Photo-luminescent hetero-trinuclear Zn ₂Ln (Ln = Nd, Yb, Er or Gd) complexes based on the binuclear Zn ₂L precursor

With the binuclear Zn ₂L ([Zn ₂L(OAc) ₂(H ₂O)], 1) compound from the Salen-type Schiff-base ligand H ₂L (H ₂L: N,N′-bis(salicylidene)cyclohexane- 1,2-diamine) with the flexible linker as the precursor, a series of hetero-trinuclear Zn ₂Ln complexes (Ln = Nd (2), Ln = Yb (3), Ln = Er (4) or Ln = Gd (5)) have been obtained by further reaction with Ln(OAc) ₃·6H ₂O. The result of their photophysical properties shows that the strong and characteristic NIR luminescence with emissive lifetimes in the ranges of microseconds for each of the complexes 2-4, has been sensitized from the excited state ( ¹LC and ³LC) of the Salen-type Schiff-base ligand with the flexible linker. Especially, the almost planar arrangement of two energy donors of ZnL components around the Ln ^{3 +} ion for the larger conjugation effect, endowing the energy level's good match between the excited state ( ³LC) of the chromophore to the corresponding Ln ^{3 +} ion's exciting state, results in the enhancement of their NIR luminescence.