Cross-conjugated bis(porphryin)s: Synthesis, electrochemical behavior, mixed valency, and biradical dication formation

The synthesis and characterization of seven (Zn-II)(2)bis(porphyrin) molecu les 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 benzen e ring (1-4), two have porphyrin rings attached in a gem-fashion to a carbo n- carbon double bond (6 and 7), and one bis(porphyrin) (5) has a p-phenyle ne coupler. Bis(porphyrin)s 1, 4, and 5 contain tetraaryl-type porphyrins, while 2, 3, 6, and 7 contain triarylethynylporphyrins. Except for exciton c oupling in the unoxidized species, interaction between porphyrins is greate r in the triarylethynyl-type bis(porphyrin)s than in the tetraaryl-type bis (porphyrin)s regardless of the number of bonds through which the interactio n propagates. The cyclic volammetry of the bis(porphyrin)s was examined, an d a varying degree of interaction between electrophores within the series w as found. Redox splitting was observed for porphyrin ring oxidations of 3, 6, and 7, suggesting interaction between the oxidized porphyrin rings beyon d 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 pathwa y is short (Delta E-1/2(6/7) > Delta E-1/2(2/3)), (2) pi-overlap between th e electrophore and coupler is maximized (minimal bond torsions: Delta E-1/2 (6/7) > Delta E-1/2(4/5)), and (3) the electron demand of the coupler match es that of the spin carrier (Delta E-1/2(3) > Delta E-1/2(2)). One-electron oxidized triarylethynyl-type bis(porphyrin)s exhibit spectral features cha racteristic of mixed-valent compounds. In two cases (6(.+) and 7(.+)), near -IR bands near 8300 cm(-1) were observed and are tentatively assigned to in tervalence transitions. Singly oxidized tetraaryl-type bis(porphyrin)s exhi bit no such near-IR transitions, and electronic absorption spectra recorded during electrochemical oxidations are marked by isosbestic points, suggest ing negligible interaction between the two halves of the molecule, consiste nt with cyclic voltammetric results. Two-electron oxidation of 2-7 yields b iradical dications whose frozen solution EPR spectra lack fine structure, b ut exhibit Delta m(s) = 2 transitions characteristic of exchange-coupled S = 1 states. Oxidation of 1 yields a biradical in which both exchange coupli ng and dipolar interaction between unpaired electrons are presumably very w eak; consequently no Delta m(s) = 2 is observed. The results of variable-te mperature EPR spectroscopy of 2(2.2+)-7(2.2+) suggest either a triplet grou nd state or a singlet-triplet degeneracy. As a consequence of our results, we hypothesize that the exchange interaction, and therefore the singlet-tri plet gap, in a biradical di-ion can be adjusted to favor the triplet state by simple substituent effects.