Experimental and theoretical approaches toward anion-responsive tripod-lanthanide complexes: mixed-donor ligand effects on lanthanide complexation and luminescence sensing profiles

A new series of tripods were designed to form anion-responsive, luminescent lanthanide complexes. These tripods contain pyridine, thiazole, pyrazine, or quinoline chromophores combined with amide carbonyl oxygen and tertiary nitrogen atoms. Crystallographic and EXAFS studies of the 10-coordinated tripod-La(NO(3))(3) complexes revealed that each La(3+) cation was cooperatively coordinated by one tetradentate tripod and three bidentate NO(3)(-) anions in the crystal and in CH(3)CN. Quantum chemical calculations indicated that the aromatic nitrogen plays a significant role in lanthanide complexation. The experimentally determined stability constants of complexes of the tripod with La(NO(3))(3), Eu(NO(3))(3), and Tb(NO(3))(3) were in good agreement with the theoretically calculated interaction energies. Complexation of each tripod with lanthanide triflate gave a mixture of several lanthanide complex species. Interestingly, the addition of a coordinative NO(3)(-) or Cl(-) anion to the mixture significantly influenced the lanthanide complexation profiles. The particular combination of tripod and a luminescent Eu(3+) center gave anion-selective luminescence enhancements. Pyridine-containing tripods exhibited the highest NO(3)(-) anion-selective luminescence and thus permit naked-eye detection of the NO(3)(-) anion.     

Lanthanide polyoxocationic complexes: experimental and theoretical stability studies and Lewis acid catalysis

The [ε-PMo(V)(8)Mo(VI)(4)O(36)(OH)(4){Ln(III)(H(2)O)}(4)](5+) (Ln=La, Ce, Nd, Sm) polyoxocations, called εLn(4), have been synthesized at room temperature as chloride salts soluble in water, MeOH, EtOH, and DMF. Rare-earth metals can be exchanged, and (31)P NMR spectroscopic studies have allowed a comparison of the affinity of the reduced {ε-PMo(12)} core, thus showing that the La(III) ions have the highest affinity and that rare earths heavier than Eu(III) do not react with the ε-Keggin polyoxometalate. DFT calculations provide a deeper insight into the geometries of the systems studied, thereby giving more accurate information on those compounds that suffer from disorder in crystalline form. It has also been confirmed by the hypothetical La→Gd substitution reaction energy that Ln ions beyond Eu cannot compete with La in coordinating the surface of the ε-Keggin molybdate. Two of these clusters (Ln=La, Ce) have been tested to evidence that such systems are representative of a new efficient Lewis acid catalyst family. This is the first time that the catalytic activity of polyoxocations has been evaluated.     

Experimental and theoretical study of the photophysics and structures of europium cryptates incorporating 3,3'-bi-isoquinoline-2,2'-dioxide.

A detailed photophysical study of [Eu within (biqO2.2.2)(CF3SO3)](CF3SO3)2. CH3CN.H2O (Eu within 1) and two other types of cryptates incorporating three 3,3'-biisoquinoline-2,2'-dioxide units has been performed. Structural crystallographic data of Eu within 1, electronic structure calculations and theoretical models were used to obtain the intramolecular energy transfer rates and the appropriate set of rate equations, which was solved numerically. Quantum yields and decay lifetimes were obtained from these results and compared to the experimental data. The role of the ligand-to-metal charge transfer (LMCT) states was ascertained. A theoretical ligand field and intensity analysis was carried out and the results agree very well with the emission spectra. The molecular structures of the lanthanide cryptates were successfully modelled by the YIII ion using the restricted Hartree-Fock (RHF) method, with the advantage of dealing with closed-shell systems. These molecular structures were used to explain the drastic differences in the photophysics of the three EuIII cryptates.     

