Theoretical study of the lowest pi-->pi* excitation energies for neutral and doped polyenes

In earlier theoretical studies, it has been widely noticed that the electron correlation effect played an important role in determining the excitation energies of low-lying pi-->pi(*) excited states for neutral polyenes and their radical cations and dications. In this paper, neutral and doped polyene oligomers of medium to large sizes are investigated with the Pariser-Parr-Pople model, and the pi-electron correlation effect is fully taken into consideration by virtue of the density-matrix renormalization group method. The excitation properties in the polymer limit are also obtained by exponential extrapolation from the finite oligomers. The reasonable agreement of our results with the available experimental observations and advanced ab initio calculations is witnessed. It is also observed that while charge doping can significantly lower the exciting energy, the odd-charged oligomers show lower excitation energies than the even-charged ones.