Ligand-Ligand Interaction in the β-Diketonato Complexes of Be2+, Al3+, and Zn2+

The fluorescence yield (ΦF) and the phosphorescence yield (ΦP) of acetylacetone, benzoylacetone, and dibenzoylmethane were enhanced by coordination of Zn2+, Al3+, and Be2+ in an increasing order. In the bis- and the tris-bidentate complexes, not only did the ratios of ΦP to ΦF increase, but also the lifetimes of the phosphorescences became a little longer than that of the corresponding mono-bidentate complex. This change originates from some kinds of interaction between the ligands in the excited and ground states. Such an inter-ligand interaction was supported by the measurement of the polarization. However, the interaction energy is smaller than the expected values in the analyses of the absorption spectra and the circular dichroism in the region of the ligand band. A difference in the absorption spectra between the mono- and the bis- or the tris-bidentate complexes was seen only in the Franck-Condon factors; this finding supports the extension of the excited state through the two or the three ligands due to the weak inter-ligand interaction.