Electronic decoupling in ground and excited states of asymmetric biaryls

New asymmetric biaryls have been synthesized in order to clarify the condit ions necessary for charge-transfer transitions in photoexcited neutral comp ounds and biradical formation in doubly charged ground-state species. A par allel behavior for both types of approaches is observed and explained with a simple coupling model allowing prediction of the intermoiety coupling str ength. It is shown that for weakly coupled biaryls the monoions are connect ed with charge localization and the dications and dianions form thermally e xcited biradicals while the fluorescence data indicate biradicaloid excited states connected with partial charge separation. More strongly coupled bia ryls, on the other hand, result in diamagnetic species for the doubly charg ed ground-state ions, and a charge-transfer contribution in the fluorescenc e spectra of the neutral compound is absent.