Hetero-trinuclear near-infrared (NIR) luminescent Zn₂Ln complexes from Salen-type Schiff-base ligands

With the Zn–Schiff-base [ZnL¹(Py)] or [ZnL²(Py)] from the simple Salen-type Schiff-base ligand H₂L¹ or H₂L² (H₂L¹: N,N′-bis(salicylidene)ethylene-1,2-diamine; H₂L²: N,N′-bis(salicylidene)phenylene-1,2-diamine) without the outer O₂O₂ portion as the precursor, two series of eight trinuclear Zn₂Ln arrayed complexes (Ln = Nd (1 or 5), Yb (2 or 6), Er (3 or 7) or Gd (4 or 8)) have been obtained by the further reaction with Ln(NO₃)₃·6H₂O, respectively. The results of photophysical studies show that the strong and characteristic NIR luminescence with emissive lifetimes in microsecond ranges, has been sensitized from the excited state (¹LC and ³LC or only ¹LC) of the ligand, whereas the Zn(II)-based visible luminescence is mostly quenched because of quite effective intramolecular energy transfer from the ligand-centered excited state of the Zn(II)–Schiff base complex to Ln(III) ions.