Color chromonics

Flavonoids are more or less intensely yellow colored pigments belonging either to the group of 2-phenyl-chromones 39 (flavones, flavonols) or 2-phenyl-chromanones 391 (flavanones, flavanonols) both groups have a backbone of 2-phenyl-l,4-benzopyrones ... 

The furyl chromone, which is a nonfluorescent compound, is smoothly converted by irradiation to the isomeric 1-arylfuro [3,4-b]chromone-3-acrolein. The starting material absorbs strongly at 295 nm where the reaction product does not absorb. As the reaction proceeds, the absorption in this region decreases. At longer wavelengths the product absorbs more than the precursor and shows two maxima at 368 nm and at 430 nm. It has an orange color and fluoresces Intensively at 527 nm (Xmax 7 nm ... 

CigHigOg, Mr 362.34, prisms, mp. 158-I60°C, L. shows the following color reactions KOH yellow, KOH+NaOCl green, and FeClg red, turning green. A chromone from the lichens Lecanactis latebrarum and Roccella fuciformis. 

Figure 8.5, Photoalignment of chromonic LC/silica nanohybrid. Chemical struc-ture of the component and a scheme of B67 columns on a photoaligned PPLC film. See color insert.

The first pyrylium salt without hydroxy or alkoxy substituents made by the protonation or alkylation of 4-pyrones was obtained in 1911 by Baeyer and Piccard fi"om 4-pyrones and Grignard reagents. Isolation of these compounds was often difficult, and their characterization was confiised by a lack of a clear understanding of their electronic structure. However, there was much interest in benzopyrenes (coumarins, flavones, and chromones) and benzopyrylium (flavylium, chromylium) salts isolated fi om plants (anthocyanins, anthocyanidines) to which they conferred a large diversity of colors. 

Chromone 12 has been employed as two-color fluorescent probe for albumin serum protein, see. S. Ercelen, A. S. Klymchenko and A. P. Demchenko, FEB, Letters, 2003, 538, 25. 

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