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Fluorescence data of 3-ethoxycarbonyl-, 3-acetyl-, 3-phephyl-, 4-methyl-3-phenyl-, 3-(2-benzothiazolyl) -and 3-cyanosubstituted 7-hydroxycoumarines are reported as a function of acidity (p H  0 - 9). Unlike umbelliferone or its 4-methyl derivative they do not exhibit significant photoautomerism in acidified solutions, but with the exception of the benzothiazolyl derivative. This behaviour can be interpreted in terms of a) steric hindrance by the substituent and b) enhanced charge delocalisation in the first excited singlet state, leading to lower basicities at the ring carbonyl oxygen.

p H -Dependent Fluorescence Spectra of 3-Substituted Umbelliferones