Dynamic electron-hole screening in conjugated polymers

Semi-empirical quantum chemical models, such as PPP or INDO, provide a reasonable description of the 2Ag state of polyenes. This suggests that these models accurately treat the effects of electron-electron interactions. However, in conjugated polymers, such models substantially overestimate the exciton binding energy (the gap between the 1Bu state and states with well-separated charges). The calculations yield binding energies of about 3eV while experimental estimates range from 0.2 to 0.9eV. This has led many researchers to substantially weaken the electron-electron interactions in models of conjugated polymers, thereby breaking the connection with molecular studies. By including the dielectric screening effects caused by adjacent chains in the solid-state, we have obtained exciton binding energies in reasonable agreement with solid-state experiments. Our model explicitly calculates the polarization induced in the surrounding chains, and includes both the time scale of the electron-hole motion on the molecule and the time scale of the induced polarization.