Highly redox-active oligomeric porphyrins

Facile and Reversible Electrogeneration of Porphyrin Trianions and Tetraanions in Nonaqueous Media

“…We had initially anticipated that the reduction of NiTTP-(PE) 8 at low temperature in THF might be characterized by a conversion of the porphyrin dianion to its tetraanionic and hexaanionic forms, as earlier demonstrated by Heinze, Mullen, and co-workers for structurally related Zn(II) and free base porphyrins which lacked the electron-withdrawing NO 2 and PE substituents. 15,16 The voltammetric data in Figure 3 for the reduction of MTPP(NO 2 )(PE) 6 and MTTP(PE) 8 indicate that a conversion of the doubly reduced porphyrin to its quadruply reduced form occurs via two one-electron transfers in pyridine, and the same is seen for NiTTP(PE) 8 in THF (Figure 5). The four one-electron transfers are extremely well defined in THF at room temperature (Figure 5), with virtually identical separations in E 1/2 values of 0.27 and 0.28 V, respectively, between the first and second reductions at E 1/2 = −0.72 and −0.99 V and the third and fourth reductions at E 1/2 = −1.59 and −1.87 V.…”

“…Thus, a broadbased search for monomeric porphyrins with highly reduced πring systems has, to our knowledge, never been attempted, despite reports more than 25 years ago that zinc and free base tetraphenylporphyrins could be reversibly reduced to the tetraand hexa-anionic forms in dimethylamine at −65 °C. 15,16 We recently described the synthesis and characterization of porphyrins containing multiple phenylethynyl (PE) and/or other highly electron-withdrawing substituents at the β-pyrrole positions of the macrocycle. 17−19 The electrochemistry of these porphyrins was examined in CH 2 Cl 2 and the potentials recorded for the stepwise formation of π-anion radicals and dianions after the addition of two electrons.…”

“…Most electrochemical studies of porphyrins in nonaqueous media have been carried out in solvents whose negative potential limit extends to −2.0 V vs SCE or more, but this limit was often not reached when characterizing the redox reactivity of porphyrins having highly electron-withdrawing substituents, sometimes because only the first (or first and second) reduction was of interest and other times for the simple reason that no further electrode reactions of the porphyrin macrocycle were expected to occur at potentials beyond formation of the dianion. Thus, a broad-based search for monomeric porphyrins with highly reduced π-ring systems has, to our knowledge, never been attempted, despite reports more than 25 years ago that zinc and free base tetraphenylporphyrins could be reversibly reduced to the tetra- and hexa-anionic forms in dimethylamine at −65 °C. , …”