They found nearly linear polyiodide chains in-between stacks of pyrroloperylene. The material is crystalline, and therefore, the team was able to determine its structure using X-ray crystallography. The team investigated a related system, a pyrroloperylene–iodine complex, to study its properties as an organic electronic conductor. However, Ram Seshadri, Fred Wudl, and colleagues, University of California, Santa Barbara, USA, have found evidence that infinite polyiodide chains I n x– are contained in the amylose-iodine complex.
It has been proposed that the species inside the helix are repeated I 3 – or I 5 – units. The amylose-iodine complex is amorphous (i.e., it does not form ordered crystals), which has made it difficult to determine its structure. The exact structure of the polyiodides inside the amyloid helix is not clear. This complex absorbs light of a different wavelength than polyiodide, and the color turns dark blue. Once amylose is added, it forms another CT complex, Here, the amylose acts as a charge donor and the polyiodide as an acceptor. In the case of the aqueous solution of polyiodides, the absorptions of the different species lead to an overall brownish color. The light is absorbed in the process and its complementary color is observed by the human eye. Electrons in such charge-transfer complexes are easy to excite to a higher energy level by light. The negatively charged iodide in these compounds acts as charge donor, the neutral iodine as a charge acceptor. Together, they form polyiodide ions of the type I n –, for example, I 3 –, I 5 –, or I 7 –. Molecular iodine (I 2) is not easily soluble in water, which is why potassium iodide is added. The colors are caused by so-called charge transfer (CT) complexes.
Its chain forms a helix shape, and iodine can be bound inside this helix (pictured below). One is the linear amylose and the other is the branched amylopectin (pictured below).Īmylose is the compound that is responsible for the blue color. It consists of two different types of polysaccharides that are made up of glucose units which are connected in two different ways. Starch is a carbohydrate found in plants. If it is added to a sample that contains starch, such as the bread pictured above, the color changes to a deep blue. A solution of iodine (I 2) and potassium iodide (KI) in water has a light orange-brown color. Using iodine to test for the presence of starch is a common experiment.