains high thermal stability until 390 °C. Luminescence investigation shows that it emits characteristic green light of Tb^III ions.     

Luminescence of Salen Lanthanide Bimetallic Complexes: Dual Emission and Energy Transfer

The luminescence properties of the tetranuclear bimetallic lanthanide complexes Sm₂Eu₂ ( 1 ) and Eu₂Tb₂ ( 2 ), were compared with those of the analogous homometallic complexes [Sm₄(μ₃‐OH)₂(salen)₂(acac)₆(CH₃OH)₂] · CH₃OH ( 3 ) and [Eu₄(μ₃‐OH)₂(salen)₂(acac)₆(CH₃OH)₂] ( 4 ) [H₂salen = N, N′‐ethylenebis(salicylideneimine), Hacac = acetylacetonate]. X‐ray crystallographic analysis reveals that complexes 3 and 4 have planar tetranuclear structures. For the Eu₂Tb₂ configurational isomer, the Tb^III ion in complex 2 mainly serves as a sensitizer. The quantum yields and lifetime measurements for 2 support the premise that Ln/Ln energy transfer occurs in such lanthanide bimetallic complexes, along with the usual ligand‐to‐metal triplet energy pathways. Complexes 3 and 4 exhibit the characteristic metal‐centered emission.     

A Lanthanide‐Complex‐Based Ratiometric Luminescent Probe Specific for Peroxynitrite

A lanthanide‐complex‐based ratiometric luminescence probe specific for peroxynitrite (ONOO^?), 4′‐(2,4‐dimethoxyphenyl)‐2,2′:6′,2′′‐terpyridine‐6,6′′‐diyl]bis(methylenenitrilo)tetrakis(acetate)‐Eu³⁺/Tb³⁺ ([Eu³⁺/Tb³⁺(DTTA)]), has been designed and synthesized. Both [Eu³⁺(DTTA)] and [Tb³⁺(DTTA)] are highly water soluble with large stability constants at ≈10²⁰, and strongly luminescent with luminescence quantum yields of 10.0 and 9.9 %, respectively, and long luminescence lifetimes of 1.38 and 0.26 ms, respectively. It was found that the luminescence of [Tb³⁺(DTTA)] could be quenched by ONOO^? rapidly and specifically in aqueous buffers, while that of [Eu³⁺(DTTA)] did not respond to the addition of ONOO^?. Thus, by simply mixing [Eu³⁺(DTTA)] and [Tb³⁺(DTTA)] in an aqueous buffer, a ratiometric luminescence probe specific for time‐gated luminescence detection of ONOO^? was obtained. The performance of [Tb³⁺(DTTA)] and [Eu³⁺/Tb³⁺(DTTA)] as the probes for luminescence imaging detection of ONOO^? in living cells was investigated. The results demonstrated the efficacy and advantages of the new ratiometric luminescence probe for highly sensitive luminescence bioimaging application.     

Experimental and DFT Evidence for the Fractional Non‐Innocence of a β‐Diketonate Ligand

New compounds [Ru(pap)₂(L)](ClO₄), [Ru(pap)(L)₂], and [Ru(acac)₂(L)] (pap=2‐phenylazopyridine, L^?=9‐oxidophenalenone, acac^?=2,4‐pentanedionate) have been prepared and studied regarding their electron‐transfer behavior, both experimentally and by using DFT calculations. [Ru(pap)₂(L)](ClO₄) and [Ru(acac)₂(L)] were characterized by crystal‐structure analysis. Spectroelectrochemistry (EPR, UV/Vis/NIR), in conjunction with cyclic voltammetry, showed a wide range of about 2 V for the potential of the Ru^III/II couple, which was in agreement with the very different characteristics of the strongly π‐accepting pap ligand and the σ‐donating acac^? ligand. At the rather high potential of +1.35 V versus SCE, the oxidation of L^? into L^. could be deduced from the near‐IR absorption of [Ru^III(pap)(L^.)(L^?)]²⁺. Other intense long‐wavelength transitions, including LMCT (L^?→Ru^III) and LL/CT (pap^.?→L^?) processes, were confirmed by TD‐DFT results. DFT calculations and EPR data for the paramagnetic intermediates allowed us to assess the spin densities, which revealed two cases with considerable contributions from L‐radical‐involving forms, that is, [Ru^III(pap⁰)₂(L^?)]²⁺?[Ru^II(pap⁰)₂(L^.)]²⁺ and [Ru^III(pap⁰)(L^?)₂]⁺?[Ru^II(pap⁰)(L^?)(L^?)]⁺. Calculations of electrogenerated complex [Ru^II(pap^.?)(pap⁰)(L^?)] displayed considerable negative spin density (?0.188) at the bridging metal.     

Exchange coupling and magnetic anisotropy in a family of bipyrimidyl radical‐bridged dilanthanide complexes: Density functional theory and ab initio calculations

The origin of the magnetic anisotropy energy barriers in a series of bpym^? (bpym = 2,2′‐bipyrimidine) radical‐bridged dilanthanide complexes [(Cp*₂Ln)₂(μ‐bpym)]⁺ [Cp* = pentamethylcyclopentadienyl; Ln = Gd^III ( 1 ), Tb^III ( 2 ), Dy^III ( 3 ), Ho^III ( 4 ), Er^III ( 5 )] has been explored using density functional theory (DFT) and ab initio methods. DFT calculations show that the exchange coupling between the two lanthanide ions for each complex is very weak, but the antiferromagnetic Ln‐bpym^? couplings are strong. Ab initio calculations show that the effective energy barrier of 2 or 3 mainly comes from the contribution of a single Tb^III or Dy^III fragment, which is only about one third of a single Ln energy barrier. For 4 or 5 , however, both of the two Ho^III or Er^III fragments contribute to the total energy barrier. Thus, it is insufficient to only increase the magnetic anisotropy energy barrier of a single Ln ion, while enhancing the Ln‐bpym^? couplings is also very important. © 2014 Wiley Periodicals, Inc.