Preparation and Structural Evaluation of the Conformational Polymorphs of α-[(4-Methoxyphenyl)methylene]-4-nitrobenzeneacetonitrile

Crystals of α−[(4-methoxyphenyl)methylene]-4-nitrobenzeneacetonitrile (C16H12O3N2) are shown by X-ray crystallography to exist in three polymorphic trans forms (I−III) and one cis configuration (IV). Calorimetric studies show that at low temperature form I is thermodynamically the most stable phase, followed by form III and form II. On heating, form III undergoes a phase transformation to form II. There is no direct thermal transformation between forms II and I. Form I melts at 164.5 °C. Mechanical damage of form II initiates a phase transformation into form III, and structural studies confirm that forms II and III, both of which show attractive second-order nonlinear optical behavior, are closely related. Though clearly conformational polymorphs, they also exhibit the essence of orientational polymorphism. Although form I is the most stable of the three trans isomers, nevertheless forms II and III are indefinitely stable and may be useful for development for NLO purposes. Crystal growth studies show that growth from the melt and vapor phases could be the only routes for the preparation of specimens for optical examination. The results show well the difficulties involved in the preparation and isolation of the different polymorphic forms of such materials.