Holes in this crystal ‘sponge’ change shape when zapped with light

A crystal material that’s filled with holes could act like a sponge, capturing molecules of specific sizes and shapes in its pores. The crystals have the potential to deliver drugs or lock in toxins, researchers say.

Those possibilities are what Jason Benedict, an assistant professor of chemistry at the University at Buffalo, had in mind when he led the design of the new material. It’s called UBMOF-1 and it’s a metal-organic framework, or MOF.

Swiss cheese-like MOFs are not new, but Benedict’s material has a few remarkable qualities. The crystal’s pores change shape when hit by ultraviolet light. This is important because changing the pore structure is one way to control which compounds can enter or exit the pores.

You could, for instance, soak up a chemical and then alter the pore size to prevent it from escaping. Secure storage is useful in applications like drug delivery, where “you don’t want the chemicals to come out until they get where they need to be,” Benedict explains.

The crystal also changes color in response to ultraviolet light, going from colorless to red. This suggests that the material’s electronic properties are shifting, which could affect the types of chemical compounds that are attracted into the pores. The team’s findings are published in the journal Chemical Communications.

From static to active

“MOFs are like molecular sponges—they’re crystals that have pores,” Benedict says. “Typically, they are these passive materials. They’re static. You synthesize them, and that’s the end of the road,” he adds. “What we’re trying to do is to take these passive materials and make them active, so that when you apply a stimulus like light, you can make them change their chemical properties, including the shape of their pores.”

To force UBMOF-1 respond to ultraviolet light, Benedict and colleagues used some clever synthetic chemistry.

MOF crystals are made from two types of parts—metal nodes and organic rods—and the researchers attached a light-responsive chemical group called a diarylethene to the organic component of the material. Diarylethene is special because it houses a ring of atoms that is normally open but shuts when exposed to ultraviolet light.

In the UBMOF, the diarylethene borders the crystal’s pores, which means the pores change shape when the diarylethene does.

Reverse the process

The next step in the research is to determine how, exactly, the structure of the holes is changing, and to see if there’s a way to get the holes to revert to their original shape.

Rods containing diarylethene can be forced back into the “open” configuration with white light, but this tactic only works when the rods are alone. Once they’re inserted into the crystal, the diarylethene rings stay stubbornly closed in the presence of white light.

Chemists from Penn State Hazelton collaborated on the project.

Source: University at Buffalo

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