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. KGaA, Weinheim.