Solvatochromic pyrogallarenes and pyrogallarene crystals
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Pyrogallarenes have the ability to self-assemble into a variety of structures. Crystal forms ranging from bilayers to dimers and hexamers have been isolated. One unique cocrystal, PgC[subscript 3]-(CH[subscript 3])[subscript 2]CO-0.5C[subscript 6]H[subscript 4]Cl[subscript 2]-nH[subscript 2]O, is shown here. This cocrystal of C-propyl pyrogallol[4]arene and the pharmaceutical gabapentin has the potential to alter gabapentin's stability, bioavailability, and dissolution rate. During the cocrystallization process unusual chromogenic behavior was observed, which led to the isolation of crystal 1, C-propyl pyrogallol[4]arene-3acetone-3H[subscript 2]O. Crystal 1 possessed a visible chromophore, but upon vacuum drying the color was quenched. PXRD of the colorless solid proved that vacuum drying resulted in removal of solvent molecules in the crystal structure, a hallmark of solvatochromism. Titration of C-propyl pyrogallol[4]arene over the entire pH range showed that the solvatochromism was pH-sensitive. C-propyl pyrogallol[4]arene operates as a pH indicator but is chemically distinct from common pH indicators. Mapping of hydrogen bonding interactions in crystal 1 reveals that water acts primarily as a base in crystal 1, yet the hydroxyl rim is fully protonated. Crystal 2 corroborates solvatochromism found in crystal 1. Finally, solution-phase thermodynamic shifts in absorbance suggest that self-assembly may be traced through solvatochromic interactions. Pyrogallarene solvatochromism is an area that has been previously overlooked but holds great potential to expand understanding of supramolecular structures and self-assembly.
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