Two Crystal Structures Towards the Discotic Columnar Mesophase of (1,4,8,11,15,18,22,25-Octahexylphthalocyaninato)nickel

(1,4,8,11,15,18,22,25-Octahexylphthalocyaninato)nickel is a thermotropic liquid crystal, undergoing a transition to a discotic columnar mesophase at 418 K. Its crystals are weakly diffracting needles. Single-crystal X-ray diffraction studies at 103 K, using intense synchrotron radiation of wavelength 0.504 Å at Station F2, CHESS and the `Princeton 1k' CCD area detector, and at 295 K using a Cu Kα rotating anode diffractometer, have respectively shown that the symmetry of the crystals is increased from primitive monoclinic (space group P2₁/n) to C-centred monoclinic (space group C2/c) when the temperature is raised. Solution of the structure at each temperature shows that the increase in symmetry at 295 K arises from slight movement of the molecules, as well as increased thermal motion of the hexyl groups, and small conformational changes in these groups, indicating the first stage of the transition to the discotic columnar mesophase, when the hexyl groups are thought to become completely mobile. The quality of the refinement of the structure is indicative of what can now be realized by employing multipole wiggler synchrotron radiation and a CCD area detector, as well as cryocooling, for difficult small molecule samples. The temperature of the space group transition was determined at the Daresbury SRS, on Station 7.2, using a CCD detector to monitor an area of the diffraction pattern, whilst varying the temperature systematically. A gradual appearance of spots for h + k = 2n + l occurred between 205 and 195 K. Tests were also made using an image plate system at 195 and 293 K, which confirmed the transition from primitive to C-centred lattice symmetry with temperature. No other changes were apparent on the CCD or IP detectors.

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