Effect of substitution on the redox behaviour and EPR spectra of zinc(II) substituted tetraphenylporphyrin cation radicals

Abstract

Substituted tetraphenylporphyrinatozinc(II) complexes have been synthesized and characterized by various physicochemical techniques. The effects of substitution and counterions on the redox behaviour and EPR spectra have been examined. The EPR spectra revealed that chemical oxidation with bromine yields ZnT ( X - P ) P + Br - (species I) and a brominated tetraphenylporphyrin cation radical (species II). The former exhibits a labile electronic ground state (with 2 A 2 u ground state transforming into 2 A 1 u at 175 K), while the latter is characterized by a 2 A 1 u state. The lability in the electronic ground state for species I is discussed in terms of the change in molecular geometry as the temperature is lowered. The EPR spectra also revealed for the first time the formation of dimeric agglomerates of TPP cation radicals in low concentration below the electronic transition (i.e. near 170 K) wherein the paramagnetic sites are antiferromagnetically coupled.