Effect of substitution on the ESR spectra and electronic ground state of ZnTPP cation radicals

Zinc(II) complexes of substituted tetraphenylporphyrins, Zn[T(X-P)P], where X = H, p-F, p-Cl, m-Cl, p-CH3O, and m-CH3O, have been synthesized and char acterized. Peripheral substitution shows marked changes in the chemical shi fts of the phenyl proton resonances and E-1/2 values of the redox couples i n the anodic region while the UV/Vis spectra are unaffected. The monocation radicals of these complexes are generated by chemical oxidation with bromi ne. The ESR spectra reveal the formation of two types of radical species, Z n[T(X-P)P]Br-+(-) (species I) and Zn[T(X-P)(Br-n-P)]Br-+(-) (species II): S pecies I at 298 K shows a spectrum, with well-resolved bromine hyperfine fe atures, characteristic of a (2)A(2u) electronic ground state while species II, except for the complex with X = m-CH3O, shows a featureless, isotropic resonance attributable to a (2)A(1u) state. Zn[T(m-CH3O-P)(Br-n- P)]Br-+(-) , on the other hand, exhibits nine resolved nitrogen hyperfine features cor responding to a (2)A(2u) state. Variable temperature ESR spectra (77-298 K) indicate reduction in the bromine and nitrogen hyperfine coupling constant s and an increase in the g value of species I from 2.0049 to 2.0060 with lo wering temperature and suggest a labile electronic ground state for species I. The p-CH3O substituted complex exhibits an electronic transformation fr om (2)A(2u) to (2)A(1u) while the remaining complexes, including m-CH3O, sh ow a transformation from (2)A(2u) to admired (2)A(1u)/(2)A(2u) state. The e ffect of substitution on the variable temperature ESR spectra are discussed .