Design, synthesis, structural, spectral, and redox properties and phenoxazinone synthase activity of tripodal pentacoordinate Mn(II) complexes with impressive turnover numbers

Catechol oxidase (CO) and phenoxazinone synthase (PHS) are two enzymes of immense significance due to their capability to oxidize catechols and o-aminophenols to o-quinones and phenoxazinones, respectively. In this connection two mononuclear manganese complexes with the molecular framework [Mn^II(Ln)Cl]Cl {L1: tris((1H-benzo[d]imidazol-2-yl)methyl)amine; n = 1 and L2: tris(N-methylbenzimidazol-2-ylmethyl)amine; n = 2} have been designed to be potential catalysts for OAPH (o-aminophenol) oxidation. Both the ligands and their corresponding metal complexes have been successfully synthesized and thoroughly characterized by different spectroscopic and analytical techniques such as FT-IR, ¹H NMR, UV-vis spectroscopy, EPR spectroscopy and ESI mass spectroscopy. The molecular structures of [Mn^II(L1)Cl]Cl (1) and [Mn^II(L2)Cl]Cl (2) have been revealed by a single-crystal X-ray diffraction study. The spectral properties and redox behaviour of both the complexes were examined. Under ambient conditions, 1 and 2 show excellent phenoxazinone synthase activity as both are very susceptible to oxidize o-aminophenol to phenoxazinone. The kinetic parameters for both complexes have been determined by analyzing the experimental spectroscopic data. The turnover numbers (k_cat value) of these two complexes are extremely high, 440 h^-1 and 234 h^-1 for 1 and 2, respectively. The present report offers a thorough overview of information involving the role of the metal ions and their extent of phenoxazinone synthase mimicking activity. The oxidation of o-aminophenol to 2-amino-3H-phenoxazine-3-one (APX) by catalytic oxidation of oxygen (O₂) by the reaction with transition metal complexes has been an important study for the last few decades. The current study evidently showed better performance of our synthesized Mn(II) complexes than all the predecessors. The plausible mechanism has been reiterated based on the experimental data via ESI-MS spectra and considering the concepts from the previously reported mechanisms involved in the formation of hydrogen peroxide (H₂O₂) as an intermediate substrate is fairly indicating the involvement of molecular oxygen in the catalytic cycle.

A Versatile Tripodal Ligand for Sensitizing Lanthanide (LnIII) Ions and Color Tuning

Lanthanide (LnIII) ions were successfully chelated and sensitized with a tripodal ligand. The absolute LnIII-centered emission efficiencies were ~3% for both the europium(III) (EuIII) and terbium (TbIII) complexes and up to 54% for the cerium(III) (CeIII) complex. The differences in emission quantum yields for the early lanthanides (CeIII) and the mid lanthanides (EuIII and TbIII) were attributed to their d–f and f–f nature, respectively. Despite the low quantum yield of the EuIII complex, the combination of the residual ligand fluorescence and the red EuIII emission resulted in a bluish-white material with the Commission Internationale de l’Eclairage (CIE) coordinates (0.258, 0.242). Thus, metal complexes of the ligand could be used in the generation of single-component white-light-emitting materials.

Seven-Coordinate Tb3+ Complexes with 90% Quantum Yields: High-Performance Examples of Combined Singlet- and Triplet-to-Tb3+ Energy-Transfer Pathways

