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 ββ takes the value 0.04, the calculated charged soliton excitations (1.15 eV for S+S+ and 1.34 eV for S−S−) are in good agreement with the low-energy photo-induced absorption peak (1.0 and 1.3 eV).