Charge storage states in polyenes

Changes in the geometric and electronic structure of polyenes upon sodium doping have been studied in detail. A short polyene containing three double bonds, hexatriene, forms a charge transfer complex with a maximum of one sodium ion. The interaction results in the formation of a polaron (radical anion) on the polyene chain. This type of charge storage state is shown to be unstable relative to the formation of a hexatriene dimer when interaction between two polyenes is allowed for; the two polyenes are then bound to each other via a carbon-carbon single bond. The charge storage states in this case correspond to two singly charged, spinless defects, one on each polyene segment of the dimer molecule. Decapentaene, which contains five carbon-carbon double bonds, is shown to be able to accept two electrons from a pair of sodium atoms. Similar to the case of doped trans-polyacetylene, the charges are stored as a soliton-antisoliton pair. However, when there is an excess of the number of polyene molecules relative to the number of sodium atoms, i.e., below saturation doping, this type of charge storage state is also unstable towards the formation of a decapentaene dimer of the same type as in hexatriene.