Four solid-crystal forms of paraazoxyanisole and the thermodynamic relationships between them

The crystal structure of a metastable polymorph of para-azoxyanisole

“…In 1968, Robinder and Poirier 5 reported the existence of a third crystal form using DSC data; and recently, a fourth form has been observed through spectral and calorimetric measurements by Ogorodnik. 6 Summarising these data, it appears that there is a stable form (III) which melts at 117 uC and three metastable polymorphs I, II and IV which melt at 103, 113 and 'slightly below' 6 117 uC, respectively. Despite these significant studies on PAA in the past 70-80 years, no crystal structure has been reported for any of its metastable phases.…”

DFT-Based Study of Physical, Chemical and Electronic Behavior of Liquid Crystals of Azoxybenzene Group: <i>p</i>-azoxyanisole, <i>p</i>-azoxyphenetole, ethyl-<i>p</i>-azoxybenzoate, ethyl-<i>p</i>-azoxycinnamate and <i>n</i>-octyl-<i>p</i>-azoxycinnamate

“…In a solid state, it appears as a white powder, but when heated it forms a liquid crystal. As one of the first known and most readily prepared liquid crystals, PAA has been played an important role in the development of liquid crystal displays [5,6]. Presently, a number of azoxy based liquid crystals are well known.…”

Probing Crystal Nucleation from Fluid Phases: The Nucleation of para-Azoxyanisole from Its Nematic Liquid Crystalline State

“…The DSC data (not shown), were entirely consistent with these microscopic observations and the previous reports of Crowfoot and Bernal 13 and Robinder and Poirier. 19 The first phase to crystallize from the nematic at around 75 °C (Form I) transformed on heating to the nematic at 103.3 °C ((0.5 °C) with a heat of transition ∆H (1) ) 24.9 kJ/mol. The phase present at room temperature (Form III) melted to the nematic at 117 °C ((0.5 °C) with ∆H (3) ) 31.9 kJ/mol.…”

“…The crystal structure of the metastable polymorph (I) has only been reported recently and this is an important factor in enabling further understanding of nucleation in this system . More extensive thermal studies , of this material have revealed the existence of two additional, so far uncharacterised, polymorphs. Delord and Malet, proposed a model of the nematic phase derived from X-ray scattering data in which molecules are essentially aligned with their molecular axes parallel to the director and packed in a hexagonal array.…”

“…16 The crystal structure of the metastable polymorph (I) has only been reported recently 17 and this is an important factor in enabling further understanding of nucleation in this system. 18 More extensive thermal studies 19,20 of this * To whom correspondence should be addressed. E-mail: roger.davey@ manchester.ac.uk.…”