Metal-organic structures become flexible

The application potential of metal-organic structures was first discovered around 20 years ago and nearly 100,000 of these hybrid porous materials have been identified since. There are high hopes for technical applications, especially for flexible MOFs. As shock absorbers, for example, they could react to sudden high pressures by closing their pores and losing volume, i.e. plastically deforming. Or they could separate chemicals from each other like a sponge by absorbing them into their pores and releasing them again under pressure. “This would require much less energy than the normal distillation process,” explains Rochus Schmid. However, only a few of these flexible MOFs have been identified so far.

MOF under pressure

To get to the bottom of the mechanisms behind these materials, the Munich team carried out a more detailed experimental analysis of an already widely known MOF. To this end, the researchers subjected it to uniform pressure from all sides, observing what happens inside using X-ray structure analysis.

“We wanted to know how the material behaves under pressure and which chemical factors are the driving force behind the phase transitions between the open and closed pore state,” says Gregor Kieslich. The experiment showed that the closed-pore form is not stable; under pressure the system loses its crystalline order, in short: it breaks down.

This is not the case with a variant of the same basic structure: if the team attached flexible side chains of carbon atoms to the connecting organic pieces of the MOF that protrude into the pores, the material remained intact when compressed and regained its original shape when the pressure has decreased. The carbon arms transformed the inflexible material into a flexible MOF.

The secret of phase transformation

Bochum’s team studied the underlying principles using computer chemistry simulations and molecular dynamics. “We have shown that the secret lies in the degrees of freedom of the side chains, the so-called entropy,” points out Rochus Schmid. “Every system in nature strives for the greatest possible entropy, to put it simply, the greatest possible number of degrees of freedom to distribute the energy of the system.”

“The large number of possible arrangements of the carbon arms in the pores ensures that the open pore structure of the MOF is entropically stabilized,” continues Schmid. “This facilitates a phase transformation from the open pore structure to the closed pore structure and vice versa, instead of breaking down when the pores are squeezed together as would be the case without the carbon arms.” In order to calculate such a large system composed of many atoms and to search for the many possible arm configurations in the pores, the team developed a precise and numerically efficient theoretical model for simulation.

The key result of the study is the identification of another chemical option for controlling and modifying the macroscopic response behavior of an intelligent material by means of a thermodynamic factor. “Our results open up new ways to specifically achieve structural phase transformations in porous MOFs,” concludes Gregor Kieslich.