Playing Catch and Release with Molecules

Targeted molecules plucked from solution, held, and then released on command

Tyler Josephson

Figure 1. The transition from the trans azobenzene derivative to the cis orientation can be reversibly switched using ultraviolet or blue light.

Figure 2. A cage structure with the azobenzene units in the trans orientation. The orange solid shows the cuboctahedral cavity inside the cage.

Figure 3. When the cages are in the trans orientation (shown in red), the bulky groups interact with other cages and assemble into a matrix, where guest molecules can be trapped inside. After shining ultraviolet light, the cages switch to the cis orientation (shown in green), where the bulky groups are curled in and do not interact, and the trapped molecules are released into the solution.

When gathering biofuel molecules from a complex mixture, the challenge is trapping the desired molecules and releasing them on command. Light is an excellent command, as it does not usually create waste, damage the materials involved, or add excessive cost. Researchers from the Center for Gas Separations Relevant to Clean Energy Technologies have developed a new technique for chemical separations with potential applications in biofuel separations, molecular machines, adaptable coatings, and even drug delivery. To open and close these molecular "boxes," all you need to do is switch on a light.

The core of this capture technique is a molecule, specifically an azobenzene derivative, that switches between two forms: a trans­ and cis­ arrangement (Figure 1). Shining ultraviolet light on the trans material causes a rotation around the central nitrogen or N=N bond, leading to the ­­cis conformation, and shining blue light on the cis changes it back to trans. When reacted with copper(II) acetate, these photosensitive molecules form a cuboctahedral cage, like a 14-sided die (Figure 2).

When the parts of this molecular cage are in the trans conformation, the bulky groups act like arms and stick out, interacting with the "arms" of other cages, so that they interlink to form an array (Figure 3a), which aggregates into a solid that falls out of solution as green crystals. However, when ultraviolet light shines on the cages, the trans linkages convert to cis linkages, and the "arms" are tucked inward. They let go of the neighboring cages, release from the solid, and dissolve back into the solution (Figure 3b). The spaces in between the cuboctahedral cages can "catch" molecules in the trans state and release them in the cis state; the cages themselves remain intact throughout this transition.

The researchers tested this behavior using an easy-to-analyze guest molecule, methylene blue (MB), a malaria drug so-named for its vibrant blue color when dissolved in water. They added MB to a solution of cages in the trans state, and when these trans cages solidified into the structure depicted in Figure 3, the MB was trapped in the crystals. Next, they separated the solid that had incorporated MB, and transferred it to a fresh solution. When they shined ultraviolet light on this new solution for 30 minutes, the trans cages converted into cis cages, releasing the MB into solution. Then, shining blue light on the solution for 1 hour, the cis cages converted into trans cages, and the lattice came together once again, pulling the MB out of solution. The recapture rate was over 96 percent, even after five cycles of catch and release.

Further research will be needed to develop related materials that effectively operate in solvents other than the methanol, chloroform, and acetone mixtures used in this study. Nonetheless, these successful first steps show that the future is bright for light-activated chemical separations.