Electronic decoupling in ground and excited states of asymmetric biaryls

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