DYEING CRYSTALS Bart Kahr, Department of Chemistry, Purdue University, West Lafayette, IN 47907-1393

The future of small molecule crystallography is wedded more closely to the past than many of us might suspect. It is our purpose here to demonstrate that many important crystalline materials were studied and abandoned before their time. For example, scientists for 150 years have studied crystals that adsorb and overgrow organic dyes during growth from solution. A wide variety of scientific and technological questions have motivated these researches including: the nature of pleochroism, limitations of the law of isomorphism, photosensitization of silver halides for photography, crystallization of ceramics, colloid stabilization, habit modification, epitaxy, preparation of explosives, and fertilizer identification. While mixed crystals of salts and dyes have not been objects of systematic study for a half century, they will undoubtedly appeal to a new generation of scientists because of their rich stereochemistry, and because crystals of simple ionic salts containing oriented, monodispersed, organic dyes promise spectroscopic and photonic applications. This review assembles previous observations that make up of the history of dyed crystals, while translating the descriptive crystallographic observations of past generations into a language that may be readily applied by the contemporary researcher. Next, we present our recent contributions and those of others according to host counteranion and including the following ions: sulfates, phosphates, carboxylates, carbonates, nitrates, halates, and halides. We show how structural studies of some historical dye inclusions are used in the design of new dye inclusions with prescribed physical properties. Both diastereoselective and enantioselective recognition process are illustrated. Dyed salt crystals are introduced as new solid state lasers and other potential optoelectronic devices. Comparisons with contemporary strategies for the matrix isolation of organic chromophores and biomolecules are presented. Here, we hope to restore a substantial body of chemical research to the contemporary scientific dialogue. We conclude by asking whether diffraction techniques of the future such as intense synchrotron x-ray sources coupled with careful analyses of diffuse scattering can contribute to structural studies of the aforementioned crystals.