Seven-coordinate, pentagonal-bipyramidal (PBP) complexes [Ln(bbpen)Cl] and [Ln(bbppn)Cl], in which Ln = Tb³⁺ (products I and II), Eu³⁺ (III and IV), and Gd³⁺ (V and VI), with bbpen^2- = N,N'-bis(2-oxidobenzyl)-N,N'-bis(pyridin-2-ylmethyl)ethylenediamine and bbppn^2- = N,N'-bis(2-oxidobenzyl)-N,N'-bis(pyridin-2-ylmethyl)-1,2-propanediamine, were synthesized and characterized by single-crystal X-ray diffraction analysis, alternating-current magnetic susceptibility measurements, and photoluminescence (steady-state and time-resolved) spectroscopy. Under a static magnetic field of 0.1 T, the Tb³⁺ complexes I and II revealed single-ion-magnet behavior. Also, upon excitation at 320 nm at 300 K, I and II presented very high absolute emission quantum yields (0.90 ± 0.09 and 0.92 ± 0.09, respectively), while the corresponding Eu³⁺ complexes III and IV showed no photoluminescence. Detailed theoretical calculations on the intramolecular energy-transfer rates for the Tb³⁺ products indicated that both singlet and triplet ligand excited states contribute efficiently to the overall emission performance. The expressive quantum yields, Q_Ln^L, measured for I and II in the solid state and a dichloromethane solution depend on the excitation wavelength, being higher at 320 nm. Such a dependence was rationalized by computing the intersystem crossing rates (W_ISC) and singlet fluorescence lifetimes (τ_S) related to the population dynamics of the S₁ and T₁ levels. Thin films of product II showed high air stability and photostability upon continuous UV illumination, which allowed their use as downshifting layers in a green light-emitting device (LED). The prototypes presented a luminous efficacy comparable with those found in commercial LED coatings, without requiring encapsulation or dispersion of II in host matrixes. The results indicate that the PBP environment determined by the ethylenediamine (en)-based ligands investigated in this work favors the outstanding optical properties in Tb³⁺ complexes. This work presents a comprehensive structural, chemical, and spectroscopic characterization of two Tb³⁺ complexes of mixed-donor, en-based ligands, focusing on their outstanding optical properties. They constitute good molecular examples in which both triplet and singlet excited states provide energy to the Tb³⁺ ion and lead to high values of Q_Ln^L.

Effect of the aromatic substituent on the para-position of pyridine-bis(oxazoline) sensitizers on the emission efficiency of their EuIII and TbIII complexes

Two efficient lanthanide ion sensitizers 2,6-bis(oxazoline)-4-phenyl-pyridine (PyboxPh, 1) and 2,6-bis(oxazoline)-4-thiophen-2-yl-pyridine (Pybox2Th, 2) were synthesized. 1 crystallizes in the monoclinic space group P21/c with cell parameters a = 16.3794(4) Å, b = 7.2856(2) Å, c = 11.7073(3) Å, β = 97.229(1)° and V = 1385.97(6) Å3. 2 crystallizes in the monoclinic space group P21/n with cell parameters a = 5.9472(2), b = 16.0747(6), c = 14.3716(5) Å, β = 93.503(1)° and V = 1371.35(8) Å3. Photophysical characterization of 1 shows that its triplet state energy is located at 22 250 cm-1 and efficient energy transfer is observed for EuIII and TbIII. Solutions of [Ln(PyboxPh)3]3+ in dichloromethane display an emission efficiency of 37.2% for Ln[double bond, length as m-dash]Eu and 24.0% for Ln[double bond, length as m-dash]Tb. The excited state lifetimes for EuIII and TbIII are 2.227 ms and 723 μs, respectively. The triplet state energy of 2 is located at 19 280 cm-1 and is therefore too low to efficiently sensitize TbIII emission. However, the sensitization of EuIII is effective, with an emission quantum yield of 14.5% and an excited state lifetime of 714 μs. This shows that the derivatization of the chelator is strongly influenced by the aromatic substituents on the para-position of the pyridine ring. New isostructural 1 : 1 complexes of PyboxPh with EuIII (3) and TbIII (4) were also isolated and crystallize in the triclinic space group P1[combining macron] with cell parameters a = 9.1845(2) Å, b = 10.3327(2) Å, c = 11.9654(2) Å, α = 98.419(1)°, β = 108.109(1)°, γ = 91.791(1)°, V = 1064.08(4) Å3 and a = 7.8052(1) Å, b = 11.8910(1) Å, c = 14.2668(2) Å, α = 72.557(1)°, β = 86.355(1)°, γ = 77.223(1)°, V = 1231.95(3) Å3, respectively.

