Packing and uniaxial alignment of liquid crystalline oligofluorenes

Liquid crystalline oligomers of 9,9-bis(2-ethylhexyl)fluorene of defined degree of polymerization 4, 5, 6, and 7 were investigated by X-ray diffraction in the non-oriented and in the aligned state. The diffraction data give evidence for a smectic B type phase for all of the oligomers. Quenching below the glass transition does not change the structure of the liquid crystalline phase. This allows to align spin-coated films of these oligomers on rubbed polyimide substrates to give monodomain films. These are stable against thermal disordering below T-g, e.g. at room temperature. The degree of alignment is characterized by the dichroic spectra and polarized fluorescence spectra. Dichroic ratios and polarization ratios increase substantially with the chain length and values as high as D = 23 and P = 41 are obtained for the heptamer. The type of packing of the oligomers in the LC phase is discussed based on the X-ray single crystal structure of models. In one such model the packing of the 2-ethylhexyl side chains could be fully resolved, while the other model reveals the torsional angle between adjacent fluorene units in the same molecule as 144.2 degrees which corroborates earlier work based on fiber diffraction of corresponding polyfluorenes.