Conformational and substituent effects on spin distributions in porphyrin cation radicals

Optical, redox, EPR, and ENDOR results are presented for zinc meso-tetraphenyltetrabenzoporphyrin (ZnTPTBP). The molecule is a hybrid of the common synthetic porphyrins zinc tetraphenylporphyrin (ZnTPPP) and zinc tetrabenzoporphyrin (ZnTBP). The consequences of the additional peripheral substituents in the hybrid are evident in a previously obtained x-ray structure of ZnTPTBP that shows the macrocycle to be severely puckered to minimize steric interactions between the substituents. The cumulative effects of conformational changes and substituent addition are reflected in the red-shifted optical spectrum and ease of oxidation of the neutral compound, and in a shift of unpaired spin density from a{sub 1u} (ZnTBP) to a{sub 2u} (ZnTPP) character in the radical. Significant spin density is found to delocalize onto the phenyl substituents of ZnTPTBP{sup {sm bullet}+} and illustrates the shortening of the effective distance between donor and acceptor that may occur in biomimetic complexes used to investigate the distance and energy dependence of electron transfer.