Novel High-Spin Molecules: π-Conjugated Polyradical Polyanions. Ferromagnetic Spin Coupling and Electron Localization

The three- and four-site spin coupled chains, based on polyarylmethyl high-spin tri- and tetraradicals, with additional electrons, i.e., diradical anions, radical dianions, and diradical dianions, are prepared and studied. These π-conjugated polyradical polyanions have their charge and spin localized at the separate triarylmethyl sites. In diradical anions, modification of the aryl substituents at the center triarylmethyl sites allows for control of charge/ spin localization at the center or terminal sites. It is shown that when the charge (electron pair) is localized at the terminal site, ferromagnetic coupling between the remaining “unpaired” electrons is found; i.e., ferromagnetic coupling of the spin chain is unaffected by the additional electron. However, when the charge (electron pair) is localized at the center site, antiferromagnetic coupling between the remaining “unpaired” electrons is observed. Therefore, a control of spin coupling via control of electron localization is attained. Also, a new stable triradical, with quartet (S = 3/2) ground state, is prepared. Electrochemistry, UV-vis and ESR spectrosopy, and magnetic data of these novel species are reported.