Localized and delocalized singlet excitons in ladder-type poly(paraphenylene)

We have performed electron-energy-loss measurements in transmission to study the lowest lying electronic excitations in ladder-type poly(paraphenylene). Our results show that two different types of singlet excitons exist near the absorption onset. They differ considerably in the size of their dispersion. While one of them is nondispersive, i.e., strongly localized, the other one is delocalized with a dispersion relation similar to those reported previously in the literature for π-π^ excitations in conjugated polymers. The existence of two different low-energy excitations in ladder-type poly(paraphenylene) may have important consequences for potential applications of this material in optical or electronic devices. We discuss possible reasons that might cause the formation of two different electron-hole excitations at the absorption onset.