Magnetic Circular Dichroism of the Iron(III) Porphins

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Abstract
Magnetic circular dichroism (MCD) spectra were measured of some low-spin and high-spin Fe(III) porphins in a rigid matrix of polymethyl methacrylate(PMMA) for a variety of temperature. Some of the most prominent MCD, extrema of the low-spin Fe(III) porphins showed a remarkable enhancement of the intensity at reduced temperatures, while no appreciable increase was observed in the absorption intensity. The temperature dependence of the MCD dispersion of the low-spin Fe(III) porphins arises, in principle, from a change in the Boltzmann distribution between the ground Kramers doublet splitting in a magnetic field, however, the MCD enhancement observed in the Soret region can correctly be interpreted only when the excited states described by a product wave function of the porphin B state and the spin-orbit coupling substates of low-spin iron(III) d5 system are split by a configuration interaction with the “porphin to iron(III)” charge-transfer excited states.

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