Fluorescence Properties of Hydroxy- and Methoxyflavones and the Effect of Shift Reagents

Abstract

The fluorescence spectra of 42 hydroxy- and methoxyflavones in methanol solution have been investigated. The following findings are considered to be useful in structure elucidation and identification of flavonoids: (a) The maxima of the absorption and fluorescence bands give, in most cases, a unique combination; (b) flavones exhibit exceptionally large Stokes’ shifts (6,800 to 10,000 cm-1); (c) most flavones fluoresce blue, but flavonols fluoresce yellow or green; the fluorescence of flavonols consists frequently of two bands; (d) fluorescence is zero or very weak for 5-hydroxyflavones; (e) 5-hydroxyflavones become increasingly fluorescent with in increasing number of oxygen functions being present in the molecule; a 3-hydroxy group has a particular beneficial effect; (f) hydroxyflavones fluoresce less intense than the corresponding m ethoxyflavones; (g) fluorescence intensity is distinctly higher in polar protic than in apolar solvents. The effects of three groups of shift reagents on the spectra has also been investigated. The first group involves water, 10% and 50% sulphuric acid. The second group consists of basic reagents (sodium acetate, sodium hydrogen carbonate and sodium carbonate), and the third group of com plexing reagents such as aluminum trichloride, borax and magnesium sulphate. The following generalisations may be made: (a) Water causes a bathochromic shift in emission with 7-hydroxyflavones, but a hypsochromic shift with flavonols; (b) 50% sulphuric acid is able to protonate (and thus to longware-shift the em ission maximum) of all flavones except for the 3-, 5-, and 8-hydroxy isomers; (c) sodium acetate and hydrogen carbonate cause characteristic shifts with 7-hydroxyflavones and, occasionally, with 4 '-hydroxyflavones lacking a 7-hydroxy group; (d) aluminum trichloride produces a longware shift and an enorm ous increae in fluorescence intensity with flavonols; (e) borax can be used to detect sensitively the presence o f a 3 ',4 '-dihydroxy group