Cation Radicals and n Dications

Easy accessibility of rather stable porphyrin cation radicals, however, as well as their detailed analysis, was only achieved when esr spectra, absorption spectra and redox-potentials had been quantitatively related to the oxidative formation of these radicals and crystalline material had been obtained. 

The oxidation of magnesium porphyrins in organic solvents to their cation radicals were studied in great detail because of their interest for some reactions of the chlorophylls in photosynthesis. Chemical or electrochemical oxidation of various magnesium porphyrins yields stable, crystallizable jr-cation radicals and is fully reversible [Fuhrhop (76, 78)]. 

A comparison of the midpoint potentials of some typical complexes (Table 8) shows that the potentials of metallochlorins lie about 300 mV 

The electronic spectrum of the MgOEP cation radical contains an unusually intense solvent- and counterion-dependent absorption band at 685 nm, which can be explained either by distortion of the macrocycle and interaction of a methine bridge with a soft counterion or by a change in the symmetry of the porphyrin orbital that contains the electron hole [Felton (59)]. 

Zn OEP undergoes a fully reversible one-electron oxidation at 525 mV [Fuhrhop (78)] and ZnTPP at 710 mV [.Felton (59)]. The visible spectra of the resulting cr-cation radicals consist of one broad band covering the whole visible range. Zn OEP+ forms a stable n-n dimer, which produces a very strong, broad absorption band with a maximum around 930 nm [Fuhrhop (79)]. This diamagnetic dimer has exactly the same redox behavior as the monomer and is also formed when the monomeric radical is crystallized. 

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