Contrasting Solid-State Structures for Two Nematogenic Benzylideneanilines. Crystal Structures of p [(p'-Ethoxybenzylidene)amino] -benzonitrile and p- [(p'-Methoxybenzylidene)amino] -phenyl Acetate

Abstract

The crystal structures of two nematogens, p-[(p'-ethoxybenzylidene)aminolbenzonitrile (EOBABN) and p-[(p'-methoxybenzylidene)amino]phenyl acetate (MOBAPA), have been determined as part of a study of the relations between crystal structure and liquid crystallinity. In EOBABN the molecules are arranged in a head-to-tail fashion in rows parallel to the crystal b-axis, with which the molecular long axes make angles of ± 12.5°. This structure is of a common type for nematogens, and could transform to the nematic state by means of a simple displacive transition where the molecules acquire three translational, but at most one rotational, degrees of freedom. In MOBAPA the crystal structure is made up of bimolecular sheets in which the long molecular axes lie in parallel planes normal to the sheet surface, but are inclined to the surface at ± 19°. The director axes of successive sheets lie in the sheet planes, but are mutually perpendicular from one sheet to the next. To proceed to a classical nematic phase from this solid-state arrangement would require a reconstitutive transition involving an initial rotation of the molecules about an axis other than the long axis. A reversible solid-solid transition is observed for this material about 3° below the solid-nematic transition, so that it is not certain at which transition such motions occur, if at all.

Crystals of EOBABN have space group P2₁/c, with a = 7.817(2), b = 16.208(5), c = 11.490(4) Å, β = 112.76(5)°, and Z = 4. Crystals of MOBAPA have space group Pna2₁, with a = 19.833(5), b = 5.858(2), c = 12.210(4) Å, and Z = 4. Each structure was solved by direct methods. Least-squares refinement gave R = 0.040 for 1362 independent significant reflections in EOBABN, and R = 0.054 for 962 reflections in MOBAPA.