Effect of metal-ligand ratio on fluorescence properties of zinc(II) and beryllium(II) Schiff base complexes

Many excellent fluorometric determination methods of metal ions by using the fluorescent Schiff base complexes have been reported. Most of them employed complexes which have a metal-Schiff base ratio of 1:1. We studied the effect of the metal-Schiff base ratio and of the alkyl group in the phenyl ring of the Schiff base on the fluorescence properties for Zn-and Be-salicylidene-alkylamine complexes. The shape and maximum wavelength of the absorption and fluorescence spectra of the 1:2 (metal-ligand ratio) complexes were similar to those of the 1:1 complexes. But the fluorescence quantum yields of the 1:1 complexes were larger than those of the 1:2 complexes, while the phosphorescence quantum yields of the 1:1 complexes were smaller to those of the 1:2 complexes. These results meant that the interaction between two ligands in the 1:2 complexes caused the increase of the spin-orbit coupling and remarkably affected the decay process of the excited electrons, while it slightly affected the electronic states of each ligands. The effect of the metal-ligand ratio mentioned above were more advantageous to the Be complexes rather than the Zn complexes. This is due to the difference of the ionic radius between Be and Zn. Regarding the effect of the alkyl group in the phenyl ring of the Schiff base, the alkyl group at the 5-position to azomethine group increased the fluorescence of the complexes, while the alkyl group at 4-position increased the phosphorescence. These phenomena corresponded to the energy difference between the excited singlet state and excited triplet state. For the fluorometric determination of metal ions using metal-Shiff base complexes, especially for small ions. like Be, the 1:1 (metal-Shiff base) complexes were superior to the 1:2 complexes in the sensitivity.