Cross-Conjugated Bis(porphryin)s:  Synthesis, Electrochemical Behavior, Mixed Valency, and Biradical Dication Formation

The synthesis and characterization of seven (ZnII)2bis(porphyrin) molecules are described. The molecular structures of two bis(porphyrin)s (6 and 7) were determined by X-ray diffraction methods. Four of the compounds have their porphyrin moieties attached in a meta-fashion to a substituted benzene ring (1−4), two have porphyrin rings attached in a gem-fashion to a carbon−carbon double bond (6 and 7), and one bis(porphyrin) (5) has a p-phenylene coupler. Bis(porphyrin)s 1, 4, and 5 contain tetraaryl-type porphyrins, while 2, 3, 6, and 7 contain triarylethynylporphyrins. Except for exciton coupling in the unoxidized species, interaction between porphyrins is greater in the triarylethynyl-type bis(porphyrin)s than in the tetraaryl-type bis(porphyrin)s regardless of the number of bonds through which the interaction propagates. The cyclic voltammetry of the bis(porphyrin)s was examined, and a varying degree of interaction between electrophores within the series was found. Redox splitting was observed for porphyrin ring oxidations of 3, 6, and 7, suggesting interaction between the oxidized porphyrin rings beyond simple electrostatic repulsions. No such redox splitting was observed for any of the tetraaryl-type bis(porphyrin)s (1, 4, and 5). We conclude that two porphyrin radical cations interact best when (1) the interaction pathway is short (ΔE1/2(6/7) > ΔE1/2(2/3)), (2) π-overlap between the electrophore and coupler is maximized (minimal bond torsions: ΔE1/2(6/7) > ΔE1/2(4/5)), and (3) the electron demand of the coupler matches that of the spin carrier (ΔE1/2(3) > ΔE1/2(2)). One-electron oxidized triarylethynyl-type bis(porphyrin)s exhibit spectral features characteristic of mixed-valent compounds. In two cases (6•+ and 7•+), near-IR bands near 8300 cm-1 were observed and are tentatively assigned to intervalence transitions. Singly oxidized tetraaryl-type bis(porphyrin)s exhibit no such near-IR transitions, and electronic absorption spectra recorded during electrochemical oxidations are marked by isosbestic points, suggesting negligible interaction between the two halves of the molecule, consistent with cyclic voltammetric results. Two-electron oxidation of 2−7 yields biradical dications whose frozen solution EPR spectra lack fine structure, but exhibit Δms = 2 transitions characteristic of exchange-coupled S = 1 states. Oxidation of 1 yields a biradical in which both exchange coupling and dipolar interaction between unpaired electrons are presumably very weak; consequently no Δms = 2 is observed. The results of variable-temperature EPR spectroscopy of 22•2+−72•2+ suggest either a triplet ground state or a singlet−triplet degeneracy. As a consequence of our results, we hypothesize that the exchange interaction, and therefore the singlet−triplet gap, in a biradical di-ion can be adjusted to favor the triplet state by simple substituent effects.