Molecular Packing in Crystalline Poly(9,9-di-n-octyl-2,7-fluorene)

Presented are transmission electron microscopic observations of micron-sized single crystals of poly(9,9-di- n -octyl-2,7-fluorene) (PFO) prepared from thin films in the melt state. A preliminary determination of unit cell dimensions and molecular packing (orthorhombic, a = 2.56 nm, b = 2.34 nm, c = 3.32 nm, 8 chains, with space group P 2 1 2 1 2 1 and density 1.041 g mL -1 ) was made via combined considerations of the selected-area electron diffraction (SAED) pattern obtained along the [00 l ] zone of the single crystals, the SAED “fiber” pattern obtained from shear-oriented films, and the “powder” pattern from X-ray diffraction of melt-crystallized thick films in the absence of preferred orientation. In this model, PFO backbones are generally separated by transversely extended alkyl side chains, consistent with the dominance of single-chromophore emissions and the general lack of interbackbone delocalization of PFO chains in this ordered state as indicated by earlier photophysical studies. In addition, microscopic evidence for the presence of a vast number of nanograins was presented, and its implications in the crystallization process of semirigid PFO chains were discussed.