Fluorescence properties of zinc(II) and beryllium(II) salicylidene-alkylamine Schiff base complexes and effect of methyl group on fluorescence characterization.

Abstract

The cause of fluorescence emission in metal Schiff base complexes have been explained mainly by two hypotheses, one is attributed to a quinoid structure complex, the other an anion structure of a Schiff base. Structural effects on the fluorescence intensity of Zn and Be salicylidene-alkylamine complexes have been studied in a mixed solvent of methanol and ethanol. The neutral Schiff bases exist as a tautomer of the enolimine and the ketoamine forms, the latter of which has a quinoid structure. Fluorescence from neutral Schiff base molecules were not observed even at temperatures as low as 77 K. On the other hand, the anionic form of Schiff bases fluoresce at room temperature. The fluorescence spectra of metal-Schiff base complexes were similar to those of Schiff base anions measured at 77 K. The absorption spectra of Zn and Be complexes were in accordance with those of the anionic form Schiff bases. The enhancement on the fluorescence intensity by replacing the hydrogen atom at the 3- or 5-position of a salicylaldehyde component of a Schiff base with a methyl group is ascribable to the delocalization of π-electrons of the anionic form. These results suggested that the fluorescence properties of the metal Schiff base complexes are not due to the quinoid structure of the Schiff base, but due to maintaining the planar structure of the anionic form of the Schiff bases. The substituent effect on the fluorescence intensity of metal Schiff base complexes would be affected by the extent of the delocalization of the π-electrons on the anionic form of Schiff bases. © 1987, The Japan Society for Analytical Chemistry. All rights reserved.