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)center dot 2H(2)O reacted with an equimolar amount of asymmetric salen-type Schiff base ligand, generated in situ front 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)center dot 6H(2)O in a 1:1 molar ratio gave the cyclic hetero-tetranuclear complexes [Zn-2(L-1)(2)(py)(2)Ln(2)(NO3)(4)(dmf)(2)]center dot 3Et(2)O (Ln = Nd, 1; Yb, 2; Er, 3; and Gd, 4) or [Zn-2(L-2)(2)Ln(2)(NO3)(4)(dmf)(4)] (Ln = Nd, 5; Yb, 6; Er, 7; and Gd, 8), respectively, in moderate yields. Photophysical studies of these cyclic Zn(2)Ln(2) tetranuclear complexes showed that, upon photoexcitation in the 200-550 nm range corresponding to the intraligand pi ->pi* transition of the Schiff base, strong and characteristic NIR luminescence of Ln(3+) ions with emissive lifetimes in the microsecond ranges were observed, whereas the ligand-centered singlet ((LC)-L-1) visible fluorescence was mostly quenched as a result of effective intramolecular energy transfer from the (LC)-L-1 excited state to the Ln(3+) 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.