Asymmetry in Mössbauer spectra of FeIII-azaporphyrin complexes

Temperature dependence of asymmetry as well as isomer shift and quadrupole splitting values of Mössbauer spectra of trivalent iron complexes coordinated to porphyrin ligands such as octaethylporphyrin, monoazaethioporphyrin, diazaoctaethylporphyrin and phthalacyanine is discussed in this paper. All the above mentioned complexes have the chloride axial ligand at the fifth coordination site. Different temperature changes of asymmetry and other parameters of Mössbauer spectra of these complexes can be correlated with a number of methine bridges CH substituted by nitrogen atoms in the porphyrin macrocycle. The form of this asymmetry is such that the higher-energy component of the doublet is more broadened than the lower-energy one, while the areas (intensities) under each of two components are the same. The most distinct asymmetry is observed at room temperature. When temperature is decreasing, the spectrum becomes successively more symmetrical and at liquid helium temperature linewidths of both components of the quadrupole splitting are very close one another. The Mössbauer spectrum of the FeCl-monoazaethioporphyrin complex exhibits reversed asymmetry at liquid helium temperature in comparison with the spectra of the remaining complexes i.e. the lower-energy component of the doublet is slightly more broadened than the higher-energy one. The spin-spin and spinlattice relaxation mechanisms in the context of quantum mechanically mixed spin states S = 5/2 + 3/2 of FeIII ions are considered as a source of the observed asymmetry.