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Synthesis and Spectral Properties of 7-(N-Arylsulfonyl)aminocoumarins, A New Class of Fluorescent pH Indicators
Wolfbeis, Otto S.,Baustert, John H.
, p. 1215 - 1218 (2007/10/02)
7-Arylsulfonylamino-coumarins 3 and 5, with electron-delocalizing substituents in position 3 were obtained by condensation of 4-arylsulfonylamino-2-hydroxybenzaldehyde with various (hetero)arylacetic esters and by other methods.Oxidative cyanation using potassium cyanide and elemental bromine gave the corresponding 4-cyano derivatives 6a,b. The absorption and emission maxima of coumarins 3 and 5 in organic solvents and in water of different pH were determined.All showed bright blue fluorescence in organic or acidic aqueous solution, which is shifted to the green in alkaline solution.The 4-cyano derivatives 6a,b exhibit green fluorescences, which change to yellow in alkaline solution.Simultaneously the color changes from yellow to orange. The pKa values of compounds 3 and 5 were found to lie between 6.33 and 7.06, whereas for the 4-cyano derivatives they were distinctly lower (5.66-5.95).The spectral changes observed in the near neutral pH range may render the new compounds useful indicators for the measurement of physiological pH's.
