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

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.