Ion-Initiated Cross-Talk

In biological systems, the chemical chatter of electron transfer is often initiated by ions. But until now, few, if any, simple chemical systems were known to exhibit such ion-mediated cross-talk. In a new report, researchers have developed and characterized what they believe is the first example of a supramolecular assembly that undergoes electron transfer in the presence of particular ions (Science 2010, 329, 1324).

The system provides a new way to control electron-transfer reactions that could make it useful in the development of organic batteries, artificial photosynthesis systems, or molecular-scale information storage devices, according to Jonathan L. Sessler, one of the report’s four principal investigators.

Sessler, who holds positions at the University of Texas, Austin, and Yonsei University, in South Korea, teamed up with Karl M. Kadish of the University of Houston; Shunichi Fukuzumi of Osaka University, in Japan, and Ewha Womans University, in South Korea; Christopher W. Bielawski, also of UT Austin; and their respective research groups to create and study the new supramolecular system.

“This project had its start with beer,” Sessler tells C&EN. He and Bielawski were discussing chemistry during happy hour one evening when it occurred to them that a tetrathiafulvalene calix[4]pyrrole (TTF-C4P) molecule prepared in Sessler’s lab might make an interesting complex with a bisimidazolium quinone dication (BIQ²⁺ ) recently synthesized by Bielawski’s group. TTF-C4P features four arms that, in the presence of certain ions, come together to form a bowl shape.

Two such bowls could fit like caps around BIQ²⁺ , which the researchers reckoned might accept an electron from the TTF-C4P’s arms. The researchers observed that such an interaction does take place, Sessler says, and the resulting cations produced by electron transfer “diffuse apart to allow formation of a long-lived electron-transfer state.” Many electron-transfer reactions involve neutral molecules and create oppositely charged species that quickly recombine, Sessler notes.

Furthermore, he adds, the system can switch back and forth with the addition of different ions. Chloride and bromide, for example, prompt the electron-transfer complex to come together. Tetraethylammonium cation, on the other hand, displaces BIQ ²⁺ from the TTF-C4P caps. This leads to back electron transfer and restores the initial oxidation states of the donor and acceptor pair.

“The biggest challenge was showing that electron transfer was occurring—namely, that an ion-switchable reaction was taking place and that this gave rise to stable radical products,” Sessler says. To that end, Jung Su Park, Sessler’s graduate student, crystallized “just about every species involved,” the UT Austin professor points out, in some cases using the painstaking “separation method of Pasteur” to isolate individual crystals. Another student of Sessler’s, Elizabeth Karnas, traveled to Fukuzumi’s lab in Japan to carry out the electron paramagnetic resonance analysis that provided the key evidence of electron transfer.

The study “exposes the subtleties that are at work during inter- as well as intramolecular electron processes in an elegantly designed, multicomponent, integrated system,” comments J. Fraser Stoddart, a supramolecular chemistry expert at Northwestern University.

“What puts the research on a pedestal is the fact that here at last is a wholly synthetic ensemble that provides some real insight into just how complicated electron-transfer processes are in the biological world,” Stoddart adds.