Effect of Film Morphology on the Energy Transfer to Emissive Green Defects in Dialkyl Polyfluorenes

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The formation of a ketone defect at the 9-site along the backbone of dialkyl polyfluorenes has been shown to be directly involved in the degradation of the polymer's emission from blue to an undesirable green. Films of poly(9,9‘-dihexylfluorene) (PFH) with and without ketone defects were annealed above their liquid crystalline phase transition in an inert argon atmosphere, and their emission spectra were collected in order to study the effect of morphology on the energy transfer to ketone defects. The annealing was performed in situ in the fluorometer, allowing for a direct comparison of the absolute changes in the emission spectra. Annealing of the films resulted in regions of highly aligned polymer chains as confirmed by atomic force microscopy. After annealing, the fluorescence spectra of pristine films (without ketone defects) exhibited no green emission, indicating the lack of thermal oxidation in the inert atmosphere. However, these films did show an increase in fluorescence quantum yield, revealing that high polymer order does not lead to interchain electronic species that quench the excited states. Annealing of partially photobleached PFH films revealed that an increase in the polymer chain order of a film containing a few defects resulted in an increase in green emission and decrease in blue without the creation of further defects. The increase in green emission combined with the decreased blue can only be the result of increased energy transfer from pristine chromophores to ketone sites, as the aligned polymer chains increase exciton diffusion. PFH films containing defects that were annealed beneath the LC temperature of the polymer did not result in any spectral changes, indicating that alignment of polymer chains was necessary for the increased energy transfer to the defect sites.