jp0572481_si_001.pdf (1.16 MB)
Electronic Excitation of Polyfluorenes: A Theoretical Study
journal contribution
posted on 2006-05-25, 00:00 authored by WanZhen Liang, Yi Zhao, Jin Sun, Jian Song, Shuanglin Hu, Jinlong YangWe 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. β-phase
oligofluorenes (β-(FL)n) and ladder-type poly(p-phenylene) (LPPP) oligomers show a red shift compared to
the spectra of α-(FL)n. The asymmetry of the absorption and fluorescence spectra in α-(FL)n and the fluorenone
(FLO) defect oligofluorenes α-(FL)n-m(FLO)m is significantly more pronounced than that in planarized β-(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.