Resonance Raman spectroscopy of Mn(III) etioporphyrin I at the π→π* and charge transfer bands: The use of charge transfer bands to monitor the complexation state of metalloporphyrins

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Abstract
Electronic transitions of Mn(III) etioporphyrin I (MnETP) are assigned by the use of resonance Raman spectroscopy (RRS). Dramatic differences are found in the RR spectra of MnETP upon excitation within different absorption bands. RRS supports the assignment of the strong absorption band of Mn(III) porphyrins between 460–490 nm to a charge transfer transition. The two bands between 540–600 nm are assigned to vibronic components of a π→π* transition. The RR spectra of MnETP–X (X=F−,Cl−,Br−,I−, or butanol) show large differences in the low frequency Raman spectrum (100–500 cm−1) depending on the axial ligand. Pure Mn–halide vibrations are assigned. An explanation is proposed to account for the differences between the RR spectra excited in the Q bands and the charge transfer band. Some of the vibrational Raman bands in the low frequency region may serve as probes for the degree of out‐of‐plane distortion of the metal from the porphyrin plane.