FLUORESCENCE-SPECTRA OF 3-CYANO-4-METHYL-7-HYDROXYCOUMARIN AND ITS ACID-BASE BEHAVIOR IN THE EXCITED-STATE

Fluorescence spectra of 3-cyano-4-methyl-7-hydroxycoumarin were measured in ethanolic solutions containing various amounts of water and acid. Spectra composed of two bands, one from the undissociated molecule and the other from the phenolate anion, were observed by excitation of the undissociated molecule with most of the solutions studied. From variation of the fluorescence intensities pKa*'s were determined according to Weller's method. The value varied from 2.3 to 0.01 with the variation of water content from 0.2 to 80%. The rate constants of excited-state dissociation and its reverse reaction were also determined from the fluorescence intensity and lifetime data. With highly acidic solutions the fluorescence band due to the excited-state proton adduct at the carbonyl oxygen was observed in the spectral region between the two bands mentioned above. The green fluorescence band characteristic of most 7-hydroxycoumarins, which is attributed to the zwitterionic tautomer, was not observed. This fact was attributed to the effect of the electron-withdrawing cyano group at the 3 position. © 1979 American Chemical Society.