Sensitization of lanthanide complexes through direct spin-forbidden singlet → triplet excitation

Different mechanisms for lanthanide(III) sensitization and Yb-field-induced single-molecule magnet behaviour in a series of pentagonal bipyramidal and octahedral lanthanide complexes with axial phosphine oxide ligands

Seven mononuclear lanthanide complexes have been isolated and structurally characterised. Four of them are cationic, whose charges are balanced by chloride counteranions, and exhibit pentagonal bipyramidal coordination geometry, whereas the rest of them are neutral and display octahedral coordination environment. In all cases, the coordination sphere of the LnIII ions consists of two di(1-adamantyl)benzylphosphine oxide ligands in axial positions, whereas in the equatorial plane the former contains a chloride and four water molecules and the latter a solvent molecule and three chloride ligands. We report a detailed photophysical investigation, including time-dependent density functional theory (TD-DFT) calculations and intramolecular energy transfer (IET) analysis, which reveals two distinct lanthanide sensitization mechanisms. Compound-specific energy transfer pathways occur through either the S1 or T1 states, as supported by calculated IET rates and resonance with lanthanide acceptor transitions. In addition, dc and ac magnetic properties were measured on complexes 1 and 2, showing that compound 1 behaves as a bi-functional compound, exhibiting field-induced single molecule magnet behaviour together with YbIII-centred NIR luminescence. The relaxation of the magnetization in this pentagonal bipyramidal complex takes place through Raman and direct processes, as supported by ab initio calculations.

Difluorocarbene Generation via a Spin-Forbidden Excitation under Visible Light Irradiation

The generation of difluorocarbene from difluoromethane bis(sulfonium ylide) 1 through spin-forbidden excitation under irradiation with 450 nm blue light was reported. The formation of difluorocarbene was confirmed by its reaction with styrene derivatives for the generation of difluorocyclopropanation and insertion into RX-H bonds (X = O, S) for the generation of RXCF2H. The spin-forbidden excitation mechanism for the formation of difluorocarbene from difluoromethane bis(sulfonium ylide) was supported by spectroscopic and kinetic studies as well as computational chemistry. The homolytic cleavage of two S-C bonds in compound 1 under irradiation was confirmed by time-resolved EPR spectroscopic studies of the precursor's free-radical-capturing reaction, as well as the isolation of the dimer of dimethyl (phenylthiol)malonyl radical. Further studies showed that the homolytic cleavage process occurred asynchronously in the solvent cage based on the isotope-labeled scrambling experiments and DFT calculations.

EuIII and TbIII upconversion intermediated by interparticle energy transfer in functionalized NaLnF4 nanoparticles

Lanthanide (LnIII)-doped sodium gadolinium tetrafluoride (NaGdF4) nanoparticles have been excelled as attractive upconversion systems for anti-counterfeiting or energy conversion for instance, with a special interest in the visible upconversion of EuIII and TbIII. The core@shell architecture has enabled the bright upconversion of EuIII and TbIII in this matrix by interfacial energy transfer sensibilized by the TmIII/YbIII pair. Another approach to enable EuIII and TbIII upconversion could be the interparticle energy transfer (IPET) between LnIII-doped sensitizer and acceptor nanoparticles. Yet, the low molar absorptivity of the LnIII through 4f ↔ 4f electronic transitions and the large distance between the nanoparticles are shortcomings that should decrease the energy transfer efficiency. On the other hand, it is feasible to predict that the association of organic ligands displaying large molar absorptivity on the acceptor nanoparticle surface could help to overcome the absorption limitation. Inspired by this exciting possibility, herein, we present the EuIII/TbIII upconversion intermediated by IPET between the donor TmIII, YbIII-doped NaGdF4 nanoparticle and the acceptor LnIII-doped NaGdF4 (Ln = Eu and/or Tb) nanoparticles functionalized with a series organic ligands on the surface (tta- = thenoyltrifluoroacetonate, acac- = acetylacetonate, or 3,5-bbza- = 3,5-dibromebenzoate). Either in solid state or in suspension, upon excitation at 980 nm, visible EuIII/TbIII upconversion could be observed. This emission comes from the absorption of the TmIII, YbIII pair in the donor nanoparticle, followed by IPET from the TmIII excited levels to the ligand singlet/triplet states on the acceptor nanoparticle surface, ligand-to-EuIII/TbIII energy transfer, and upconversion emission. Spectroscopic evidences from the analysis of the donor level lifetimes indicate the contribution of non-radiative energy transfer for the IPET mechanism; the radiative mechanism also contributes for the IPET. Moreover, the design herein introduced enables the development of luminescence temperature probes with relative thermal sensitivity as high as 1.67% K-1 at 373 K. Therefore, this new upconversion pathway opens an avenue of possibilities in an uncharted territory to tune the visible upconversion of LnIII ions.

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.