Evolution of charge-induced gap states in short diphenylpolyenes as studied by photoelectron spectroscopy

The evolution of doping-induced electronic states within the otherwise forbidden energy gap has been studied as a function of the polyene length in a series of diphenylpolyenes. The chemical and electronic structures have been studied using both X-ray and ultraviolet photoelectron spectroscopies. The results are interpreted with the help of quantum chemical calculations, performed using the semiempirical Austin Model 1 and valence effective Hamiltonian methods. The molecules studied are a series of diphenylpolyenes, DPx, with x = 1-7 C=C double bonds in the polyene part of the molecule. Since the frontier orbitals of the diphenylpolyenes are localized on the polyene chain portion of the molecule, there is a high degree of separation of the phenyl and polyene parts of the π-systems. Hence, many chemical and electronic properties of diphenylpolyenes are expected to be similar to those of short-chain trans-polyacetylene. For the longer molecules, n = 6 or 7, the present results indicate the presence of doubly charged, interacting soliton-antisoliton pairs, which appear as two new energy levels in the otherwise forbidden energy gap. In diphenyldecaheptaene to stilbene, i.e. 1 ≤ x ≤ 5, however, a singly charged state is formed at intermediate doping levels, after which the soliton-antisoliton pairs appear for the fully doubly charged systems. These results show that, remarkably, even for very short polyene segments, charges transferred are stored in the form of (confined) solitons.