Gas-Phase Structures of Potassium Tetrakis(hexafluoro- acetylacetonato) Lanthanide(III) Complexes [KLn(C5HF6O2)4] (Ln=La,Gd, Lu)

The molecular structures of potassium tetrakis(hexafluoroacetylacetonato)lanthanide(III) complexes [KLn(hfa)₄] (Ln=La, Gd, Lu; hfa=C₅HF₆O₂,) were studied by synchronous gas-phase electron diffraction/mass spectrometry (GED/MS) supported by quantum-chemical (DFT/PBE0) calculations. The compounds sublime congruently and the vapors contain a single molecular species: the heterobinuclear complex [KLn(hfa)₄]. All molecules are of C₁ symmetry with the lanthanide atom in the center of an LnO₈ coordination polyhedron, while the potassium atom is coordinated by three ligands with formation of three K−O and three K−F bonds. One hfa ligand is not bonded to the potassium atom. Topological analysis of the electron-density distributions confirmed the existence of ionic-type K−O and K−F bonding. The structures of the free [KLn(hfa)₄] molecules are compared with those of the related compounds [KDy(hfa)₄] and [KEr(hfa)₄] in their crystalline state. The complex nature of the chemical bonding is discussed on the basis of electron-density topology analyses.     

A New Mode of Energy Transfer between Mn2+ and Eu2+ in Nitride‐Based Phosphor SrAlSi4N7 with Tunable Light and Excellent Thermal Stability

In this work, reciprocal energy transfer between Mn²⁺ and Eu²⁺ ions in nitride SrAlSi₄N₇ has been found and investigated in detail. In contrast to Mn²⁺‐ and Eu²⁺‐activated oxide‐based phosphors, the red light centered at 608 nm is ascribed to 4f–5d transitions of Eu²⁺ ions and Mn²⁺‐activated SrAlSi₄N₇ emits a cyan light peaking at 500 nm. Additionally, the special broad excitation band of SrAlSi₄N₇:Mn²⁺ centered at 362 nm has been covered by that of Eu²⁺ ions ranging from 300 to 550 nm. The overlap of the energy level of Mn²⁺ and Eu²⁺ ions creates the conditions for reciprocal energy transfer between Eu²⁺ and Mn²⁺ ions. A series of SrAlSi₄N₇:0.002 Mn²⁺,xEu²⁺ (0≤x≤005) with tunable light emission have been synthesized and the decay curves of samples prove the reciprocal occurrence of the energy transfer between Mn²⁺ and Eu²⁺ ions. This mode of energy transfer not only prevents the loss of energy, but also improves the thermal stability, and the intensity of SrAlSi₄N₇:Mn²⁺,Eu²⁺ at 150 °C is still beyond 92 % of the initial intensity. The results provide a new mode of energy transfer, which is expected to reduce the drawbacks existing in energy transfer.     

Understanding the Unconventional Effects of Halogenation on the Luminescent Properties of Oligo(Phenylene Vinylene) Molecules

It is commonly known that halogenation tends to decrease the luminescence quantum yield of an organic dye, owing to the high electronegativity and heavy‐atom effect of the halogen atom. However, based on an investigation of the effects of halogenation on the luminescence of the oligo(phenylene vinylene) (OPV) framework, we demonstrate that halogenation can have positive impact on the solid‐state fluorescence and electrochemiluminescence (ECL) properties of OPV derivatives. The chlorinated OPV exhibits a very high solid‐state fluorescence quantum yield (91 %), whilst the brominated analogue gives the highest ECL emission intensity. Time‐dependent density functional theory calculations, natural bond orbital analysis, and natural transition orbital analysis were performed to assist the understanding of the origin of these positive halogenation effects, which provide insight into the rational design of highly luminescent halogenated organic materials for solid‐state devices and ECL applications.     

The mechanism of the (bispidine)copper(II)-catalyzed aziridination of styrene: a combined experimental and theoretical study

Experimental and DFT-based computational results on the aziridination mechanism and the catalytic activity of (bispidine)copper(I) and -copper(II) complexes are reported and discussed (bispidine=tetra- or pentadentate 3,7-diazabicyclo[3.1.1]nonane derivative with two or three aromatic N donors in addition to the two tertiary amines). There is a correlation between the redox potential of the copper(II/I) couple and the activity of the catalyst. The most active catalyst studied, which has the most positive redox potential among all (bispidine)copper(II) complexes, performs 180 turnovers in 30 min. A detailed hybrid density functional theory (DFT) study provides insight into the structure, spin state, and stability of reactive intermediates and transition states, the oxidation state of the copper center, and the denticity of the nitrene source. Among the possible pathways for the formation of the aziridine product, the stepwise formation of the two N-C bonds is shown to be preferred, which also follows from experimental results. Although the triplet state of the catalytically active copper nitrene is lowest in energy, the two possible spin states of the radical intermediate are practically degenerate, and there is a spin crossover at this stage because the triplet energy barrier to the singlet product is exceedingly high.     

