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(mu-L-1)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(mu-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 (ZnL1 and ZnL2) were measured and compared with those of the corresponding zinc( II) ethylene-bridged complexes (ZnL3 and ZnL4). Our results revealed that, at 77 K, both ligand-centered triplet ((LC)-L-3) and singlet ((LC)-L-1) states existed for the ethylene-bridged complexes (ZnL3 and ZnL4), whereas only the (LC)-L-1 state was detected for the phenylene-bridged complexes (ZnL1 and ZnL2). NIR sensitization studies of [Zn(mu-L')Nd(NO3)(3)(H2O)(n)] (L' = L-1-L-4) complexes further showed that Nd3+ 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.