Conductive organic materials often derive their electronic properties from highly organized crystal structures. Now, researchers have developed an organic polymer that conducts electricity like a metal even when its strands are in a disordered jumble (Nature 2022, DOI: 10.1038/s41586-022-05261-4).
Metals such as copper are exemplary electrical conductors because the electronic structure of the substituent atoms create continuous energy bands that act like highways for electrons to zip through the material. Because organic materials are composed of individual molecules with discrete electronic structures, electrons within the material run up against detours and dead ends that stifle conductivity. Materials scientists can mitigate these disruptions by fixing the molecules in an organized lattice to ensure an uninterrupted pathway for electron flow, but doing so can make crystalline organic conductors difficult to shape and fabricate into electronic components. A research team led by John S. Anderson of the University of Chicago and Jiaze Xie of Princeton University have devised a clever strategy for making an organic polymer that retains its conductive properties without needing to have an ordered structure.
“It’s an absolutely fascinating material,” says Theodore Betley, a chemist at Harvard University who was not involved in the study.
Anderson and his team were initially inspired by conductive organic materials made with tetrathiafulvalene (TTF). This molecule is made from conjugated rings of sulfur and carbon which allow electrons to delocalize across the structure, making TTF a “voracious π-stacker,” he says. Anderson notes that the conductive properties of polymers made with TTF and metals have been studied before, but the mechanisms underpinning their remarkable conductivity were difficult to parse because of outstanding questions about the molecule’s electronic structure. To address this challenge, Xie, then a graduate student at the University of Chicago, and his colleagues developed a method to synthesize a polymer made of nickel and modified TTF linkers that gave the researchers precise control over the charge state of the metal and other features that affect the distribution of electrons within the material. This innovative synthesis yielded a grey powder that could be pressed into pellets. When they tested the physical properties of these amorphous pellets, the researchers found that the polymer was highly conductive in all directions and stable in air, heat, and even strong acid.
Next, the researchers created computational simulations of the electronic structure of the polymer. These experiments suggested that individual strands exhibit semimetallic conductivity, and the metallic behavior emerges when the strands are bunched together. Xie points to multiple factors that contribute to this metallic property, including strong π-stacking interactions that are unperturbed by the twisting, sliding, and staggering of individual strands. As long as the polymers are touching, their electronic structures form a pipeline for delocalized electrons, much like the band structure of metals, Xie says. An electron can skip over defects in the material “because there’s good molecular overlap in the system,” Anderson says. “The electron can move further than any structural repeat unit in the material, and that’s weird,” he adds.
It’s a simple design that creates “beautiful characteristics” in the macroscopic material that are greater than the sum of each individual part, Betley says. “What’s absolutely wild is that they’re able to achieve that band-like construction using purely molecular building blocks. It’s a remarkable milestone.”
Mircea Dincă, a chemist at the Massachusetts Institute of Technology who was not involved in the research, says “there is long range disorder, but it doesn’t seem to impact the electronic properties at the same extent. So that, to me, is a fundamental question that is interesting and deserves further study.” It’s unlikely that this material will replace metal conductors, but it could open new avenues for applications where existing metal conductors aren’t as good, Dincă says.
Anderson and his team continue to demystify the principles behind their polymer’s rule-breaking properties. He is also working on analogs using other metals and linkers to coax more intriguing properties from this system. “Let’s see how much we can push the material,” Anderson says.