Redox potential porphyrin substituents

An important class of porphyrins is that constituted by confor-mationally distorted porphyrins, which mimic the non-planar geometry of the porphyrins present in photosynthetic systems.89 Obtainment of such non-planar distortions is associated with the introduction into the macrocyclic frame of proper crowding substituents, which therefore not only cause structural distortion but also affect, through their electronic effects, the redox potentials. A typical case is that constituted by [Cun(OETPP)] (OETPP = 2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetraphenylporphyrin), the saddle-distorted molecular structure of which is illustrated in Figure 56.102... 

The different cytochromes display rather similar redox activity, i.e. the reversible Fe(III) zzt Fe(II) process, but their redox potentials change with the peripheral substituents of the Fe(III)-porphyrin (particularly, as a function of the axially coordinated groups). 

Table 2 contains redox potentials for representative porphyrin, SB, and related complexes in nonaqueous solvents. These data primarily illustrate the influence of axial ligation and porphyrin substituents on the Mn / potential. Planar macro-cyclic ligands such as phthalocyanine and corrole provide an equatorial coordination environment similar to that of porphyrins. 

Relationships between the nature of porphyrin and the redox potentials have been found. For example, The reduction potentials of 3,8-disubstituted DPIX DME and their pJsT, values are correlated by equation (24).145 Kadish et a/.146 have summarized porphyrin substituent effects by applying the Hammett-Taft equation (equation 25). 

H), i, 2(X) redox potentials for the parent and substituted porphyrins Hammett substituent constant < EMP sensitivity parameter... 

The effect of substitution is in the order meso > /S > phenyl, as expected from the shapes of HOMO and LUMO and the distances from the porphyrin n system. The ring substituent effects on the metal redox potential are smaller than those on the ring redox for comparison, gEMP = 18 mV (Nim/I1), 38 mV (Fe111 1), 54mV (Mn111/n) for MCT PP) in CH2C12. 

Substituents on the porphyrin affect the redox potential, in that reduced basicity of the pyrrole N donor atoms results in a destabilization of the Fe111 state. 

Several reports have called into question the correlation between sterically enforced, out-of-plane deformation and the electronic properties, redox potential and spectroscopic signatures. Based on UV-visible spectra, structural analysis, and TD-DFT calculations, it has been suggested that the shifts in UV-visible transitions result from electronic effects of the peripheral substituents on the aromatic porphyrin core. These effects drive a bond-alternating rearrangement of the porphyrin core atoms called in-plane nuclear reorganization. ... 

Fullerene derivatives functionalized with pyridine or imidazole substituents can coordinate with several metal porphyrins, Pc, and Nc in solution by means of self-assembly. Such electron-rich macrocyclic dyes are good electron donors as they absorb light in the visible near-infrared (IR) region and exhibit a usable redox potential. 

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