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More bang for your bond

Size-controllable bulky ligands stabilize multiple bonds of heavy elements into photoactive materials

Organic compounds containing double or triple bonds can pack a powerful punch. By sharing electrons between atoms through a process called pi conjugation, unsaturated molecules often have exceptional photonic and electronic behavior, making them essential components in state-of-the-art products such as polymer light-emitting displays.

One way to boost the usefulness of multiply bonded materials is to add heavy elements other than carbon into organic frameworks. Now, Kohei Tamao and colleagues from the RIKEN Advanced Science Institute in Wako and Kyoto University have developed a size-controllable molecular ring system that enables double bonded silicon–phosphorus units (Si=P) to be securely incorporated into pi conjugated networks1—unlocking previously unseen photo-absorption and emission activity.

Because Si=P bonds are extremely reactive, chemists typically attach them to geometrically large molecules known as bulky ligands that protect the double bonded elements. Unfortunately, most bulky ligands cause the Si=P double bond to twist, disrupting the critical pi conjugation.

Tamao and his team designed a new type of bulky molecule—the so-called ‘Rind’ ligands—to address this rotational problem. Based on a rigid, symmetric skeleton of three fused rings known as s-hydrindacene, Rind groups also contain alkyl side chains that can be tailored in length to control ligand bulkiness.

Adding Rind ligands to Si- and P-based starting materials produced molecules with highly coplanar Si=P bond with aromatic groups on Si that maintained pi conjugation in the solid state and allowed a unique room temperature fluorescence to emerge. According to Tsukasa Matsuo, a co-author of the study, the Rind ligands interlock with each other to enforce the favored planar geometry for pi conjugation.

“Rind groups can make planar arrangements out of a variety of conjugated systems,” says Matsuo. “But the electronic effect of Rind itself is small, because they are perpendicular to the conjugated electron system.”

The researchers also discovered that Rind ligands produced startling results when used to stabilize molecules containing copper atoms and organic groups2. While organocopper compounds are extremely useful in synthetic chemistry, their structures remain largely unknown because of continuous aggregation and dissociation processes in solution. By intoducing Rind ligands to copper bromide, the team isolated stable compounds containing remarkable internal architectures, such as four copper atoms arranged into a planar square.

“We were surprised when we found the beautiful structures of the organocopper materials,” says Matsuo. The four-copper framework also gave new luminescent qualities to these complexes—another reason why the researchers are continuing to explore ways to make functional materials using innovative bulky Rind ligands.

The corresponding author for this highlight is based at the Functional Elemento-Organic Chemistry Unit, RIKEN Advanced Science Institute

Journal information
1. Li, B., Matsuo, T., Hashizume, D., Fueno, H., Tanaka, K. & Tamao, K. ð-Conjugated phosphasilenes stabilized by fused-ring bulky groups. Journal of the American Chemical Society 131, 13222–13223 (2009).

2. Ito, M., Hashizume, D., Fukunaga, T., Matsuo, T. & Tamao, K. Isolated monomeric and dimeric mixed diorganocuprates based on the size-controllable bulky ‘Rind’ ligands. Journal of the American Chemical Society 131, 18024–18025 (2009).

Saeko Okada | Research asia research news
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