Absorption and magnetic circular dichroism spectra of nitrogen homologues of magnesium and zinc phthalocyanine

Absorption and magnetic circular dichroism (MCD) spectra are reported for the metallophthalocyanine-N-isologs: magnesium-3,4-pyridinoporphrazine (MgPcN4(−2)), zinc-3,4-pyridinoporphyrazine (ZnPcN4(−2)), and zinc-3,4-pyridazinoporphyrazine (ZnPcN8(−2)). Band deconvolution calculations, which couple both the absorption and the MCD spectra, are reported for each complex. The presence of the peripherally fused pyridine rings in MgPN4(−2) and ZnPcN4(−2) reduces the molecular symmetry and splits the degenerate Q band into its x and y components with zero field splitting parameters (ΔQxy) of 224 and 279 cm−1, respectively. Band fitting results for ZnPcN8(−2) show that with the fused pyridazine rings the degeneracy in the 1Eg excited state is retained and the MCD envelope in the region of the Q00 transition can be described by an MCD A term. Comparison of the fitted band energies with the results reported for ZnPc(−2) (T. Nyokong, Z. Gasyna, and M.J. Stillman. Inorg. Chem. 26, 1087 (1987)) and MgPc(−2) (E.A. Ough, T. Nyokong, K.A.M. Creber, and M.J. Stillman. Inorg. Chem. 27, 2725 (1988)) shows that as the number of nitrogens substituted increases from 0 to 8, the Q band blue shifts from 671 nm in ZnPc(−2) to 664 nm in ZnPcN4(−2) (midpoint between the x and y components) to 654 nm in ZnPcN8(−2) and from 672 nm in MgPc(−2) to 666 nm in MgPcN4(−2) (midpoint). These results demonstrate that chemical modification (symmetric and asymmetric) of the peripherally fused benzene rings influences both the energy and the symmetry of the states that form the inner 18-π-electron system.