Neocuproine-extended porphyrin coordination complexes. 2. Spectroscopic properties of the metalloporphyrin derivatives and investigations into the HOMO ordering.
The synthesis of a porphyrin compound, 1, containing a 2,9-dimethyl-1,10-phenanthroline moiety that is fused at the beta-pyrrole positions is reported. The absorption spectra of the free-base, copper(II), and zinc(II) derivatives have been studied. On the basis of absorption band intensities, the HOMO of the free base (H21) and its copper and zinc complexes (Cu1 and Zn1) was determined to be of a1u symmetry. Relative to H21, compounds Cul and Znl show enhanced spectral changes upon external metal ion binding. Although the HOMO is the same in all three compounds, the energy gap between the two highest occupied orbitals is greater for Cu1 and Zn1 than it is for the free-base compound. Several metal ions (Ni2+, Cu+, Cu2+, Zn2+, Li+) were examined in their binding to the phenanthrolinic group by measuring the resulting changes in the absorption spectra. It is shown that the observed changes in the absorption spectra are insensitive to the nature of the metal ion coordinated by the phenanthroline moiety. Significant differences in the absorption and emission spectra between Zn1 and [Zn(Zn1)2]2+ clearly demonstrate that the porphyrin pi-system is strongly affected by the binding of metal ions at the fused phenanthrolinic moiety.