Frontier orbitals of porphyrin electron donors in biomimetic model compounds — partial lifting of orbital degeneracy in asymmetric porphyrins studied by EPR on anion radicals

Abstract

The electronic structure of three porphyrins is investigated by EPR. These porphyrins are used as photoinduced electron donors in biomimetic model compounds for the light induced charge separation in photosynthesis. For efficiency of the model compound the distribution of the LUMO and HOMO over the porphyrin are important. Thus, cation radicals probing the HOMO, and anion radicals probing the LUMO are studied. The effect of lowering the porphyrin symmetry on the Jahn‐Teller distortions in anion radicals (J. Seth, D. F. Bocian; J. Am. Chem. Soc. 116 , 143–153 (1994)) are studied. Jahn‐Teller effects severely limit the information obtainable by EPR. Strong perturbations to the symmetry in 2,8,12,18‐tetrabutyl‐3,7,13,17‐tetramethyl‐5,15‐diphenylpor‐phyrinato‐magnesium cause sufficient lifting of degeneracy to allow to determine details of the electronic structure in the anion radical. The cation radical of this porphyrin is found to have an A 1u , or a mixed A 1u /A 2u ground state. On the basis of both results an optimized design of model compounds based on this porphyrin is discussed quantitatively: linking of an electron acceptor to the phenyl group bearing positions of this porphyrin should result higher efficiency than linking at the other two meso positions.