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(4I11/2 !4I15/2)
at 801 nm or Er(4I13/2 !4I15/2) 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.