A comparative study of optical properties of poly(9,9-dioctylfluorene) and poly (p -phenylenevinylene) oligomers

A systematic study comparing the optical properties of poly(9,9-dioctylfluorene) oligomer (PFO) of different sizes with those of the well-studied poly (p -phenylenevinylene) oligomer (PPV) was carried out using a time-dependent localized density-matrix approach based on intermediate neglect of differential overlap/spectroscopy Hamiltonian. The derived theoretical optical gap for PFO of infinite size is about 2.9 eV, while that of PPV is about 2.7 eV, agreeing well with the experimental data (2.95 and 2.2-2.5 eV, respectively). The actual physical sizes of the lowest excited-state exciton (Wannier exciton) were obtained to be ~2.5 nm (about four repeating units) of PFO while ~2.7 nm for PPV (about five repeating units). The result indicates that the optical properties of PFO would saturate to its bulk behavior at a smaller size than that of PPV. Furthermore, the intensity of band-edge absorption of PFO is similar to that of PPV, suggesting that the PFO oligomer can perform as efficiently as PPV in optoelectronic application. © 2005 American Institute of Physics.