1,5-Diphenyl-3-vinyl-Δ2-pyrazoline (1) has a fluorescence quantum yield [Formula: see text] which is strongly controlled by the hydrogen bond acidity of the solvent (α) and to a minor extent by the dipolarity of the solvent (π*) according to the equation:[Formula: see text]The wavenumber of the fluorescence band maximum [Formula: see text] is controlled nearly equally by π* and a according to the equation:[Formula: see text]Thus the fluorescence spectrum of 1 delivers two distinct and independent parameters of its microenvironment while its electronic absorption spectrum is essentially solvent invariant.

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