FLUORESCENCE STUDY OF 3-ACETOACETYL-7-METHYL-2H,5H-PYRANOPYRAN-2,5-DIONE - EVIDENCE FOR THE 2 CIS-ENOL FORMS

The behaviour of the title compound has been investigated in different solvents by absorption and fluorescence spectroscopy. Absorption spec tra showed that the enol form largely predominates in organic solvents , as confirmed by NMR data. Fluorescence was observable at room temper ature (the quantum yield ranging from 10(-2) to 10(-3) according to th e solvent) and was totally attributed to the enol tautomer. The emissi on spectrum was strongly shifted towards the blue, and its shape under went significant changes when passing from toluene to water. This beha viour was mainly attributed to the involvement of two different specie s, assumed to be two cis-enol isomers, whose relative proportions vary with the nature of the solvent. Attributions were performed with the help of NMR data and MNDO calculations.