Excited state acidity of bifunctional compounds 9. Excited state intramolecular proton transfer in 3,5-dihydroxy-7,4 '-dimethoxyflavone and 3,5-dihydroxy-7,4 '-dimethoxyflavanone: spectral and fluorescence decay results

摘要

Both title compounds show two fluorescence bands, with maxima at approximately 290 nm and 340 nm in non-protic (hexane, cyclohexane) and protic (ethanol) solvents. Approximate molecular orbital calculations suggest that these two bands can be associated with an excitation partially localized on the anisolyl moiety and a more delocalized excitation, respectively, present in both compounds. In cyclohexane both compounds exhibit a fluorescence band with a maximum at 550 nm, which can be associated with the proton transferred species. In the case of the flavanone this band is very weak and attributed to Excited State Intramolecular Proton Transfer (ESIPT) of the proton at the 5-position. In the flavone the corresponding band is considerably more intense and the theoretical calculations point to the proton being transferred as originating from the 5-position also.