Assignment of the charge-transfer bands in some metal phthalocyanines. Evidence for the S= 1 state of iron (II) phthalocyanine in solution

Abstract

Magnetic circular dichroism spectroscopy has been used to investigate the assignment of the charge transfer bands in a number of metal phthalocyanines containing first row transition metals. It has been shown that the band in cobalt(III) phthalocyanine dicyanide at 22.5 kK is not a charge transfer band but must be a transition of the phthalocyanine ring. Similarly doubt is shed upon the charge-transfer assignment of the band in iron(II) phthalocyanine dicyanide at 22.5 kK. However, it has been confirmed that the band system in cobalt(I) phthalocyanine at ∼22 kK is a metal-ligand charge-transfer transition which may be assigned as e_g→b_1u, b_2u. Finally, the low energy band of iron(II) phthalocyanine which appears in solvents of low co-ordinating ability, such as dichlorobenzene, has been studied. The m.c.d. spectrum enables a choice to be made for the ground state of the compound. It is shown to be ³A_2g, a result which is the first demonstration of an intermediate spinstate of iron(II) in solution. The low energy band in the spectrum has been assigned as a metal to ligand charge transfer band, ³A_2g→³E_u. This evidence throws considerable doubt upon the recent assignment of the ground state of crystalline iron(II) phthalocyanine to ³B_2g based upon single crystal anisotropy measurements.