Structural and electronic properties of poly(fluorene-vinylene) copolymer and its derivatives: Time-dependent density functional theory investigation

The ground state and the lowest singlet excited-state geometries of poly-(9,9-dialkylfluorene-2,7-vinylene) copolymer or PFV and its derivatives (NH2-FV)n, (CN-FV)n, (OMe-FV)n and (OH-FV)n (n = 2-5) were investigated based on density functional theory (DFT) and time-dependent DFT using B3LYP functional. The ground state and the lowest singlet excited-state geometries of the oligomers were optimized at the B3LYP/6-31G∗ and TD-B3LYP/SVP levels, respectively. The calculated ground state geometries favor the aromatic type structure, while the electronic excitations lead to quinoid type distortion, which exhibited a shortening of the inter-ring bonds (about 0.03 Å). Absorption and fluorescence energies were extrapolated to infinite chain length making use of their good linearity with respect to 1/n. Extrapolated values of 2.19 eV for (FV)n was obtained by TD-B3LYP/TZVP method and 2.37, 2.11, 2.35 and 2.11 eV for its derivatives, (NH2-FV)n, (CN-FV)n, (OMe-FV)n and (OH-FV)n, respectively. The predicted energy gaps of the copolymer derivatives were calculated and compared to available experimental data. Fluorescence energies are 1.78, 1.74, 2.00 and 1.73 eV and the predicted radiative lifetime are 0.6, 1.0, 0.8 and 1.0 ns. for (FV)n, (CN-FV)n, (OMe-FV)n and (OH-FV)n, respectively. These fundamental structural and electronic informations can be useful in designing of novel conducting polymer materials.