Theoretical analysis of the charge-storage states in diphenylpolyenes with one to seven double bonds

The semiempirical Austin Model 1 (AM1) and the nonempirical pseudopotential valence effective Hamiltonian (VEH) methods have been applied for the investigation of the doping-induced electronic and geometrical changes in a series of conjugated molecules. The series consists of diphenylpolyenes with an even number of carbons and extends from stilbene to diphenyltetradecaheptaene, i.e., one to seven double bonds in the polyene part of the molecule. The densities of valence states, as calculated using VEH, are directly compared with the experimental valence band spectra, as recorded by ultraviolet photoelectron spectroscopy, as a function of increasing sodium exposure of the molecular solids. The charge-storage states in the series are discussed in terms of soliton-antisoliton- and polaronlike states, induced upon doping (reduction). In the cases of diphenyltetradecaheptaene and diphenyldodecahexaene (i.e., seven and six double bonds in the polyene part of the molecules), charge-storage states in the form of soliton-antisoliton pairs are induced even at the lowest doping levels, up to saturation at about two sodiums per molecule. In contrast, polaronlike charge storage states are formed at low to intermediate doping levels in diphenyldecapentaene down to stilbene (i.e., molecules with five to one double bonds in the polyene part of the molecules). However, at saturation, all the molecules in the series are able to accommodate two sodium atoms, resulting in soliton-antisoliton-like charge storage states.