Researchers in Europe have threaded their chemical needles to create a fabric woven, not from fibers, but from individual organic threads.
Many woven materials, such as the fabric used in clothing and the reed used in wicker baskets, contain two sets of interlaced threads that are perpendicular to one another. One set of threads is called the warp, and the other, which interlaces with it to lock in the pattern, is called the weft. To mimic such biaxial woven materials in the molecular world, scientists have in the past used metals to set crossing points at which organic strands interlace.
Now, Helma Wennemers of ETH Zurich and coworkers have taken this technology to another level by creating the first organic woven material made of three sets of organic strands (Nat. Chem. 2017, DOI: 10.1038/nchem.2823). This triaxial weave has two sets of overlaid, but not interlaced, warp threads and one set of weft threads that lock into the other two. The threads are all a single type of derivatized oligoproline.
In addition to the material’s fundamental significance as a new form of matter, it can host nanoparticle or molecular guests in voids between its three sets of threads and could thus be useful for making sensor arrays and gas or nanoparticle storage devices.
The work is “a clever illustration of molecular weaving” and the material “an elegant topologically woven structure,” says Omar M. Yaghi of the University of California, Berkeley. Because the structure is much tougher than the molecular threads from which it is made, it could also be the basis for “materials of unusual strength and resiliency,” he says. Yaghi’s group made the first interwoven organic material last year by using copper ions as templates for benzidine/bisphenanthroline self-assembly and then removing the copper (Science 2016, DOI: 10.1126/science.aad4011).
It might seem like Wennemers and coworkers would’ve needed magic to perfectly interlace the oligomeric threads in the new triaxial material. But they hit on a simple strategy free of spells—designing the oligomer so it was destined to self-assemble into the woven triaxial structure.
The trick involved adding extensively aromatic perylene-monoimide groups to proline oligomers. The aromatic appendages associate with one another by π-π interactions, creating longer oligomers with periodic alternating up-and-down gaps. CH-π interactions between the aromatic groups induce one warp thread to overlay another and the weft thread to weave into the alternating gaps, forming crossing points that lock in the triaxial architecture. Wennemers and coworkers used transmission electron microscopy, atomic force microscopy, and grazing-incidence wide-angle X-ray scattering to confirm the structure.
“The potential functionality is similar to that of metal- and covalent-organic frameworks, which have been of great interest in the past few years in materials chemistry,” comments Samuel I. Stupp, an expert in self-assembling organic materials at Northwestern University. “However, the work demonstrates the possibility of accessing such supramolecular structures without the need to use metals or difficult chemical reactions,” which are needed to produce those framework compounds.
In principle, chemists could use solvents to easily disassemble and recycle the triaxial structures, Stupp says. “Also, their woven topology and strong noncovalent interactions mimic polymer entanglements and thus offer mechanical robustness: A lot of energy per unit volume is required to fracture them, a property important for any function.”