Visible and near-infrared luminescence of helical zinc(II)-lanthanide(III) trinuclear complexes having acyclic Bis(N2O2) oxime ligand

Luminescent properties of helical zinc(II)-lanthanide(III) complexes [LZn2Ln] (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu), which are obtained by the reaction of acyclic bis(N2O2) tetraoxime ligand with zinc(II) and lanthanide(III) ions, were studied. The H NMR spectral study indicated that the complexes [LZn2Ln] adopt a helical conformation in solution, where two terminals are in an equivalent environment. In the absorption spectra, the * transitions of ligand L were observed at around 330–340 nm associated with two shoulder bands between 370–450 nm. Upon excitation at 425 nm, [LZn2La] showed relatively intense ligand-centered emission at 540 nm (quantum yield, 0.052). The corresponding luminescence of the complexes of all other lanthanides [LZn2Ln] (Ln = Ce –Lu) was much weaker. The complexes [LZn2Ln] (Ln = Sm , Eu, Tb, Dy) did not show detectable metal-centered emission, although these ions are potentially emissive in the visible region. On the other hand, [LZn2Nd] and [LZn2Yb] exhibited metal-centered emission due to f-f transition in the near infrared region in addition to the weak ligand-centered emission. Consequently, the ligand L can work as a sensitizer for Nd and Yb luminescence in the near infrared region.