Direct Detection of 17O in [Gd(DOTA)]− by NMR Spectroscopy

The ¹⁷O NMR spectrum of the non‐coordinated carboxyl oxygen in the Gd^III–DOTA (DOTA=tetraazacyclododecanetetraacetic acid) complex has been observed experimentally. Its line width is essentially unaffected by paramagnetic relaxation due to gadolinium, and is only affected by the quadrupole pathway. The results are supported by the relevant parameters (hyperfine and quadrupole coupling constants) calculated by relativistic DFT methods. This finding opens up new avenues for investigating the structure and reactivity of paramagnetic Gd^III complexes used as contrast agents in magnetic resonance imaging.     

Exploring Excited‐State Tunability in Luminescent Tris‐cyclometalated Platinum(IV) Complexes: Synthesis of Heteroleptic Derivatives and Computational Calculations

The synthesis, structure, electrochemistry, and photophysical properties of a series of heteroleptic tris‐ cyclometalated Pt^IV complexes are reported. The complexes mer‐[Pt(C^N)₂(C′^N′)]OTf, with C^N=C‐deprotonated 2‐(2,4‐difluorophenyl)pyridine (dfppy) or 2‐phenylpyridine (ppy), and C′^N′=C‐deprotonated 2‐(2‐thienyl)pyridine (thpy) or 1‐phenylisoquinoline (piq), were obtained by reacting bis‐ cyclometalated precursors [Pt(C^N)₂Cl₂] with AgOTf (2 equiv) and an excess of the N′^C′H pro‐ligand. The complex mer‐[Pt(dfppy)₂(ppy)]OTf was obtained analogously and photoisomerized to its fac counterpart. The new complexes display long‐lived luminescence at room temperature in the blue to orange color range. The emitting states involve electronic transitions almost exclusively localized on the ligand with the lowest π–π* energy gap and have very little metal character. DFT and time‐dependent DFT (TD‐DFT) calculations on mer‐[Pt(ppy)₂(C′^N′)]⁺ (C′^N′=thpy, piq) and mer/fac‐[Pt(ppy)₃]⁺ support this assignment and provide a basis for the understanding of the luminescence of tris‐cyclometalated Pt^IV complexes. Excited states of LMCT character may become thermally accessible from the emitting state in the mer isomers containing dfppy or ppy as chromophoric ligands, leading to strong nonradiative deactivation. This effect does not operate in the fac isomers or the mer complexes containing thpy or piq, for which nonradiative deactivation originates mainly from vibrational coupling to the ground state.     

Ruthenium(IV)-catalyzed isomerization of the C=C bond of o-allylic substrates: a theoretical and experimental study

A general mechanism to rationalize Ru(IV) -catalyzed isomerization of the C=C bond in O-allylic substrates is proposed. Calculations supporting the proposed mechanism were performed at the MPWB1K/6-311+G(d,p)+SDD level of theory. All experimental observations in different solvents (water and THF) and under different pH conditions (neutral and basic) can be interpreted in terms of the new mechanism. Theoretical analysis of the transformation from precatalyst to catalyst led to structural identification of the active species in different media. The experimentally observed induction period is related to the magnitudes of the energy barriers computed for that process. The theoretical energy profile for the catalytic cycle requires application of relatively high temperatures, as is experimentally observed. Participation of a water molecule in the reaction coordinate is mechanistically essential when the reaction is carried out in aqueous medium. The new mechanistic proposal helped to develop a new experimental procedure for isomerization of allyl ethers to 1-propenyl ethers under neutral aqueous conditions. This process is an unique example of efficient and selective catalytic isomerization of allyl ethers in aqueous medium.     

Excited-state properties and environmental effects for protonated schiff bases: a theoretical study.

Complete active space self-consistent field (CASSCF), multireference configuration interaction (MRCI), density functional theory (DFT), time dependent DFT (TDDFT) and the singles and doubles coupled-cluster (CC2) methodologies have been used to study the ground state and excited states of protonated and neutral Schiff bases (PSB and SB) as models for the retinal chromophore. Systems with two to four conjugated double bonds are investigated. Geometry relaxation effects are studied in the excited pipi* state using the aforementioned methods. Taking the MRCI results as reference we find that CASSCF results are quite reliable even though overshooting of geometry changes is observed. TDDFT does not reproduce bond alternation well in the pipi* state. CC2 takes an intermediate position. Environmental effects due to solvent or protein surroundings have been studied in the excited states of the PSBs and SBs using a water molecule and solvated formate as model cases. Particular emphasis is given to the proton transfer process from the PSB to its solvent partner in the excited state. It is found that its feasibility is significantly enhanced in the excited state as compared to the ground state, which means that a proton transfer could be initiated already at an early step in the photodynamics of PSBs.     

