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

By using thermal and pressure measurements, it was found that in the bis(di methylamino)squaraine (SQ)-water system near room temperature and pressure two hydrates exist, one containing two and the other four molecules of wate r per SQ unit. Their equilibrium vapor pressure is the key to their apparen tly erratic behavior under laboratory conditions. X-ray diffraction at 293 +/- 2 K established that the space group of the dihydrate (SQ2) is the mono clinic P2(1)/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 interpl ay of intermolecular hydrogen bonds. Such interactions are weak enough to l eave virtually unchanged the geometry of the organic moieties with respect to that of the anhydrous compound. These results are fully compliant with t hose drawn from the infrared spectra and thermal and pressure measurements.