This work illustrates a simple approach for optimizing long-lived near-infrared lanthanide-centered luminescence using trivalent chromium chromophores as sensitizers. Reactions of the segmental ligand L2 with stoichiometric amounts of M(CF₃SO₃)₂ (M = Cr, Zn) and Ln(CF₃SO₃)₃ (Ln = Nd, Er, Yb) under aerobic conditions quantitatively yield the D₃-symmetrical trinuclear [MLnM(L2)₃](CF₃SO₃)_n complexes (M = Zn, n = 7; M = Cr, n = 9), in which the central lanthanide activator is sandwiched between the two transition metal cations. Visible or NIR irradiation of the peripheral Cr(III) chromophores in [CrLnCr(L2)₃]⁹⁺ induces rate-limiting intramolecular intermetallic Cr→Ln energy transfer processes (Ln = Nd, Er, Yb), which eventually produces lanthanide-centered near-infrared (NIR) or IR emission with apparent lifetimes within the millisecond range. As compared to the parent dinuclear complexes [CrLn(L1)₃]⁶⁺, the connection of a second strong-field [CrN₆] sensitizer in [CrLnCr(L2)₃]⁹⁺ significantly enhances the emission intensity without perturbing the kinetic regime. This work opens novel exciting photophysical perspectives via the buildup of non-negligible population densities for the long-lived doubly excited state [Cr*LnCr*(L2)₃]⁹⁺ under reasonable pumping powers.

The connection of two Cr^III sensitizers around a central Er^III acceptor in a self-assembled cation provides high local metal concentrations that favor efficient nonlinear energy transfer upconversion luminescence (see picture). Upon selective low-energy near-infrared irradiation of Cr^III-centered transitions, 1 displays an unprecedented molecular two-photon upconverted green Er^III-centered emission.

The electronic structure and the photophysical properties of the vanadium(III)ion in pseudo-octahedral oxygen coordination is reviewed. V³⁺ has received much interest from spectroscopists in recent years due to the advancement of state-of-the-art experimental techniques such as inelastic neutron scattering and high-field electron paramagnetic resonance spectroscopy that directly interrogate its large ground state zero-field splittings (ZFSs) and to rational parameterization of the ligand fieldp arameters using the angular overlap model. However, for V³⁺ these ZFSs can be large enough to also be probed directly by high-resolution electronic absorption spectroscopy of intra-configurational (t²_2g → t²_2g) spin-forbidden transitions in the near-IR and visible regions. The luminescent properties of V³⁺ with hexa-oxo and tris-bidentate di-oxo-coordination are quite disappointing compared to its neighbor in the periodic table, Cr³⁺, in similar environments. The efficient non-radiative pathways in these compounds are reviewed and compared to recent work on V³⁺ doped into NaMgAl(ox)₃â‹…9H₂O. The poor luminescence quantum efficiencies of V³⁺ oxo complexes is explained by strong coupling of multi-phonon processes with a dynamic Jahn-Teller distortion originating from the ³E trigonal component of the ³T_1g ground state.

The ground-state electronic structures of K₃V(ox)₃·3H₂O, Na₃V(ox)₃·5H₂O, and NaMgAl_1–xV_x(ox)₃·9H₂O (0 < x <= 1, ox = C₂O₄^2–) have been studied by Fourier–transform electronic absorption and inelastic neutron scattering spectroscopies. High-resolution absorption spectra of the ³Î“(t_2g²) → ¹Î“(t_2g²) spin-forbidden electronic origins and inelastic neutron scattering measurements of the pseudo-octahedral [V(ox)₃]^3– complex anion below 30 K exhibit both axial and rhombic components to the zero-field-splittings (ZFSs). Analysis of the ground-state ZFS using the conventional S = 1 spin Hamiltonian reveals that the axial ZFS component changes sign from positive values for K₃V(ox)₃·3H₂O (D ≈ +5.3 cm^–1) and Na₃V(ox)₃·5H₂O (D ≈ +7.2 cm^–1) to negative values for NaMgAl_1–xV_x(ox)₃·9H₂O (D ≈ –9.8 cm^–1 for x = 0.013, and D ≈ –12.7 cm^–1 for x = 1) with an additional rhombic component, |E|, that varies between 0.8 and 2 cm^–1. On the basis of existing crystallographic data, this phenomenon can be identified as due to variations in the axial and rhombic ligand fields resulting from outer-sphere H-bonding between crystalline water molecules and the oxalate ligands. Spectroscopic evidence of a crystallographic phase change is also observed for K₃V(ox)₃·3Y₂O (Y = H or D) with three distinct lattice sites below 30 K, each with a unique ground-state electronic structure.

High-resolution Fourier transform absorption and luminescence spectroscopy reveal axial and rhombic zero-field splittings of the spin-forbidden electronic origins of V³⁺ in NaMgAl(ox)₃·9H₂O (ox=oxalate) single crystals below 25 K. The temperature dependence of the integrated absorption of the split features display behavior consistent with a Boltzmann distribution within the zero-field split ³Ãƒâ€š₂ ground state of V³⁺. Weak luminescence is observed in the near-IR from the lowest energy spin-forbidden transition with a luminescence lifetime of less than 0.5 μs at 11 K and an estimated quantum efficiency of the order of 10^-5