The fluorescence and photostabilities of naturally occurring isoflavones

The 5,7-dihydroxyisoflavones do not emit detectable fluorescence and are significantly more photostable than the two 7-hydroxy-5-deoxyisoflavones, daidzein and formononetin. The latter isoflavones emit fluorescence that depends on the basicity of the solution and the polarity of the solvent. Comparison of the spectral distributions of the emission in methanol in the presence and absence of base indicates that it originates from the conjugate anion excited singlet states. The fluorescence of daidzein and formononetin exhibit large Stokes shifts to longer wavelengths in methanol, ethanol, hexanol and acetonitrile and the magnitudes of these shifts increase with solvent polarity. This indicates that the emission originates from excited singlet states with much larger dipole moments than those of the corresponding ground states. Values of 8.3 D and 6.5 D were determined for daidzein and formononetin respectively. The photostabilities of formononetin and daidzein both increase with increasing solvent polarity and in corresponding solvents, the photostability of daidzein is higher than that of formononetin, behaviour which is attributed to the greater antioxidant activity of daidzein.