Solvent and Acidity Dependence of the Absorption and Fluorescence of Coumestrol*

The UV absorption and fluorescence of the plant estrogen coumestrol (1) are markedly dependent on solvent and acidity. In neutral non‐aqueous solvents, emission occurs from the uncharged first excited singlet state. In aqueous solutions of 1, five species—the neutral form, the mono‐ and dianions, and the mono‐ and dications—were found and characterized by their UV absorptions, fluorescence (including lifetimes and quantum yields), and pK's of the ground and excited singlet states. The assignments are based on absorption, fluorescence and pK(*) data of coumestrol dimethyl ether (2), 3‐phenyl‐7‐hydroxy coumarin (3), 34′‐hydroxyphenyl coumarin (4), and appropriate model compounds from the literature. Variations in øF and, consequently, τF values are ascribed to excited state proton transfer processes. Fluorescence quantum yields as high as ca. 0.7 in most organic solvents and in certain pH ranges should render fluorimetry a suitable method for quantitative assays of this estrogen which is responsible for ‘clover disease’ in sheep.