The negatively charged iodide in these compounds acts as charge donor, the neutral iodine as a charge acceptor. Electrons in such charge-transfer complexes are easy to excite to a higher energy level by light. The light is absorbed in the process and its complementary color is observed by the human eye.
In the case of the aqueous solution of polyiodides, the absorptions of the different species lead to an overall brownish color. Once amylose is added, it forms another CT complex, Here, the amylose acts as a charge donor and the polyiodide as an acceptor.
This complex absorbs light of a different wavelength than polyiodide, and the color turns dark blue. The exact structure of the polyiodides inside the amyloid helix is not clear.
The amylose-iodine complex is amorphous i. It has been proposed that the species inside the helix are repeated I 3 — or I 5 — units. The team investigated a related system, a pyrroloperylene—iodine complex, to study its properties as an organic electronic conductor. The material is crystalline, and therefore, the team was able to determine its structure using X-ray crystallography.
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