Room-Temperature Linear Light Upconversion in a Mononuclear Erbium Molecular Complex.

To date, the piling up of successive photons of low energies (near infrared; NIR) using a single lanthanide center and linear optics to ultimately produce upconverted visible emission was restricted to low-phonon solid materials and nanoparticles. Now we show that the tight helical wrapping of three terdentate N-donor ligands around a single nine-coordinate trivalent erbium cation provides favorable conditions for a mononuclear molecular complex to exhibit unprecedented related upconverted emission. Low power NIR laser excitations into the metal-centered transitions Er( ⁴ I _11/2 ← ⁴ I _15/2 ) at 801 nm or Er( ⁴ I _13/2 ← ⁴ I _15/2 ) at 966 nm result in upconverted blue-green emissions, where two or three photons respectively are successively absorbed by a molecular lanthanide complex possessing high-energy vibrations.
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