Spectroscopic studies on unsymmetrical redox-activetetra arylporphyrins

Abstract

A series of unsymmetrical meso-tetra-arylporphyrins, substituted with redox active (3,5 di-tertiary-butyl-4- hydroxyphenyl (DtB4HP» and funct'ionalisable (4-hydroxyphenyl (4HP), 3-hydroxyphenyl (3HP) and 4-nitrophenyl (4NP)) substituents were synthesised and characterized by nuclear magnetic resonance spectroscopy, elemental analysis and fast atom bombardment mass spectroscopy. Their aerial oxidation in basified dichloromethane solutions (using 1 molar potassium hydroxide in methanol (1M KOH/MeOH) and l molar tetra-n-butylammonium hydroxide in methanol (1M tnBAH/MeOH») was followed by UV/visible spectroscopy, electron spin resonance spectroscopy and cyclic voltammetry. Basification with. 1M tnBAH/MeOH produced very little aerial oxidation for all the porphyrins in this study, but basification with 1M KOH/MeOH lead to substantial aerial oxidation of the 4HP series and the mono-4NP and trans-4NP porphyrins. Little oxidation in the case of the 3HP series occurred with either base. Cyclic voltammetry showed that in basified DCM solutions, one electron oxidation becomes more difficult as the DtB4HP groups are replaced by 4HP. In contrast, the trend in oxygen reactivity for the 3HP and 4NP porphyrins was not correlated with the first half-wave potentials obtained from cyclic voltammetry. An explanation for these differences is suggested in terms of: (1) the difference in the strength of electron releasing power of the three types (DtB4HP, 4HP and 3HP) of phenolic substituent. DtB4HP groups release electron density onto the porphyrin macrocycle more effectively than 4HP groups, which in turn release electron density more effectively than 3HP groups; (2) the relative availability to oxygen of DtB4HP electron density in the various series of porphyrins¡ (3) the relative ability in certain 4NP-substituted porphyrins to stabilise unpaired electron density over the macrocycle and its substituents