Orbital reduction factors in the lowest excited state of the phthalocyanine ring and their measurement by magnetic circular dichroism spectroscopy

Abstract

Magnetic circular dichroism (m.c.d.) spectroscopy has been used to measure the angular momentum of the lowest excited state of the phthalocyanine ring as a function both of the central metal ion and of the axial ligands bound to the metal. The magnetic moments have been extracted from experiment using a moments analysis thus providing numbers which are independent of any distortions, static or vibronic, within the excited state. It is shown that on changing the metal ion from cobalt(II) to zinc(II) a gradual increase in angular momentum occurs whereas changing the axial ligand on iron(II) phthalocyanine from dimethyl sulphoxide to cyanide produces a large change in magnetic moment from 3.24 to 4.58 Bohr magnetons. A simple model has been proposed to account for these observations. It involves molecular orbital formation between the e_g(d_{xz, yz}) metal d-orbitals and the empty 6e_g(π*) orbitals of the phthalocyanine ring. It is shown that this necessarily leads to a reduction of the orbital angular momentum. With simple assumptions it is possible to estimate the molecular orbital coefficients for the metal d-orbital participation in the excited orbitals for a series of iron(II) phthalocyanines with different axial ligands.