Electrochemistry and spectroelectrochemistry of β-pyrazino-fused tetraarylporphyrins in nonaqueous media

Abstract

Bis-porphyrins containing a β,β′-fused pyrazino (Pz) linking group were examined by electrochemistry and thin-layer UV-visible spectroelectrochemistry in PhCN containing 0.1 M tetra-n-butylammonium perchlorate (TBAP) as supporting electrolyte. The investigated compounds are represented as M(TPP)-Pz-(TPP)M, where TPP is the dianion of tetraphenylporphyrin and M = Zn(II) , Cu(II) or Ag(II) . The effect of the linking Pz group on the redox potentials and UV-visible spectra of the neutral, electroreduced and electrooxidized bis-porphyrins is discussed and the data compared to what is observed for related monoporphyrins and earlier characterized bis-porphyrins containing a tetraazaanthracene (TA) linking group and the same central metal ions. The Cu(II) and Zn(II) Pz linked bis-porphyrins exhibit a relatively strong interaction between the two equivalent porphyrin macrocycles as evidenced by UV-visible spectra of the neutral compounds and characteristic splitting of redox processes for three of the four electron transfer reactions, the one exception being the first oxidation, where no splitting of potentials is observed in the formation of the bisporphyrin bis-cation radical, [M(TPP)-Pz-(TPP)M]2+. The first oxidation and first reduction of the bis-porphyrin with two Ag(II) central metal ions occurs via two overlapping one-electron transfer steps in each process indicating equivalent, but non-interacting redox centers. This difference in redox behavior is due to differences in the site of electron transfer, metal centered for the Ag(II) bisporphyrins, which undergo Ag(II) / Ag(III) and Ag(II) / Ag(I) processes as compared to only ring centered electron transfers for the Cu(II) and Zn(II) derivatives. The Pz-linked Zn(II) bis-porphyrins have a larger average HOMO–LUMO gap (2.01 V) as compared to related tetraazaanthracene linked bis-porphyrins (1.67 V), which were earlier characterized in the literature, but the gap is smaller than that for the mono-porphyrins with the same central metal ions (2.17 V). Each redox reaction of the investigated bisporphyrins was characterized by thin-layer spectroelectrochemistry.