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The pH‐dependent absorption and fluorescence spectra of 4‐phenylumbelliferone (4‐PU) are reported. 4‐PU, which is a model for several natural compounds, shows an unusual behaviour in fluorescing due to a species formed in acidic solutions by an adiabatic photoreaction during the lifetime of the excited state. This fluorescent species is shown to be either a phototautomer or, more likely, a quinoid ketene phototautomer. The latter can be trapped by H2 18O to form an 18O‐enriched coumarin dye, which is identified using high resolution mass spectrometry.
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Wolfbeis et al. (1980) studied this question.
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