NIR-emissive erbium-quinolinolate complexes

Studies addressed to a deep understanding of the structure/property relationship on the near infrared (NIR)-emissive erbium-quinolinolate complexes, which are of interest for low-cost photonic systems, are reviewed. The role of the 8-quinolinolate ligand (Q), which studies also as sensitizer to overcome the weak absorptivity of lanthanide ion, is discussed. Synthetic and structural aspects are reported to revise the old assumption that these complexes are analogous in structure to AlQ₃ and to point out the specificity of the lanthanide coordination chemistry. In fact depending on reaction conditions, species with high coordination numbers and differing for nuclearity and stoichiometry have been isolated and fully characterized. In some instances it has been shown that coordinated water molecules directly bounded to the emitting erbium ion definitely represent the most effective quenchers for the luminescence at 1.5μm. Additionally, the combined optical and structural investigation of water-free Er-quinolinolate complexes, allows one to conclude that the C-H groups sitting in the Er³⁺ inner coordination sphere represent a very severe limit to the near-infrared emission yield. Implementation of structural/spectroscopic data into a theoretical model based on Förster's energy transfer resonant theory provides a comprehensive analysis of the near infrared emission quenching in erbium complexes useful to predict the quenching effects in luminescent lanthanide-complexes from the measurements of the vibrational absorption spectrum of the compound, the lanthanide radiative lifetime, and the minimum distance between the emitting ion and the quenchers. On that basis, in order to significantly improve the near-infrared emission yield, ligands which do not bear NH, CH or OH groups at a distance shorter than 7-8Å from the emitting ion are required. At the same time high sensitization efficiency of near-infrared emission and population saturation of trivalent erbium is achieved in these complexes, photoexcited into the absorption band of the quinolinolate sensitizer. It is concluded that a fully halogenated quinolinolate ligand can be an optimal candidate to improve the luminescent properties of erbium complexes.