Ligand-Ligand Interaction in [Zn(1,10-phenanthroline)2(H2O)2](NO3)2 and [Zn(2,2′-bipyridyl)3](NO3)2

The interaction between ligands in the excited state and in the ground state was examined by exploring the photophysical behaviors of ligands in the bis- and the tris-bidentate complexes. Although no exciton splitting was observed in the absorption spectra of [Zn(bipy)3](NO3)2 and [Zn(phen)2(H2O)2](NO3)2 at 77 K, the fluoresccence yields of both complexes increased in comparison with those of the corresponding mono-bidentate complexes. It is probable that this change is caused by a delocalization of the excited singlet state, because a variation in Franck-Condon factors and a depolarization of the fluorescence accompanied it. In addition to the ligand-ligand interaction 1,10-phenanthroline and 2,2′-bipyridyl suffered from coordination effects. They have larger fluorescence yields and smaller phosphorescence yields than the free molecules. The decrease on the transition rate of the intersystem crossing is probably due to the changes in the energy levels of 1(n-π*) and 3(n-π*), caused by the metal-ligand bond using a “non-bonding” orbital in the free ligands.