Polymorphic crystalline forms of 8OCB

Abstract

For most alkoxycyanobiphenyls (3OCB, 5OCB, 6OCB, 7OCB and 9OCB) it has been observed that slow or delayed cooling gave rise to a lower melting crystalline polymorphic form which then slowly converted into the higher melting stable crystalline structure. Although under slow evaporation from a solvent, polymorphic crystalline structures were observed also for 8OCB, a polymorphism on cooling like that in other nOCBs has not previously been reported. We have now observed that 8OCB also shows polymorphism, which brings it into line with the other homologues. On keeping the material between 35 and 38 °C after it was cooled from above the melting point of the stable crystal form (55 °C), regular platelets grow usually from one point and fill the whole sample. The texture is stable for weeks at room temperature. Upon heating it shows a melting point at 50 °C, i.e. 5 degrees below the stable crystalline melting point. It is interesting that all platelets are non-symmetric and have the same handedness.