Heterobimetallic Zn(II)-Ln(III) phenylene-bridged Schiff base complexes, computational studies, and evidence for singlet energy transfer as the main pathway in the sensitization of near-infrared Nd³⁺ luminescence

A series of 3d-4f heterobimetallic phenylene-bridged Schiff base complexes of the general formula [Zn(μ-L¹)Ln-(NO₃) ₃(S)_n] [Ln = La (1), Nd (2), Gd (3), Er (4), Yb (5); S = H₂O, EtOH; n = 1, 2; H₂L¹ = N,N′-bis(3-methoxysalicylidene)phenylene-1,2-diamine] and [Zn(μ-L ²)Ln(NO₃)₃(H₂O)_n] [Ln = La (6), Nd (7), Gd (8), Er (9), Yb (10); n = 1, 2; H₂L² = N,N′-bis(3-methoxy-5-p-tolylsalicylidene)phenylene-1,2-diamine] were synthesized and characterized. Complexes 1, 2, 4, and 7 were structurally characterized by X-ray crystallography. At room temperature in CH₃CN, both neodymium(III) (2 and 7) and ytterbium(III) (5 and 10) complexes also exhibited, in addition to the ligand-centered emission in the UV-vis region, their lanthanide(III) ion emission in the near-infrared (NIR) region. The photophysical properties of the zinc(II) phenylene-bridged complexes (ZnL ¹ and ZnL²) were measured and compared with those of the corresponding zinc(II) ethylene-bridged complexes (ZnL³ and ZnL ⁴). Our results revealed that, at 77 K, both ligand-centered triplet (³LC) and singlet (¹LC) states existed for the ethylene-bridged complexes (ZnL³ and ZnL⁴), whereas only the ¹LC state was detected for the phenylene-bridged complexes (ZnL¹ and ZnL²). NIR sensitization studies of [Zn(μ-L′)Nd(NO₃)₃(H₂O)_n] (L′ = L¹-L⁴) complexes further showed that Nd ³⁺ sensitization took place via the ³LC and ¹LC states when the spacer between the imine groups of the Schiff base ligand was an ethylene and a phenylene unit, respectively. Ab initio calculations show that the observed differences can be attributed to the difference in the molecular vibrational properties and electron densities of the electronic states between the ethylene- and phenylene-bridged complexes.