Crystal engineering has, until now, been chiefly concerned with controlling the overall structural features of its product molecules. But in a report published in the current issue of the journal Science, researchers from the University of Minnesota describe a crystal design strategy that affords precise control of crystal symmetry and structure.
Michael Ward, K. Travis Holman and Adam M. Pivovar managed to coax a crystal to form just the way they wanted: so that all the molecules housed within the lattice framework aligned in the same direction. "What we're doing," Ward explains, "is crystal engineering, which means designing solid-state structures based on molecules by looking at the molecules themselves and asking how they'll guide themselves into a 3-D crystal lattice." The scientists assembled a framework consisting of flexible hydrogen-bonded sheets supported by banana-shaped pillars. By forcing the pillars to line up in the same direction, the researchers created channels that could only hold guest molecules in one orientation. With all these molecules aligned, the material doubles the frequency of light that strikes it, a property termed second harmonic generation (SHG).
This SHG activity may help the new crystal find use in optoelectronicsthe interconversion of electricity and lightsuch as in lasers or optical switches. According to Ward, the process of crystal generation is amenable to modifications, suggesting other crystals with the same basic framework may be made in the future. "The ability to predict crystal structure with this level of detail," the authors conclude, "is a rather unusual achievement in organic solid-state chemistry."