Stoichiometry of lanthanide(iii) complexes with tripodal aminophosphonic ligands – a new solution to an old problem

The [Eu(NP₂py)₂]⁵⁻ complex crystallized as a [C(NH₂)₃]₅[Eu(NP₂py)₂]·12 compound. However, the formation of the [Ln(NP₂py)₂]^5– species in aqueous solution starts at pH as high as 8.

Synthesis and structure of color tunable and white-light emitting lanthanide metal–organic framework materials constructed from conjugated 1,1′-butadiynebenzene-3,3′,5,5′-tetracarboxylate ligand

Seven isostructural lanthanide MOFs based on 1,1′-butadiynebenzene-3,3′, 5,5′-tetracarboxylate ligands were synthesized. Color tunable and white-light emitting materials were achieved by carefully adjusting the doping concentration of Eu³⁺ and Tb³⁺ in the Gd³⁺ compound.

A benzimidazole functionalised DO3A chelator showing pH switchable coordination modes with lanthanide ions

The synthesis of a new macrocyclic chelator incorporating a benzimidazole heterocycle is reported. Lanthanide complexes with macrocyclic chelators based on 1,4,7,10-tetra(carboxymethyl)-1,4,7,10-tetraazacyclododecane (DOTA) and 1,4,7-tris(carboxymethyl)-1,4,7,10-tetraazacyclododecane (DO3A) are of interest in luminescent, radiopharmaceutical and magnetic resonance (MR) biomedical imaging applications. The benzimidazole DO3A chelator allows for sensitisation of europium(iii), terbium(iii) and ytterbium(iii) luminescence by the heterocycle and also shows a pH dependent coordination change due to protonation of the chelator (pKa = 4.1 for the europium(iii) complex). The thermodynamic stability of the complexes has been investigated by potentiometric titration with the gadolinium(iii) complex showing significantly higher stability than the zinc(ii) complex, where log βZnLH = 28.1 and log βGdLH = 32.1.

Near-infrared (NIR) emitting Nd/Yb(iii) complexes sensitized by MLCT states of Ru(ii)/Ir(iii) metalloligands in the visible light region

Visible light sensitized NIR emission of Nd³⁺ and Yb³⁺ by Ru(ii)/Ir(iii) metalloligands is reported.

Photoluminescence and white-light emission in two series of heteronuclear Pb(ii)–Ln(iii) complexes

Tunable photoluminescence and single component white light emission is achieved in Pb(ii)–Eu(iii) heteronuclear complexes.

Three novel lanthanide metal-organic frameworks (Ln-MOFs) constructed by unsymmetrical aromatic dicarboxylatic tectonics: synthesis, crystal structures and luminescent properties

Three novel Ln(III)-based coordination polymers, {[Ln₂ (2,4-bpda)₃ (H₂O)_x]·yH₂O}_n (Ln = La (III) (1), x = 2, y = 0, Ce (III) (2), Pr (III) (3), x = 4, y = 1) (2,4-H₂bpda = benzophenone-2,4-dicarboxylic acid) have been prepared via a solvothermal method and characterized by elemental analysis, IR, and single-crystal X-ray diffraction techniques. Complex 1 exhibits a 3D complicated framework with a new 2-nodal (3,7)-connected (4²·5) (4⁴·5¹·6⁶·8) topology. Complexes 2 and 3 are isomorphous, and feature a 3D 4-connected (6⁵·8)-CdSO₄ network. Moreover, solid-state properties such as thermal stabilities and luminescent properties of 1 and 2 were also investigated. Complex 1 crystallized in a monoclinic space group P2₁/c with a = 14.800 (3), b = 14.500 (3), c = 18.800 (4) Å, β = 91.00 (3), V = 4033.9 (14) ų and Z = 4. Complex 2 crystallized in a monoclinic space group Cc with a = 13.5432 (4), b = 12.9981 (4), c = 25.7567 (11) Å, β = 104.028 (4), V = 1374.16 (7) ų and Z = 4.

Photoluminescent 3D lanthanide MOFs with a rare (10,3)-d net based on a new tripodal organic linker

Isomorphous Ln-MOFs with a rare (10,3)-d network were assembled from a dual functional ligand which sensitizes Ln-based luminescence, achieving high quantum yield for Tb-MOF.