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.