Self-Assembly of Hydrogensquarates: Crystal Structures and Properties

The self-assembly of the hydrogensquarates is elucidated by means of linear-polarized infrared (IR-LD) spectroscopy of oriented colloids in nematic host and the so-called reducing-difference procedure for polarized IR-LD spectra interpretation. The scopes and limitation are discussed on five novel derivatives of squaric acid and its anions, that is, 2-chloro-3-aminopyridinium hydrogensquarate (1), bis (1,2,3,4-tetrahydroquino-linium) squarate (2), bis hydrogensquarate dihydrate salt of 4-(aminomethyl)pyridine (3), N-(2-ammoniumethyl)piperazinium monohydrate hydrogensquarate squarate (4), and 3-nitropyridinium hydrogensquarate monohydrate (5), respectively. The structures of these compounds 1-5 were solved by means of single-crystal X-ray diffraction, and the crystallographic data were used for the experimental elucidation of the corresponding IR spectra of crystals with respect to studying Fermi-resonance (FR), Davydov splitting (DS), and Fermi-Davydov (FD) as well as Evans' hole effects. The various motifs for self-assembly of squaric acid and its anions in the organic crystals are discussed together with their IR-spectroscopic properties.