Single-crystal-to-single-crystal transformations

At the 25th ECA meeting in Istanbul in 2009, which one of us (P. N.) attended, Menahem Kaftory from Technion showed a slide containing only two photos of what appeared to be a perfect photochromic crystal before and after irradiation. Every single detail of the crystal's surface and its translucency were identical in the two images, but the crystal color (colorless before and red after exposure to light) changed. He commented that a reaction in which the crystallinity is preserved such as the one on the screen, a single-crystal-to-single-crystal (SCSC) process, “is the dream of every crystallographer”. And rightfully so.

The notion of a process that occurs with retained long-range order being “the dream process” resonates with solid-state chemists, coordination chemists and other materials scientists who seek to correlate the progress of a physical process or a chemical reaction in the solid state with the structure. When combined with X-ray diffraction analysis, SCSC reactions are an exceedingly powerful tool; they are irreplaceable in providing direct insight into the subtle changes in the structure that occur as a result of external stimulation, the effect of the process on the crystal packing, as well as the effect of the packing on the perturbation. If the structures of the reactant and the product were the opening and closing remarks of a movie, a SCSC transformation would provide a unique opportunity to look at the movie's storyline, occasionally even at time scales which are accessible with common XRD equipment. Fast and ultrafast “filming” has also been accomplished at synchrotrons. By controlling the external conditions (light, temperature, pressure, chemical gradients), one can pause, fast-forward or even reverse the movie.

Whether a certain process will occur in SCSC fashion depends on many factors, which can be generalized as factors that drive the process and factors that tend to retard it. The outcome of the process can be a state (molecular geometry and energy) that is compliant with or deviates to a significant extent from the original structure. What is the upper limit to which a process can be considered to occur in a single crystal manner? There does not appear to be universal agreement regarding to what extent, up to the theoretical maximum of 100%, a process should be classified as SCSC. In the solid state, conversions of 100% are rare, but not impossible. Incomplete conversion can be due to a multitude of factors including, but not limited to, ceasing of the process at defect sites, reversible kinetics, steric hindrance, dissipation of the energy, limited penetration of light due to unfavorable absorption cross-sections, different responses of the reactant and product phases, feedback phenomena, formation of intermediate phases, effects of the sample history, and other factors. All these factors can affect the yield and turn the classification of a process as a SCSC process into a somewhat subjective designation.

This themed issue contains an impressive collection of papers from some of the most prolific research groups in the field. The contributions span organic, metal–organic and extended metal–organic compounds, and the underlying processes include simple phase transitions, photochemical reactions, and small molecule adsorption. Being defined as a reaction class by the effect they have on the long-range order, rather than by the nature of the external effector, SCSC processes (chemical reactions or physical processes) are a subset of solid-state processes that can be induced by heat, light, mechanical force, sound, and chemical species, including gases and water, with retention of the crystal order. This is best illustrated with the diversity of systems that are the focus of the contributed articles, which include examples of SCSC transformations in organic, coordination, spin-crossover, and metal–organic frameworks.

We thank the contributors for the impressive collection of beautifully written contributions.

---