Structure and stability of bis(dimethylamino)squaraine and its hydrates: A study using XRD, IR spectroscopy, and thermodynamic measurements

Abstract

By using thermal and pressure measurements, it was found that in the bis(dimethylamino)squaraine (SQ)–water system near room temperature and pressure two hydrates exist, one containing two and the other four molecules of water per SQ unit. Their equilibrium vapor pressure is the key to their apparently erratic behavior under laboratory conditions. X-ray diffraction at 293±2 K established that the space group of the dihydrate (SQ2) is the monoclinic P2₁/c, whereas that of the tetrahydrate (SQ4) is the orthorhombic Pbcn. The squaraine molecules in both hydrate crystals form stacks parallel to the shortest crystallographic axes (b for SQ2 and c for SQ4); the water molecules lie between adjacent stacks, and are involved in a thick interplay of intermolecular hydrogen bonds. Such interactions are weak enough to leave virtually unchanged the geometry of the organic moieties with respect to that of the anhydrous compound. These results are fully compliant with those drawn from the infrared spectra and thermal and pressure measurements.