A new polymorph of ortho-ethoxy-trans-cinnamic acid: single-to-single-crystal phase transformation and mechanism

Abstract

The alpha-polymorph of ortho-ethoxy-trans-cinnamic acid (OETCA) undergoes a reversible single-crystal-to-single-crystal phase transformation at 333 K. The new high-temperature polymorph (alpha'-OETCA) is stable between 333 and 393 K with three molecules in the asymmetric unit (Z' = 3), space group P1;. Unlike the other polymorphs (and solvate) of OETCA recently reported, two of the molecules in alpha'-OETCA deviate significantly from planarity. This conformational change results in the corrugated sheet-type structure of alpha'-OETCA. The sheets are made up of ribbons, each composed of R(2)(2)(8) hydrogen-bonded pairs (via the -COOH groups), which are further connected by CH.O interactions. When exposed to UV radiation the alpha'-OETCA polymorph can be stabilized below 333 K with ca 8% of the monomer converted into the photodimer. The crystal structures of alpha'-OETCA are reported at two temperatures above the phase transition point (at 345 and 375 K) as well as the stabilized forms at 173 and 293 K. A mechanism for the phase transition involving a cooperative conformational transformation coupled with a shift of layers of OETCA molecules is proposed. The alpha'-OETCA polymorph is also an example of a cinnamic acid derivative where two different potentially photoreactive environments exist in one crystal in which each unit cell has two non-centrosymmetric predimer sites and one centrosymmetric predimer site.

On the mechanism of some first-order enantiotropic solid-state phase transitions: from Simon through Ubbelohde to Mnyukh

“…The transition could also be followed by single-crystal diffractograms. The transition is clearly first-order, although a value for ÁV at 333 K could not be extracted from the published report nor were values given for the enthalpies of transition and melting (Fernandes et al, 2004). A careful comparison was made of the crystal and molecular structures in the two phases; details are given in the publication.…”

Synthesis, characterization and phase transitions in the inorganic–organic layered perovskite-type hybrids [(C n H2n + 1NH3)2PbI4], n = 4, 5 and 6

“…Structural phase changes and polymorphism are two of the more significant and enigmatic problems in modern structural chemistry. When coupled with our lack of understanding and control of these phenomena, this results in major obstacles to the successful design of compounds with specific chemical and physical properties (Fernandes et al, 2004;Dunitz, 1995). This is seen especially in inorganic-organic layered hybrid structures, which are able to combine desirable features from both types of constituents (Mitzi, 2001).…”

Reversible Single-Crystal-to-Single-Crystal Phase Transition of Chiral Salicylidenephenylethylamine

“…Hence, the reversible SCSC phase transition is a fascinating phenomenon because of its potential applications in ferroelectric and switchable dielectric devices [1][2][3]. Several molecular crystals that undergo temperature-induced reversible SCSC phase transition have been reported [4][5][6][7][8][9][10].…”