Diphenolic isoflavonoid puerarin fluoresces in aqueous solution with maximal intensity at pH 8.5 (Phi(fl)= 0.042, T-fl = 1.91 ns). For acidic solutions, weak fluorescence is attributed to fluorescent 7-monophenolate formed via excited-state deprotonation of neutral puerarin. For pH > 8.5, fluorescence decreases monotonically with an unchanged lifetime, suggesting that excited-state acidity of 4'-hydroxyl remains similar to the ground-state one, and that the diphenolate is non-fluorescent. The crucial role of A-ring 7-phenolate for fluorescence of puerarin is substantiated by absence (presence) of fluorescence for the 7-propylpuerarin (4'-propylpuerarin). Puerarin and its derivatives with the unusual properties may be explored to be antioxidant fluorescence probes. (c) 2008 Elsevier B.V. All rights reserved.