Stress-induced domain reorientation in urea inclusion compounds

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FOR over 50 years urea inclusion compounds (UICs)-in which molecules are entrapped within the cavities of a hydrogen-bonded urea network—have intrigued chemists, crystallographers and spectroscopists1. Most previous work on these and other inclusion compounds has focused on intrachannel interactions and dynamics. We have now found that UICs can serve as useful models to probe long-range interactions and cooperative phenomena in crystals. Here we report the structure of 2,10-undecanedione/urea (1:9) (1/ urea), in which an extended hydrogen-bonded array connecting hosts and guests serves to distort the urea channel away from the hexagonal symmetry normally observed for UIC crystals. The distortion is large enough, and exhibits enough cooperativity, to generate macroscopic domains, but is small enough to allow facile domain reorientation under small compressive stresses. By incorporating a specific impurity (2-undecanone) into 1/urea, we can modify the domain size and optical properties, and make the domain reorientation spontaneously reversible when the uniaxial stress is released. All of these phenomena can be understood in terms of cooperative interactions between different channels of the hydrogen-bonded network formed by urea and its guest.