Salting Benzenes^†

Simple salt crystals, such as potassium sulfate or barium acetate, were grown in the presence of a variety of aromatic molecules, especially aniline derivatives, bearing sulfonate or carboxylate substituents. We call this process salting, borrowing the term from Michl and co-workers who coined it in a related context (Kirkor, E.; Gebicki, J.; Phillips, D. R.; Michl, J. J. Am. Chem. Soc. 1986, 108, 7106−7107). Particular growth sectors of the resultant crystals were luminescent, and the emitted light was highly polarized indicating that the benzene derivatives were oriented inside of the salt lattices. These results are presented in the context of a generalization of single crystal matrix isolation; isomorphous matching of hosts and guests need not be a constraint. We show by polarization spectroscopy and magnetic resonance that different faces of the crystals will entrap molecules in different conformations otherwise separated by small energies in solution. Metastable triplet states are remarkably long-lived at room temperature in salt matrixes. The mechanisms of benzene derivative incorporation depend highly on secondary surface structures that are imaged by differential interference contrast microscopy. Luminescent molecules identify these features by lighting-up particular substructures and as such serve as probes of crystal growth mechanisms. We show how patterns of light in crystals are used to assign absolute growth directions. We conclude by arguing that the results herein can be extended to a limitless range of guests, while encouraging the use of salts as matrixes for the study of organic compounds.