“Flava” means yellow in Greek and the collective name of flavonoids for this group of compounds was proposed by Geissman in 1952.

This is a very large group of compounds showing extraordinary diversity and variation and as the Greek root for the word suggests, many of these compounds are yellow in color. They consist of a number of structurally related groups of products, which are often identified as polyphenols. Many have a basic skeleton that contains 15 carbon atoms, which are usually subdivided into one part made up from a phenolic moiety and another which has a cinnamic acid molecule as a building block.

The group called the chalcones may be considered as the Friedel-Crafts reaction product of a (substituted) cinnamic acid and a phenol. The flavonoids in red wine (Vitis vinifera) such as quercetin, kaempferol, and anthocyanidins account for the free radical–scavenging activity.

In green tea (Camellia sinensis or Thea viridis), it is the catechins and catechin gallate esters that are shown to be effective antioxidants against free radicals. The dietary effectiveness of these materials has been known for generations and similar antioxidant effectiveness has been shown to occur when these materials are topically applied to protect skin cells.

Flavonoids also are a source of natural color, with yellows from the chalcones and flavonols, and reds, blues, and violets from the anthocyanidins. The flavones are colorless, but are still able to absorb UV strongly and so act as a beacon to pollinating insects. The exploitation of these molecules as a source of natural color in cosmetics and toiletries is just beginning, but their poor light stability is often a stumbling block. Flavonoids may be found as their glycosides. These are molecules that are substituted on one or more of the hydroxyl groups with a sugar such as galactose, glucose, mannose, or rhamnose, etc. The aglycons do not carry a sugar moiety.

Source: Dermatologic, Cosmeceutic and Cosmetic development - Kenneth A. Walters, Michael S. Roberts