Porphyrin dimers linked by conjugated butadiyne bridges: Preparations, spectra, voltammetry and reductive spectroelectrochemistry of {(M(OEP))(mu-C-4)(M(OEP))} (M-2 H-4, Co-2, Ni-2, Cu-2, Zn-2, Pd-2, Pt-2, Co/Ni, Ni/Cu, Ni/Zn)

A series of derivatives M2P2 (M-2 = H-4, Co-2, Ni-2, Cu-2, Zn-2, Pd-2, Pt-2 , Co/Ni, Ni/Cu, Ni/Zn) of the Ligand meso,meso'-bis(octaethylporphyrinyl)bu tadiyne has been prepared and characterized by H-1 NMR, FT Raman and visibl e absorption spectroscopies as well as by cyclic and a.c. voltammetry in CH 2Cl2 solution at 20 and -40 degrees C. The electronic spectra exhibit multi ple Sorer bands and the voltammetry reveals successive one-electron reducti ons indicating the accessibility of 'mixed valence' pi-radical anions and p i-dianions. Using in situ thin layer spectroelectrochemistry, the UV to nea r-IR spectra of [M2P2](1-) and [M2P2](2-) (M as above) were recorded at les s than or equal to -40 degrees C. Apart from the Co complexes (reduced at t he metal ion), the bis(porphyrin) anions have spectra which include sharp, intense near-IR bands (epsilon = 50000-200000 M-1 cm(-1)) at c. 4500 and 11 500 cm(-1) ([M2P2](1-)) and 9500 cm(-1) ([M2P2](2-)). An empirically constr ucted semiquantitative frontier orbital model explains the observed electro nic absorption bands. Inter-porphyrin conjugation, mediated by the butadiyn e bridge, is responsible for the Soret band multiplicity. The near-IR bands of the anions are assigned to long-axis polarized pi --> pi* transitions w ithin the newly occupied upper manifold of the two-porphyrin eight-orbital framework. The small comproportionation constants found for the diporphyrin monoanions contradict the usual assumption that electronically coupled dim ers should have a large voltammetric separation between the first and secon d redox steps. (C) 1999 John Wiley & Sons, Ltd.