Solvate and polymorphs of ortho-ethoxy-trans-cinnamic acid: the crystal and molecular structures

The 173K structures of the α, β and γ phases of o-ethoxy-trans-cinnamic acid (OETCA), which crystallize in $\text{P}{}_{\mathrm{<mtext>1</mtext><mtext>̄</mtext>}}$, $\text{R}{}_{\mathrm{<mtext>3</mtext><mtext>̄</mtext>}}$ and C_2/c respectively, are reported here. The common building block in all the structures is hydrogen bonded OETCA R²₂(8) centrosymmetric dimer pairs. The α polymorph forms a layered structure and the γ polymorph a herringbone structure. Both of these phases are made up of hydrogen bonded OETCA dimers that are further assembled through C-H· · · O interactions to form layers or ribbons of OETCA molecules. The main difference between these structures is the 3-D assembly of the layers. The β phase is actually a benzene solvate or host–guest complex (1:6 benzene: OETCA). Benzene, which occupies the $\text{3}\text{̄}$ site at (0,0,1/2) in the trigonal unit cell, is essential for the stabilization of this phase. It provides a template for the spiral arrangement of the OETCA molecules required for the formation of the β-photo-dimerization product.