SEMIEMPIRICAL INVESTIGATION OF STILBENE-LINKED DIRADICALS AND MAGNETIC STUDY OF THEIR BIS(N-TERT-BUTYLNITROXIDE) VARIANTS

The electronic states of open-shell stilbene diradicals were st;hied. Previously used semiempirical MO-CI methods were applied to stilbenes having dimethylene, dioxyl, and dinitroxide centers in o,o'-, o,m'-, o ,p'-, m,m'-, m,p'-, and p,p'-substitution patterns, and the various si nglet-triplet energy gaps were computed to predict qualitative ground state spin multiplicities. Singlet and triplet spin states were essent ially degenerate in the m,m'-isomers of disjoint connectivity, while n ondisjoint o,m'- and m,p'-isomers had triplet computed ground states. Nitroxide-based diradicals showed considerably smaller computational e xchange coupling energies than did other diradical models studied. As a tests of these computations, stilbenes with two N-tert-butyl nitroxi de groups at the o,m'-, m,m'-, and m,p'-positions were prepared, and t heir magnetic properties were studied by ESR and-magnetic susceptibili ty measurements. The m,m'- and m,p'-isomers were found experimentally to have singlet and triplet ground states, respectively, in accord wit h the qualitative computed predictions.