Electronic excitation of polyfluorenes: a theoretical study

We present systematic, theoretical investigations on structure-property correlations in polyfluorenes (PFs) derived mainly from the chain morphology, oligomer length, and chemical substitutent. Both the vertical absorptions and the vibrational contributions to electronic absorption and fluorescence spectra have been calculated. The effect of temperature on the nature of photoexcitations of PFs has been demonstrated. It is found that the vibronic (electronic and vibrational) structures of PFs are morphology-dependent. beta-phase oligofluorenes (beta-(FL)(n)) and ladder-type poly(p-phenylene) (LPPP) oligomers show a red shift compared to the spectra of alpha-(FL)(n). The asymmetry of the absorption and fluorescence spectra in alpha-(FL)(n) and the fluorenone (FLO) defect oligofluorenes alpha-(FL)(n)(-)(m)(FLO)(m) is significantly more pronounced than that in planarized beta-(FL)(n) and LPPP oligomers. By properly taking into account the anharmonic torsion potentials resulting from the strong electronic and nuclear coupling in the oligofluorenes, we have reasonably reproduced the experimentally observed spectroscopic features. The low-energy on-chain chemical defect sites such as FLO units act as charge-trapping sites for singlet excitations, are the predominantly lighting-emitting species, and thus alter the blue light-emitting properties of PFs whereas the blue-light-emitting properties of PFs are hardly influenced by the hole-transporting molecules. The optical properties of PFs have been predicted by the finite-size calculations. Energy gaps of PFs are estimated by extrapolations from excitation energies of oligofluorenes up to 21 FL units.