2 CRYSTAL-STRUCTURES TOWARDS THE DISCOTIC COLUMNAR MESOPHASE OF ,8,11,15,18,22,25-OCTAHEXYLPHTHALOCYANINATO)NICKEL

8,11,15,18,22,25-Octahexylphthalocyaninato)-nickel is a thermotropic l iquid crystal, undergoing a transition to a discotic columnar mesophas e at 418 K. Its crystals are weakly diffracting needles. Single-crysta l X-ray diffraction studies at 103 K, using intense synchrotron radiat ion of wavelength 0.504 Angstrom at Station F2, CHESS and the 'Princet on 1k' CCD area detector, and at 295 K using a Cu K alpha rotating ano de diffractometer, have respectively shown that the symmetry of the cr ystals is increased from primitive monoclinic (space group P2(1)/n) to C-centred monoclinic (space group C2/c) when the temperature is raise d. Solution of the structure at each temperature shows that the increa se in symmetry at 295 K arises from slight movement of the molecules, as well as increased thermal motion of the hexyl groups, and small con formational 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 employi ng multipole wiggler synchrotron radiation and a CCD area detector, as well as cryocooling, for difficult small molecule samples. The temper ature of the space group transition was determined at the Daresbury SR S, on Station 7.2, using a CCD detector to monitor an area of the diff raction pattern, whilst varying the temperature systematically. A grad ual appearance of spots for h + k = 2n + 1 occurred between 205 and 19 5 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 s ymmetry with temperature. No other changes were apparent on the CCD or IP detectors.