Reversible transformation and fluorescence modulation in polymorphic crystals of n-butylammonium 2-naphthalenesulfonate

An organic salt composed of 2-naphthalenesulfonic acid and n-butylamine yielded polymorphic crystals: the block-like Form A and the plate-like Form B. The former was obtained from 1-propanol by slow cooling, while the latter was dominantly done from 1,4-dioxane, m-xylene and so on. Interestingly, the former was transformed to the latter by mechanical grinding and the reverse occurred by heat treatment at 120 °C. X-ray crystallographic studies clarified that the former has π-π interactions among naphthalene rings and hydrogen bonding network involving two oxygen atoms of sulfonate group, while the latter has CH-π interactions and the network involving three oxygen atoms. Luminescent properties were different in these crystals and continuously switched with the transformation.