Neutral, Zwitterion, Ionic Forms of 5-Aminoisophthalic Acid in Cocrystals, Salts and Their Optical Properties

Different ionic and zwitterionic forms of amphoteric 5-aminoisophthalic acid were observed in the self-assemblies of five salts and in a cocrystal. The presence of these forms of the molecule caused differences in respective local environments, which were reflected in corresponding absorption and emission spectra in solid state. Dianion of the acid was observed in hydrated 1:2 salt with 6-methylpyridin-2-amine whereas the salt of 5-methylthiazol-2-amine had the zwitterionic anion. The mono-anion of the 5-aminoisophthalic acid was observed in the salt of 4,6-dimethylpyrimidin-2-amine. The self-assembly of the 1:1 cocrystal of caffeine with 5-aminoisophthalic acid had neutral form of the acid and the cationic form of the acid was observed in the perchlorate and bromide salts. DFT-calculation with using CAMB3LYP functional with 6-31+G (d,p) as basis set in gas-phase showed that different HOMO-LUMO gaps of the neutral, cation and anion forms and these were reflected in the corresponding UV-visible spectra of the salt. The changes in intra-molecular charge transfer (ICT) fluorescence emissions in solid samples due to the different ionic forms were demonstrated.