Experimental and Theoretical Studies on the Magnetic Anisotropy in Lanthanide(III)‐Centered Fe3Ln Propellers

Compounds [Fe₃Ln(tea)₂(dpm)₆] ( Fe₃Ln ; Ln= Tb–Yb, H₃tea=triethanolamine, Hdpm=dipivaloylmethane) were synthesized as lanthanide(III)‐centered variants of tetrairon(III) single‐molecule magnets (Fe₄) and isolated in crystalline form. Compounds with Ln=Tb–Tm are isomorphous and show crystallographic threefold symmetry. The coordination environment of the rare earth, given by two tea^3? ligands, can be described as a bicapped distorted trigonal prism with D₃ symmetry. Magnetic measurements showed the presence of weak ferromagnetic Fe ??? Ln interactions for derivatives with Tb, Dy, Ho, and Er, and of weak antiferromagnetic or negligible coupling in complexes with Tm and Yb. Alternating current susceptibility measurements showed simple paramagnetic behavior down to 1.8 K and for frequencies reaching 10000 Hz, despite the easy‐axis magnetic anisotropy found in Fe₃Dy , Fe₃Er , and Fe₃Tm by single‐crystal angle‐resolved magnetometry. Relativistic quantum chemistry calculations were performed on Fe₃Ln (Ln=Tb–Tm): the ground J multiplet of Ln³⁺ ion is split by the crystal field to give a ground singlet state for Tb and Tm, and a doublet for Dy, Ho, and Er with a large admixture of m_J states. Gyromagnetic factors result in no predominance of g_z component along the threefold axis, with comparable g_x and g_y values in all compounds. It follows that the environment provided by the tea^3? ligands, though uniaxial, is unsuitable to promote slow magnetic relaxation in Fe₃Ln species.     

Understanding Electrogenerated Chemiluminescence Efficiency in Blue‐Shifted Iridium(III)‐Complexes: An Experimental and Theoretical Study

Compared to tris(2‐phenylpyridine)iridium(III) ([Ir(ppy)₃]), iridium(III) complexes containing difluorophenylpyridine (df‐ppy) and/or an ancillary triazolylpyridine ligand [3‐phenyl‐1,2,4‐triazol‐5‐ylpyridinato (ptp) or 1‐benzyl‐1,2,3‐triazol‐4‐ylpyridine (ptb)] exhibit considerable hypsochromic shifts (ca. 25–60 nm), due to the significant stabilising effect of these ligands on the HOMO energy, whilst having relatively little effect on the LUMO. Despite their lower photoluminescence quantum yields compared with [Ir(ppy)₃] and [Ir(df‐ppy)₃], the iridium(III) complexes containing triazolylpyridine ligands gave greater electrogenerated chemiluminescence (ECL) intensities (using tri‐n‐propylamine (TPA) as a co‐reactant), which can in part be ascribed to the more energetically favourable reactions of the oxidised complex (M⁺) with both TPA and its neutral radical oxidation product. The calculated iridium(III) complex LUMO energies were shown to be a good predictor of the corresponding M⁺ LUMO energies, and both HOMO and LUMO levels are related to ECL efficiency. The theoretical and experimental data together show that the best strategy for the design of efficient new blue‐shifted electrochemiluminophores is to aim to stabilise the HOMO, while only moderately stabilising the LUMO, thereby increasing the energy gap but ensuring favourable thermodynamics and kinetics for the ECL reaction. Of the iridium(III) complexes examined, [Ir(df‐ppy)₂(ptb)]⁺ was most attractive as a blue‐emitter for ECL detection, featuring a large hypsochromic shift (λ_max=454 and 484 nm), superior co‐reactant ECL intensity than the archetypal homoleptic green and blue emitters: [Ir(ppy)₃] and [Ir(df‐ppy)₃] (by over 16‐fold and threefold, respectively), and greater solubility in polar solvents.     

