Heteroleptic [CrIIIN6] Chromophores as Partners for Lanthanide-Based Light Conversion in d-f Molecular Complexes

The connection of a dianionic 2,2’-biimidazolate (biim2−) bridging unit to cis-[Cr(N∩N)2]3+ (N∩N is a chelating didentate ligand) or cis-[Cr(N∩N∩N∩N)]3+ building blocks (N∩N∩N∩N is a chelating tetradentate ligand) produces heteroleptic pseudo-octahedral [CrN6]+ chromophores. Their reduced cationic charge is compatible with the subsequent complexation of trivalent lanthanides (Ln3+) to give d-f {[(N∩N)2Cr(biim)]nLn}(3+n)+ (n = 1–4), {[(N∩N)2Cr(biim)]Ln(Tp)2}2+ and {[(N∩N∩N∩N)Cr(biim)]Ln(Tp)2}2+ adducts (Tp is tri(1H-pyrazol-1-yl)-λ4-borate). Moving from polyaromatic N∩N (1,10 phenanthroline) to saturated N∩N∩N∩N polyamine (cyclam) receptors controls the photophysical properties and leads to tunable light conversion in the target heterometallic complexes when Eu(III) is exploited as the activator for downshifting and Er(III) as the activator for upconversion.