Effect of exciton self-trapping and molecular conformation on photophysical properties of oligofluorenes

S. Schumacher, A. Ruseckas, N.A. Montgomery, P.J. Skabara, A.L. Kanibolotsky, M.J. Paterson, I. Galbraith, G.A. Turnbull

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Abstract

Electronic absorption and fluorescence transitions in fluorene oligomers of differing lengths are studied experimentally and using density functional theory (DFT) and time-dependent DFT. Experimental values are determined in two ways: from the measured molar absorption coefficient and from the radiative rate deduced from a combination of fluorescence quantum yield and lifetime measurements. Good agreement between the calculated and measured transition dipoles is achieved. In both theory and experiment a gradual increase in transition dipoles with increasing oligomer length is found. In absorption the transition dipole follows an similar to n(0.5) dependence on the number of fluorene units n for the range of 2 <= n <= 12, whereas a clear saturation of the transition dipole with oligomer length is found in fluorescence. This behavior is attributed to structural relaxation of the molecules in the excited state leading to localization of the excitation (exciton self-trapping) in the middle of the oligomer for both twisted and planar backbone conformations. Twisted oligofluorene chains were found to adopt straight or bent geometries depending on alternation of the dihedral angle between adjacent fluorene units. These different molecular conformations show the same values for the transition energies and the magnitude of the transition dipole.
Original languageEnglish
Pages (from-to)154906-1-154906-8
JournalJournal of Chemical Physics
Volume131
Issue number15
DOIs
Publication statusPublished - 21 Oct 2009

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Keywords

  • conducting polymers
  • density functional theory
  • excited states
  • excitons
  • fluorescence
  • localised states
  • molecular configurations
  • self-focusing

Cite this

Schumacher, S., Ruseckas, A., Montgomery, N. A., Skabara, P. J., Kanibolotsky, A. L., Paterson, M. J., ... Turnbull, G. A. (2009). Effect of exciton self-trapping and molecular conformation on photophysical properties of oligofluorenes. Journal of Chemical Physics, 131(15), 154906-1-154906-8. https://doi.org/10.1063/1.3244984