Achievement of enhanced solubility and improved optics in molecular complexes based on a sulfonate–pyridinium supramolecular synthon†
In anticipation of the formation of molecular complexes based on the sulfonate⋯pyridinium interaction, organic-sulfonic acids (2-amino benzene sulfonic acid (2-ABSA-H), 4-hydroxyquinoline sulfonic acid (4-HQSA-2H) and 4-amino-3-hydroxy naphthalene sulfonic acid (4-A-3-HNSA-2H)) were reacted with a lesser reported flexible supramolecular linker, 4,4′-bipyridyl propane (4,4′-BPP), to yield [(4,4′-BPP-H)+(2-ABSA)−·2H2O] (1), [(4,4′-BPP-2H)2+(4-HQSA-H)−2·3H2O] (2) and [(4,4′-BPP-H)+(4-A-3-HNSA-H)−·H2O] (3). Variation of the functionality on the organic acid led to interesting structural modulations in 1–3, since 1 is packed as layered ionic solid, 2 as a rare double-dumbbell and 3 as an interpenetrated-reticular solid. The anticipated sulfonate⋯pyridinium supramolecular synthon manifests differently in 1–3. Charge-assisted N+–H⋯N interactions associate the bis-pyridinium ions in 1 into infinite chains, further validating the generalization that linear bis-pyridyls react with amino-aryl sulfonic acids to form layered ionic solids. In 2, the hydroxyl-assisted pyridyl⋯pyridinium bifurcated interaction takes precedence over the formation of the otherwise robust sulfonate⋯pyridinium supramolecular synthon. 3 exists as the co-crystal-salt and the bis-pyridyl linker forms the sulfonate⋯pyridinium (salt) interaction on one end and the hydroxyl⋯pyridine (co-crystal) interaction on the other. An empirical synthon-hierarchy in multi-functional organic sulfonates with pyridyls has been proposed, suggesting the preferential formation of the sulfonate⋯pyridinium synthon over the carboxylic⋯pyridine and hydroxyl⋯pyridine synthons. For the bifurcated interactions, the methoxy-assisted hydroxylate⋯pyridinium and hydroxyl-assisted pyridine⋯pyridinium interactions are formed over and above all other possible interactions. The enhanced optical behavior reported for 1 and 3 is attributed to π-stacking interactions, while 2 is devoid of any significant π-interactions and undergoes a mild blue shift in the diffuse reflectance absorption reported as the Kubelka–Munk function. The band gap values of 1–3, confirmed through the Tauc plots, were substantiated by crystal packing. Furthermore, the reported salts exhibit a striking improvement in their aqueous solubility as compared to the crystal formers. 4-A-3HNSA-2H with very poor aqueous solubility (0.02 mg mL−1) shows an astonishing solubility enhancement of 2350% in 3, while in 1 and 2 the solubility of the organosulfonates was enhanced by 138 and 245%, respectively. The products have also been characterized via spectroscopic and thermogravimetric methods.