Temperature-dependent self-assembly of near-infrared (NIR) luminescent Zn₂Ln and Zn₂Ln₃ (Ln = Nd, Yb or Er) complexes from the flexible Salen-type Schiff-base ligand

Through the self-assembly of the precursor [Zn(L)(MeCN)] (H₂L = N,N′-bis(3-methoxy-salicylidene)cyclohexane-1,2-diamine) with LnCl ₃·6H₂O (Ln = La, Nd, Yb, Er or Gd) and NaN ₃ in alcohol-containing solutions, two series of mixed anions-induced Zn₂Ln-arrayed complexes [Zn₂(L)₂(MeOH) ClLn(N₃)]·Cl (Ln = La, 1; Ln = Nd, 2; Ln = Yb, 3; Ln = Er, 4 or Ln = Gd, 5) and Zn₂Ln₃-arrayed complexes [Zn ₂(L)₃Cl₂(μ₂-OH) (μ₃-OH)₂Ln₃(N₃)₂] (Ln = La, 6; Ln = Nd, 7; Ln = Yb, 8; Ln = Er, 9 or Ln = Gd, 10) are obtained at room temperature or under reflux, respectively. In contrast to Zn ₂Ln-arrayed complexes with the two Zn²⁺ ions in the inner cis-N₂O₂ cores and one Ln³⁺ ion in the outer O₂O₂ moieties, the demetalation of partial precursors leads to the selective exchange of Zn²⁺ centers for the Ln ³⁺ ions for the formation of novel heterometallic Zn ₂Ln₃-arrayed complexes with the Ln³⁺ ions in both the inner cis-N₂O₂ core and the outer O ₂O₂ moieties of the ligands. The result of their photophysical properties shows that the characteristic near-infrared (NIR) luminescence of Nd³⁺ or Yb³⁺ ion has been sensitized from the excited state (both ¹LC and ³LC) of the ligand H ₂L, while relatively lower quantum yields for Zn₂Ln ₃-arrayed complexes than those for Zn₂Ln-arrayed complexes, correspondingly, should be due to the luminescent quenching with the involvement of OH^- oscillators around the Ln³⁺ ions.