Mesomorphism of salicylideneaniline luminophores. X-ray diffraction and differential scanning calorimetry studies of 4-ethoxy-4′-heptylsalicylideneaniline

The structure and photothermal properties of the mesomorphic Schiff base 4-ethoxy-4′-heptylsalicylideneaniline (1) were studied by X-ray diffraction analysis and differential scanning calorimetry. Compound 1 exhibits photochromism and displays luminescence in the solid state and in a mesophase. The crystal unit cell of 1 contains two crystallographically independent molecules (A and B) adopting significantly different conformations. The molecules have a benzoid configuration of the bonds; in both molecules, the active proton was unambiguously located at the oxygen atom. According to the DSC analysis, compound 1 exhibits enantiotropic mesomorphism with a broad temperature range of mesophase (48.5–96.6 °C) and a low melt—mesophase phase transition temperature. In the crystal, the molecules form centrosymmetric tetramers stacked in the B⋯A⋯A⋯B order. Two central molecules (A⋯A) of the tetramer are linked together through a π⋯π-stacking interaction. Interactions between the B⋯A molecules (and the A⋯B molecules) occur through two C—H⋯π-system secondary bonds involving both benzene rings. The crystal packing consists of alternating loosely packed aliphatic layers and closely packed aromatic layers enriched with functional groups.