Modulative magnetic interaction in π-diradicals of directly meso-linked porphyrin dimers

A series of directly meso-linked Zn(II) porphyrin dimers were prepared, and properties of their bis(π-radical cation)s were investigated. The redox potentials of the dimers bearing various meso-substituents are distributed in wide range although the potential difference (ΔE) between the first and the second oxidations were similar values. Among these dimers, di-tert-butylphenyl derivative and octyloxyphenyl derivative were chemically oxidized by the treatment with NaAuCl₄ in chloroform to afford mono(π-radical cation)s and bis(π-radical cation)s. In the mono(π-radical cation), the π-radical resides one of the porphyrin rings in view of the ESR hyperfine structure. The bis(π-radical cation)s of these dimers showed fine-structured ESR spectra due to triplet species, indicating that the orthogonal alignment of π-radicals leads a stable triplet state. The plots of the triplet ESR signal intensity of the bis(π-radical cation)s versus temperature showed an anomalous feature, in which slight change at lower temperatures and drastic change at higher temperatures were observed. The spectral behavior at lower temperatures was elucidated to be derived from the existence of several kinds of frozen dimers with various dihedral angles between two porphyrin rings. An abrupt increase of triplet signal intensity around 130–160 K can be rationalized by molecular motion around the meso–meso linkage. Such phenomena are comparable with spin-crossover. It also proved that the magnetic interaction in the π-diradical is modulated by the electron-donating ability of the meso-aryl substituents.