Sensitization of NIR luminescence of Yb3+ by Zn2+ chromophores in heterometallic complexes with a bridging Schiff-base ligand

Abstract

Herein, complexes [ZnL]₂ (1), {(H₂O)Zn(μ-L)Yb[OCH(CF₃)₂]₃} (2), {[(CF₃)₂HCO]Zn(μ-L)Yb[OCH(CF₃)₂](μ-OH)}₂ (3), and [(H₂O)Ln₂(L)₃] (Ln = Yb (4) and Gd (5)) containing a bridging Schiff-base ligand (H₂L = N,N′-bis(3-methoxy salicylidene)phenylene-1,2-diamine) were synthesized. The compounds 1–4 were structurally characterized. The ytterbium derivatives 2–4 exhibited bright NIR metal-centred photoluminescence (PL) of Yb³⁺ ion under one- (λ_ex = 380 nm) and two-photon (λ_ex = 750 nm) excitation. The superior luminescence properties of complex 2, which was suggested as a marker for NIR bioimaging, were explained via the strong absorption of the 375 nm LMCT state of the ZnL chromophore, efficient energy transfer from ZnL towards Yb³⁺ through a reversible ligand-to-lanthanide electron transfer process, and absence of luminescence quenchers (C–H and O–H groups) in the first coordination sphere of the rare-earth atom.