Crystalline forms and emission behavior of poly(9,9-di-n-octyl-2,7-fluorene)

By means of transmission electron microscopy (TEM) and X-ray diffraction (XRD) we demonstrate the presence of a previously unspecified modification (alpha') of the crystalline (alpha) phase in poly(9,9-di-n-octyl-2,7-fluorene) (PFO). The alpha' form is metastable and lower in melting temperature than the a form, and its formation is kinetically favored at temperatures lower than 130 degreesC. The modification involves mainly a slight increase in the b axis from 2.34 to 2.38 nm and lowered symmetry along this axis, whereas the a axis tends to lie preferentially normal to film surface. Using specimens composed selectively of alpha and alpha' phases, respectively, we show that films of the two forms are characteristically different in routine 1-D XRD profiles and absorption spectra. These are attributable to the oriented nature of the W form as well as minor but significant differences in molecular packing and backbone coplanarity. However, photoexcited emission spectra of these films remain highly similar in the vibronic structure to the purely nematic film, with only minute shifts in emission maxima that are inconsistent with the significantly red-shifted absorption edge upon crystallization. This appears better explained by assuming that the liquid-crystalline matrix rather than crystalline domains dominates optical emission of PFO.