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(I-4(11/2) -> I-4(15/2)) at 801 nm or Er(I-4(13/2) -> I-4(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.