• Smaller than a nanoparticle with the design of discrete polynuclear molecular complexes displaying near-infrared to visible upconversion
    Davood Zare, Yan Suffren, Laure Guénée, Svetlana V. Eliseeva, Homayoun Nozary, Lilit Aboshyan-Sorgho, Stéphane Petoud, Andreas Hauser and Claude 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(4S3/2→4I15/2) and near-infrared microsecond Er(4I13/2→4I15/2) 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.

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Aboshyan-Sorgho, Lilit Eliseeva, Svetlana V. Guénée, Laure Hauser, Andreas Nozary, Homayoun 
Petoud, Stéphane Piguet, Claude Suffren, Yan Zare, Davood 
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