Near-infrared luminescent, neutral, cyclic Zn₂Ln₂ (Ln = Nd, Yb, and Er) complexes from asymmetric salen-type schiff base ligands

In the presence of excess pyridine (py), Zn(OAc)₂·;H ₂O reacted with an equimolar amount: of asymmetric salen-type Schiff base ligand, generated in situ from the condensation of 2,3-diaminophenol with o-vanillin or 5-bromo-3-methoxybenzaldehyde, to give the d-block "complex ligand" [Zn(HL¹)(Py)] or [Zn(HL²)(py)], respectively. Interaction of [Zn(HL¹)(Py)] or [Zn(HL²)(py)] with Ln(NO₃)₃·6H₂O in a 1:1 molar ratio gave the cyclic hetero-tetranuclear complexes [Zn₂(L¹) ₂(py)₂Ln₂(NO₃)₄(dmf) ₂]·3Et₂O (Ln = Nd, 1;Yb, 2; Er, 3; and Gd, 4) or [Zn₂(L²)₂Ln₂(NO₃) ₄(dmf)₄] (Ln = Nd, 5; Yb, 6; Er, 7; and Gd, 8), respectively, in moderate yields. Photophysical studies of these cyclic Zn ₂Ln₂ tetranuclear com-plexes showed that, upon photoexcitation in the 200-550 nm range corresponding to the intraligand π→π* transition of the Schiff base, strong and characteristic NIR luminescence of Ln³⁺ ions with emissive lifetimes in the microsecond ranges were observed, whereas the ligand-centered singlet ( ¹LC) visible fluorescence was mostly quenched, as a result of effective intramolecular energy transfer from, the ¹LC excited state to the Ln³⁺ ions. In addition, the occupation of py groups at the axial position of Zn²⁺ ions and the involvement of heavy atoms seem to help to enhance the NIR luminescence to some extent.