Humidity controls supramolecular self-assembly

The weather can affect picnics and trips to the beach, but supramolecular chemists haven’t usually blamed it for influencing lab experiments. Now, scientists report that humidity can contribute trace levels of water to organic solvents, changing the structures of self-assembled supramolecular aggregates in ways that were previously difficult to understand and control. The findings have implications for designing supramolecular materials for biological, electronic, and catalytic applications.

E. W. Meijer of Eindhoven University of Technology and coworkers investigated this phenomenon when they observed unpredictable changes in helical supramolecular structures formed by self-assembly of biphenyl tetracarboxamide in organic solution. Through spectroscopic, calorimetric, light-scattering, and theoretical techniques, they identified the culprit to be trace water in the solvents added by humidity (Nature 2018, DOI: 10.1038/s41586-018-0169-0).

Self-assembly formed three different helices: one type with no water present; a second in low humidity, in which one water molecule associated with every two building blocks; and a third at higher humidity, in which two waters bound each building block. Humidity also helped explain erratic results the scientists encountered in previous self-assembly studies.

Typical alkane solvents contain less than 0.01% water at room temperature, a seemingly negligible amount that supramolecular chemists have tended to ignore. And supramolecular aggregates are flexible, dynamic materials that are hard to characterize, making it difficult to identify exactly where waters bind in a structure, even when their presence is suspected.

Meijer and coworkers propose that water intercedes in self-assembly because the organic solvent’s hydrophobicity prevents water molecules from hydrogen-bonding with one another. The frustrated water molecules are so determined to hydrogen-bond that they join the self-assembling supramolecular structure instead.

“Even one water molecule, if present in organic solution, will attack a developing assembly by forming strong hydrogen bonds, dramatically altering the properties of the system,” comments Dimitris Vlassopoulos of the University of Crete.

“This is a brilliant paper, groundbreaking in many ways,” says Songi Han of the University of California, Santa Barbara. “It will make ripples through the supramolecular chemistry community. Anybody who has worked with organic self-assembly in organic solvents will find themselves looking back and wondering what the findings mean for their own work.”