Disappearance of MCD A term in the fingerprint band of cation radicals of phthalocyanine metal complexes

The magnetic circular dichroism (MCD) spectrum and assignment of the characteristic `fingerprint bandʹ of the π-cation radical of phthalocyanine complexes are studied using a semiempirical MO method. Two types of excited configurations from occupied eg orbitals to the a1u singly occupied molecular orbital (SOMO) were present between Q band and B-band manifolds. The magnetic dipole moments of the two configurations were similar in magnitude but the signs were opposite. Because of the vicinity in energy, the two excited configurations couple to give an excited state, 2Eg, having an extremely reduced A term. The `fingerprint bandʹ is assigned to this state.