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.