This work presents the results of UV-Visible absorption and Resonance Raman spectroscopic studies of three different heme complexes. (DMSO)2Fe(II)TPP (1) and (TMSO)2Fe(II)TPP (2) were studied in neat DMSO (dimethylsulfoxide) and neat TMSO (tetramethylene sulfoxide), respectively. A third heme complex, (PSO)2Fe(II)TPP (3), required a binary solvent of benzene and PSO (phenyl sulfoxide) in order to be created and studied. Both (1) and (2) were studied in the same binary solvent as well. The investigation determined that all three complexes behave similarly in relation to their UV-VIS and resonance Raman spectra. Resonance Raman experiments at low incident laser power have shown that (1), (2), and (3) exhibit the properties of a 6-coordinate, low-spin complex in the binary solvent. (1) and (2) exhibit the same properties in neat solvent. The UV-visible absorption data also provides evidence for the presence of a 6-coordinate low-spin complex for all three species in the ground state. The UV-VIS data for (2) and (3) shows a 4 and 5 nm red-shift, respectively, in their Soret bands as compared to (1) in the binary solvent. (2) displays a 3 nm red-shift in its Soret band as compared to (1) in neat solvent. These shifts are attributed to the different sulfoxide ligands bound to the iron center.
For all three complexes, at high laser power, photolysis of an axial ligand occurs and a transient 5-coordinate, high-spin photoproduct is created. This transient photoproduct has a core-size of 2.05 Angstroms and 2.04 Angstroms for (1) and (2) in neat solvent. In binary solvent, the transient photoproducts for (1), (2), and (3) all possessed a calculated core-size of 2.05 Angstroms.
The resonance Raman data provides evidence that the sulfoxides studied are moderate sigma-donors and pi-acceptors when compared to nitrogenous ligands such as imidazole and pyridine. However, there is no evidence in the resonance Raman data to distinguish among the three sulfoxides the strongest pi-acceptor.