Hexagonal columnar mesophases from phthalocyanine Upright and tilted intracolumnar molecular stacking, herringbone and rotationally disordered columnar packing

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Abstract
Three series of metal-free phthalocyanines with alkyl, alkoxy, and alkoxymethyl side groups have been studied with X-ray diffraction. The structure of the columnar mesophases was described as a two dimensional (pseudo) hexagonal packing of indefinitely long stacks of phthalocyanine molecules. From the slope of the squared intercolumnar distance as a function of the number of methylene groups in the peripheral chains, and using the known value of the volume of one methylene group in the liquid state, the stacking period of the molecules along the columnar axis was calculated. It was found to be equal to 3·5 Å for the alkoxy and 47middot;9 Å for the alkyl and alkoxymethyl derivatives. This is interpreted to mean that in the alkoxy series the molecules are oriented upright with respect to the columnar axis, while in the alkyl and alkoxymethyl series the molecules are tilted by an angle of 46°. Taken as circular for the alkoxy, the aromatic cores of the columns may be assumed elliptical in shape for the alkyl and alkoxymethyl series. The lateral packing of the elliptical columns was found to depend upon the length of the side chains. It is achieved with herringbone rectangular symmetry when the chains are short as in the octyloxy-methyl derivative, and with complete rotational disorder when the chains are long as in the octadecyloxymethyl derivative. When the side chains are moderately long, as in the dodecyloxymethyl derivative, the packing depends upon the temperature; it is herringbone rectangular at low temperatures and rotationally disordered at high temperatures.

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