REDOX PROPERTIES OF NOVEL TETRAPYRROLES - EXPANDED PORPHYCENES

The redox properties of four novel porphyrinoid tetrapyrrolic macrocyl es (in CH2Cl2 and THF) are reported. Two of the compounds constitute e xpanded porphycenes exhibiting 22 pi main conjugation pathways but may be viewed, alternatively, as di-trans-[22]porphyrins-(2.2.2.2). Featu ring linear C-sp2(CspCsp)(n)C-sp2 structural units, two homologous com pounds match porphycene in symmetry and are termed 22 pi (n = 1) and 2 6 pi (n = 2) acetylene-cumulene porphycenes. All of these porphyrinoid s exhibit similar electrochemical behaviour in that they undergo four reversible electron transfers: two reductions and two oxidations. As e xpected, an increase in the number of pi-electrons on going from porph yrin or porphycene to the four new compounds lowers the energy barrier for the electron transfers. This redox behaviour correlates with the red shifts observed in the UV-visible absorption spectra.