The influence of the halide electrolyte on the electrochemical reduction pathway of some meso-tetrasubstituted porphyrin free bases in N, N-dimethyl formamide

Abstract

Differential pulse, cyclic and rotating disc voltammetry have been used to study the redox properties of meso-tetra-n-propylporphyrin (TPrP), meso-tetramethylporphyrin (TMP) and meso-tetraphenyl-porphyrin (TPP) at glassy carbon electrodes in tetraalkylammonium electrolyte in N,N-dimethyl form-amide solvent. Electrochemical reduction of the porphyrins is affected by the electrolyte employed. There is a large influence from the electrolyte anion and only a small one from the cation, shown by the difference in half-wave reduction potentials between tetrabutylammonium and tetraethylammonium electrolytes, both effects independent of the electrode material being glassy carbon or mercury. A systematic study on the influence of the anion was undertaken using TPrP and TPP in halide (chloride, bromide and iodide) salts of tetrabutylammonium electrolytes, including variation of the quantity of halide ion present, the behaviour in 0.1 M tetrabutylammonium perchlorate being taken as the reference. Trends in half-wave potential and wave shape from chloride to perchlorate are more accentuated for TPrP than for TPP. UV-visible spectra of the porphyrins were found to be electrolyte invariant. Rationalisation in terms of interaction between the porphyrin mononegative ion and the halide ion providing an alternative, anion concentration dependent, reduction pathway is discussed.