Abstract: The ground states of poly(9,9-diethylfluorene-co-alt-benzoselenadiazole) were full optimized with DFT at B3LYP/3-21G* level, in which the comonomer molar ratios of9,9-diethylfluorene(DEF) and benzo-selenadiazole(BSeD) are 1∶1 and 2∶1. TDDFT and ZINDO calculations of the lowest excitation energies and absorption wavelengths were performed at the optimized geometries of the ground states. Band gaps of the corresponding polymers were obtained by extrapolating HOMO-LUMO gaps and the lowest excitation energies as well as the maximal absorption wavelengths of the polymers to the infinite chain length. By the extrapolation results, the bond gaps are becoming lower with increasing the concentration of BSeD, while the maximal absorption wavelengths are red-shifted. The excited geometry is optimized by ab initio CIS/3-21G* and the emission spectra was computed based on the excited geometry. By the calculation results, the excited geometry was more planar.

Key words: Fluorene, Benzoselenadiazole, Density function theory(DFT)