The radical cations of bridged bicyclo[5.1.0]octa-2,5-dienes - Open shell bis-homoaromatic systems.

The radical cations of a series of bridged bicyclo[5.1.0] octa- 2,5-dienes have been generated by photoinduced electron transfer to strong electron acceptors (chloranil, fluoranil) in solution. Chemically induced dynamic nuclear polarization effects observed during the reactions of simple bridged systems (lb-dj) indicate that the intermediates are bis-homoaromatic (BHA) systems of B(2) symmetry. These are the first homoaromatic radical cations of any structure type. Introduction of substituted exo-methylene groups as bridging units (1e-1) leads to a composite structure type with contributions from bis-homoaromatic structures of A(2) and B(2) symmetry and/or from a vinylcyclopropane structure (VCP) with an anti-symmetric singly occupied molecular orbital.