β-Substituted copper porphyrin cations: A2u or A1u radicals?

DFT calculations have been performed on a series of β-substituted copper porphyrins, CuP–X, with particular regard to substituent effects upon the relative energies of two frontier occupied, nearly degenerate a2u and a1u orbitals. Substitution by electron-withdrawing groups only slightly enlarges the a2u–a1u separation of CuP. Electron-donating groups CH2OH and OCH3 raise the energies of a2u and a1u uniformly. In contrast, SH and NH2 reverse the normal ordering; these [CuP–X]+ cation radicals are mainly a1u in character. Electronic structures of CuP derivatives substituted at all eight β-pyrrole positions are also examined.