Helical polyiodide chains resolve starch test mystery

In the course of developing new semiconducting materials, a research team including Ram Seshadri and Fred Wudl of the University of California, Santa Barbara, has solved a historical puzzle regarding the structure of the starch-iodine complex (Angew. Chem. Int. Ed. 2016, DOI: 10.1002/anie.201601585). Iodine dissolved in potassium iodide solution has long been used as a test for the presence of starch. The helical iodide chain that forms wraps around starch molecules (amylose) to create a complex that produces a deep purple color. Although scientists have speculated that polymeric iodide chains are involved, the exact structure has remained elusive. Chemists have reported other organic polyiodide complexes, but when characterized they have been found to have chain lengths shorter than 10 iodide units and contain molecular iodine (I₂) within the chain. The perylene-iodine complex, for example, made up of stacks of the planar polycyclic aromatic hydrocarbon perylene interspersed with iodide strands, was one of the first known organic conducting materials. But like the starch-iodine complex, its structure has never been fully resolved either. Seshadri, Wudl, and coworkers prepared a similar conductive material made from an iodine solution with pyrroloperylene and were able to get a crystal structure. The team concludes that stacks of pyrroloperylene molecules are interspersed with helical polyiodide strands of infinite length with no neutral I₂ units. The researchers have shown that the starch-iodine complex also contains these extended polyiodide chains.