Synthesis and Properties of Tetraphenylporphyrin Molecules Containing Heteroatoms Other Than Nitrogen. 6. Electrochemical Studies

When the two NH groups in tetraphenylporphyrin (TPP) are replaced by the group 6A heteroatoms S, Se, and Te, bonding interactions within the porphinato core are formed as is apparent from their short distances as found in X-ray structure analysis and is also supported by MO calculations. Because of the heterosubstitution there is a change in the energies of the frontier orbitals in such a way that the LUMO is stabilized. The HOMO is stabilized because the short distances between the chalcogen atoms give rise to the formation of dπ-dπ orbitals, which interact with the frontier orbitals. These stabilization mechanisms are expressed by positive shifts of both the reduction and the oxidation potentials as measured by cyclic voltammetry. It appears that there is a linear relationship between the difference between the first oxidation and first reduction potential (which decreases with the increasing bonding interaction) and 1/λmax, (cm-1) of the Soret transition. The decrease of the first reduction potential relates linearily to the chemical shifts of the β-hydrogens in the 1HMR spectra. For the para-substituted S2TPP molecules the Hammett constant p for the first oxidation reaction is abnormally large. This is connected with the nonplanarily of the cation radicals. The optical spectrum of the Se2TPP cation radical provides support for this suggestion.