• Smaller than a nanoparticle with the design of discrete polynuclear molecular complexes displaying near-infrared to visible upconversion
    D. Zare, Y. Suffren, L. Gune, S.V. Eliseeva, H. Nozary, L. Aboshyan-Sorgho, S. Petoud, A. Hauser and C. Piguet
    Dalton Transactions, 44 (6) (2015), p2529-2540
    DOI:10.1039/C4DT02336F | unige:46187 | Abstract | Article HTML | Article PDF | Supporting Info
 
This work shows that the operation of near-infrared to visible light-upconversion in a discrete molecule is not limited to non-linear optical processes, but may result from superexcitation processes using linear optics. The design of nine-coordinate metallic sites made up of neutral N-heterocyclic donor atoms in kinetically inert dinuclear [GaEr(L1)3]6+ and trinuclear [GaErGa(L2)3]9+ helicates leads to [ErN9] chromophores displaying unprecedented dual visible nanosecond Er(4S3/24I15/2) and near-infrared microsecond Er(4I13/24I15/2) emissive components. Attempts to induce one ion excited-state absorption (ESA) upconversion upon near-infrared excitation of these complexes failed because of the too-faint Er-centred absorption cross sections. The replacement of the trivalent gallium cation with a photophysically-tailored pseudo-octahedral [CrN6] chromophore working as a sensitizer for trivalent erbium in [CrEr(L1)3]6+ improves the near-infrared excitation efficiency, leading to the observation of a weak energy transfer upconversion (ETU). The connection of a second sensitizer in [CrErCr(L2)3]9+ generates a novel mechanism for upconversion, in which the superexcitation process is based on the CrIII-sensitizers. Two successive Cr→Er energy transfer processes (concerted-ETU) compete with a standard Er-centred ETU, and a gain in upconverted luminescence by a factor larger than statistical values is predicted and observed.
  
  • Near-Infrared to Visible Light-Upconversion in Molecules: From Dream to Reality
    Y. Suffren, D. Zare, S.V. Eliseeva, L. Gune, H. Nozary, T. Lathion, L. Aboshyan-Sorgho, S. Petoud, A. Hauser and C. Piguet
    Journal of Physical Chemistry C, 117 (51) (2013), p26957-26963
    DOI:10.1021/jp4107519 | unige:34037 | Abstract | Article HTML | Article PDF
Light-upconversion via stepwise energy transfer from a sensitizer to an activator exploits linear optics for converting low-energy infrared or near-infrared incident photons to higher energy emission occurring in the part of the electromagnetic spectrum ranging from visible to ultraviolet. Stepwise excitation is restricted to activators possessing intermediate long-lived excited states such as those found for trivalent lanthanide cations dispersed in solid-state matrices. When the activator is embedded in a molecular complex, efficient non-radiative relaxation processes usually reduce excited state lifetimes to such an extent that upconversion becomes too inefficient to be detected under practical excitation intensities. Theoretical considerations suggest that the combination of millisecond timescale sensitizers with a central lanthanide activator located in supramolecular complexes circumvents this bottleneck by creating a novel pathway reminiscent of the energy transfer upconversion mechanism observed in doped solids. Application of this novel concept to chromium/erbium pairs in discrete triple-stranded helicates demonstrates that strong-field trivalent chromium chromophores irradiated with near-infrared photons produce upconverted green erbium-centered emission both in the solid state and in solution.
  • Optimizing Millisecond Time Scale Near-Infrared Emission in Polynuclear Chrome(III)-Lanthanide(III) Complexes
    L. Aboshyan-Sorgho, H. Nozary, A. Aebischer, J.-C.G. Bnzli, , K.R. Kittilstved, A. Hauser, S.V. Eliseeva, S. Petoud and C. Piguet
    Journal of the American Chemical Society, 134 (30) (2012), p12675-12684
    DOI:10.1021/ja304009b | unige:22645 | Abstract | Article HTML | Article PDF
 
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.
  • Optical sensitization and upconversion in discrete polynuclear chromium–lanthanide complexes
    L. Aboshyan-Sorgho, M. Cantuel, S. Petoud, A. Hauser and C. Piguet
    Coordination Chemistry Reviews, 256 (15-16) (2012), p1644-1663
    DOI:10.1016/j.ccr.2011.12.013 | unige:21642 | Abstract | Article PDF
Due to its extreme kinetic inertness, trivalent chromium, Cr(III), has been rarely combined with labile trivalent lanthanides, Ln(III), to give discrete self-assembled (supra)molecular polynuclear complexes. However, the plethora of accessible metal-centered excited states possessing variable lifetimes and emissive properties, combined with the design of efficient intramolecular Cr(III)  Ln(III) energy transfer processes open attractive perspectives for programming directional light-conversion within these heterometallic molecules. Efforts made to address this exciting challenge for both light-sensitization and light-upconversion are discussed in this article.
  
  • Near-Infrared to Visible Light Upconversion in a Trinuclear d-f-d ComplexVery Important Paper
    L. Aboshyan-Sorgho, C. Besnard, P. Pattison, K.R. Kittilstved, A. Aebischer, J.-C.G. Bnzli, A. Hauser and C. Piguet
    Angewandte Chemie International Edition, 50 (2011), p4108-4112
    DOI:10.1002/anie.201100095 | unige:15714 | Abstract | Article PDF
The connection of two CrIII sensitizers around a central ErIII 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 CrIII-centered transitions, 1 displays an unprecedented molecular two-photon upconverted green ErIII-centered emission.
  • Ground-State Electronic Structure of Vanadium(III) Trisoxalate in Hydrated Compounds
    K.R. Kittilstved, L. Aboshyan Sorgho, N. Amstutz, P.L.W. Tregenna-Piggott and A. Hauser
    Inorganic Chemistry, 48 (16) (2009), p7750-7764
    DOI:10.1021/ic900613p | unige:3542 | Abstract | Article HTML | Article PDF
 
The ground-state electronic structures of K3V(ox)3·3H2O, Na3V(ox)3·5H2O, and NaMgAl1–xVx(ox)3·9H2O (0 < x <= 1, ox = C2O42–) have been studied by Fourier–transform electronic absorption and inelastic neutron scattering spectroscopies. High-resolution absorption spectra of the 3Γ(t2g2) → 1Γ(t2g2) 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 NaMgAl1–xVx(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.

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