D. W. Bright, F. Galbrecht, U. Scherf
Aug 13, 2010
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Influential Citations
33
Citations
Journal
Macromolecules
Abstract
The polymer poly(9,9-di-n-octylfluorene) (PFO or PF8) has been well studied for its excellent emission properties including polarized emission and high fluorescence efficiency. The polymer is noted for the formation of a highly ordered phase, termed the β phase, upon thermal cycling from low temperature and back up to room temperature or exposure to toluene vapor. This phase is composed of polymer backbones with a more planar configuration, where the helical twist of the repeat units along the chain has unwound, which extends the mean conjugation length and thus leads to a red-shifted absorption and emission bands. This low-energy phase readily traps excitons via strong F€ orster transfer and exciton migration. This gives rise to sharp and well-resolved emission spectra and even strong amplified spontaneous emission from the β phase via pumping of the R phase. For a long time, it was thought that only the dioctyl-substituted polyfluorene, i.e. PF8, was able to form the β phase. However, we recently demonstrated the formation of the β phase in a group of polymers identical to PF8but having different linear alkyl side chain lengths ranging from 6 to 10 carbon atoms, named PF6 to PF10. In solution, X-ray studies revealed the formation of sheets of “aggregate” inMCH solution (a moderate solvent for polyfluorenes) for PF6 to PF9, but not PF10. Corresponding work on temperature-dependent optical spectra showed the β phase forms in the cases of PF7, PF8, and PF9 solutions in MCH only, leading to the proposal that while aggregation occurs alongside the formation of the β phase, the planarization of the backbone is driven by side chain interactions in the solution and the two are separate processes. For side chain lengths of 9 or more carbon atoms, the solvent begins to break up or prevent the aggregate from forming in solution, indicative of a competition between side chain van der Waals forces and the solvent or thermal disruption of these bonds. The work reported in this Note carries on from the earlier studies we have reported in refs 14 and 15 and adds new data on β phase formation in PF10 as well. In films of these polymers, there is no solvent present to prevent interactions between the longer side chain polymer, and so it might be possible that β phase may form to some extent in the polymer poly(9,9-di-n-decylfluorene) or PF10. Data presented in this Note show that indeed the β phase does form in thin films of PF10 to some extent, but the extent of planarization is limited, most likely by the hindrance of neighboring chains limiting the amount of local-scale movement that is needed to allow the interdigitation of the side chains. However, it should be remembered that the length of the alkyl chains is the main controlling factor to β phase formation. The alkyl chains lie perpendicular to the backbone of the phenyl rings, and so the phenyl rings will also be perpendicular to the plane of the βphase lamella; thus, any phenyl-phenyl ring interactionwill be from the rings “inside” the side chain lamella and will be weak through the incumbent staggering of adjacent chain phenyl rings. This then also explains why the β phase lamella are effectively pure two-dimensional entities as the only interplane interactions would be very weak. However, this new observation adds further evidence to the conclusion drawn in our earlier work that the β phase formation is controlled by side chain interactions.