Voltammetric and UV to near-IR spectroelectrochemical characterization of the meso, meso'-buta-1,3-diyne-bridged octaethylporphyrin dimers {[M(OEP)](.mu.-C4)[M(OEP)]} (M2 = H4, cobalt2, nickel2, copper2, zinc2, palladium2, platinum2, Co/Ni, and Ni/Zn), in their neutral, mononegative, and dinegative oxidation states