Differently luminescent sensing abilities for Cu2+ ion of two metal phosphonates with or without the free Lewis basic pyridyl sites

ABSTRACT Two metal phosphonates with or without the free Lewis basic pyridyl sites, namely, [Zn1.5(HL)]•2H2O (1) and [Cd0.5(H3L1)]•H2O (2) have been hydrothermally obtained from 4–pyridyl–CH2N(CH2PO3H2)2 (H4L) and 3–pyridyl–CH2N(CH2PO3H2)2 (H4L1). The pyridyl N atom of 1 is uncoordinated with Zn(II) ion, whereas the pyridyl N atom of 2 is coordinated with Cd(II) ion, which leads to the different luminescent sensing abilities for Cu2+ ion. Compound 1 shows a highly selective and sensitive luminescent sensor for sensing Cu2+ with a low detection limit of 2.7 nM in aqueous solution. However, 2 can not be used as a luminescent sensor for metal ions. The result indicates that the presence of the free Lewis basic pyridyl sites in the framework plays an important role in the recognition of metal cations. Meanwhile, the quenching mechanism for 1 is also investigated, which can be attributed to the weak interaction between the free Lewis basic sites from the HL3– ligands and Cu2+ ions.