Design of Luminescent,Heteroleptic, Cyclometalated PtII and PtIV Complexes: Photophysics and Effects of the Cyclometalated Ligands

Neutral pentafluorophenyl benzoquinolinyl Pt^II [Pt(bzq)(HC^N−κN)(C₆F₅)] ( 1 a – g ) complexes, bearing nonmetalated N-heterocyclic HC^N ligands [HC^N=2,5-diphenyl-1,3,4-oxadiazole (Hoxd) a , 2-(2,4-difluorophenyl)pyridine (dfppy) b , 2-phenylbenzo[d]thiazole (pbt) c , 2-(4-bromophenyl)benzo[d]thiazole (Br-pbt) d , 2-phenylquinoline (pq) e , 2-thienylpyridine (thpy) f , 1-(2-pyridyl)pyrene (pypy) g ], and heteroleptic bis(cyclometalated) Pt^IV fac-[Pt(bzq)(C^N)(C₆F₅)Cl] ( 2 b – g , bzq: benzo[h]quinolinyl) derivatives, generated by oxidation of 1 b – g with PhICl₂, are reported. The oxidation reaction of 1 a evolved with formation of the bimetallic Pt^IV complex syn-[Pt(bzq)(C₆F₅)Cl(μ-OH)]₂ 3 . The crystal structures of 1 a,d,f , 2 b,d,e and 3 were corroborated by X-ray crystallography. A comparative study of the absorption and photoluminescence properties of the two series of complexes Pt^II ( 1 ) and Pt^IV ( 2 ), supported by time-dependent DFT calculations (TD-DFT), is presented. The low-lying transitions (absorption and emission) of Pt^II complexes 1 a – e [solution and polystyrene (PS) films] were assigned to the IL/MLCT mixture located on the cyclometalated Pt(bzq) unit, with minor IL′/ML′CT/LL′CT contributions involving the non-metalated ligand. Complex 1 g , bearing the more delocalized pyridyl pyrene (Hpypy) as an ancillary ligand, shows dual ¹ππ* and ³ππ* (Hpypy) emission in fluid CH₂Cl₂ and dual ³IL/³MLCT [Pt(bzq)] and [³ππ*, Hpypy] phosphorescence at 77 K. Upon oxidation, Pt^IV complexes 2 b – f display (solution, PS) ligand-based phosphorescence that arises from the bzq in 2 b (³LC) or from the second C^N ligand in 2 c – f (³L′C) with some ³LL′CT in 2 f . Despite metalation of the pyrenyl group, 2 g exhibits dual emission ¹ππ*/³ππ* located on the pypy chromophore.     

Photoluminescence of Uranium(VI): Quenching Mechanism and Role of Uranium(V)

The photoluminescence of uranium(VI) is observed typically in the wavelength range 400–650 nm with the lifetime of several hundreds μs and is known to be quenched in the presence of various halide ions (case A) or alcohols (case B). Here, we show by density functional theory (DFT) calculations that the quenching involves an intermediate triplet excited state that exhibits uranium(V) character. The DFT results are consistent with previous experimental findings suggesting the presence of photoexcited uranium(V) radical pair during the quenching process. In the ground state of uranyl(VI) halides, the ligand contributions to the highest occupied molecular orbitals increase with the atomic number (Z) of halide ion allowing larger ligand‐to‐metal charge transfer (LMCT) between uranium and the halide ion. Consequently, a larger quenching effect is expected as Z increases. The quenching mechanism is essentially the same in cases A and B, and is driven by an electron transfer from the quencher to the UO₂²⁺ entity. The relative energetic stabilities of the triplet excited state define the “fate” of uranium, so that in case A uranium(V) is oxidized back to uranium(VI), while in case B uranium remains as pentavalent.     

Improvement of stability and luminescence properties in water of Eu(III) and Tb(III) complexes photosensitized by a bipyridine antenna

The convenient synthesis of a new macrocyclic octadentate chelate based on 2,2′-bipyridine chromophore and diethylenetriaminetriacetic acid core is described. In aqueous solutions, the corresponding Eu(III) and Tb(III) neutral complexes are kinetically inert and show very bright luminescence when excited with UV radiation (Φ=11 and 25% respectively). We also report the potentiality of these complexes to act as donors in delayed fluorescence resonance energy transfer (DEFRET).