Consequences of Oxidation in Nonplanar Porphyrins: Molecular Structure and Diamagnetism of the .pi. Cation Radical of Copper(II) Octaethyltetraphenylporphyrin

Abstract

Crystal structures are reported for the sterically crowded porphyrin Copper(II) 2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetraphenylporphyrin (Cu(OETPP), 1) and its [pi] cation radical Cu(OETPP)[sup [sm bullet]+]ClO[sub 4][sup [minus]] (2). 1 was chosen to assess the consequences of oxidation in a nonplanar porphyrin on the expectation that its multiple peripheral substituents not only induce an S4 saddle conformation on the macrocycle but should also prevent the dimerizations in the solid that have complicated several previous crystallographic studies of porphyrin [pi] cation radicals. Interest in the consequences of oxidation arises from the presence of nonplanar bacteriochlorophylls in photosynthetic reaction centers in which the chromophores lie in van der Waals contact so that even small structural changes induced by electron transfer would alter the electronic coupling between the [pi] cation and anion radicals generated by the primary photochemical charge separation. Oxidation of 1 does indeed result in further conformational changes in 2: an additional ruffling is imposed on the original saddle shape of 1 in which the pyrrole rings twist, the meso carbons move alternately up and down out of the porphyrin plane by approximately 0.2 angstroms, and the phenyl groups rotate further into that plane by more than 10[degree]. 61 refs., 11 figs., 4 tabs.