Near‐Infrared Luminescent, Neutral, Cyclic Zn₂Ln₂ (Ln = Nd, Yb, and Er) Complexes from Asymmetric Salen‐Type Schiff Base Ligands

Abstract

Abstract In the presence of excess pyridine (py), Zn(OAc) 2 · 2H 2 O 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(HL 1 )(py)] or [Zn(HL 2 )(py)], respectively. Interaction of [Zn(HL 1 )(py)] or [Zn(HL 2 )(py)] with Ln(NO 3 ) 3 · 6H 2 O in a 1:1 molar ratio gave the cyclic hetero‐tetranuclear complexes [Zn 2 (L 1 ) 2 (py) 2 Ln 2 (NO 3 ) 4 (dmf) 2 ] · 3Et 2 O (Ln = Nd, 1 ; Yb, 2 ; Er, 3 ; and Gd, 4 ) or [Zn 2 (L 2 ) 2 Ln 2 (NO 3 ) 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 π→π* 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 ( 1 LC) visible fluorescence was mostly quenched as a result of effective intramolecular energy transfer from the 1 LC excited state to the Ln 3+ ions. In addition, the occupation of py groups at the axial position of Zn 2+ ions and the involvement of heavy atoms seem to help to enhance the NIR luminescence to some extent.