Soliton excitations in pernigraniline-base polymer: Effects of next-nearest-neighbor hopping

Next-nearest-neighbor hopping interaction is considered to investigate the soliton excitations in a pernigraniline-base polymer on the basis of the Baranowski–Büttner–Voit model. Numerical calculations show that the valence band (VB) expands to a larger range and the conduction band (CB) acts to the contrary. For a positive combined soliton, some new shallow levels emerge at the top edge of both the VB and CB with the increase in the next-nearest-neighbor coupling strength. For a negative combined soliton, only one bound state (the soliton state ) appears in the Peierls gap and no other shallow levels exist. We find that, while the coupling strength β takes the value 0.04, the calculated charged soliton excitations (1.15 eV for S + and 1.34 eV for S − ) are in good agreement with the low-energy photo-induced absorption peak (1.0 and 1.3 eV).