Charge transfer fluorescence of some N-benzoylindoles

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Abstract
The fluorescence properties of various N-carbonyl-substituted indoles (compounds 1–7) are examined. The N-benzoyl-indole derivatives 1 and 3–5 are shown to fluoresce weakly and the effect of solvent polarity upon the energy of the emitting state and upon the quantum yield of fluorescence is described. It is concluded that the initially formed singlet excited state is non-emissive and can relax to an intramolecular charge transfer state which is weakly fluorescent. The solvent effects are consistent with a general type of solvent interaction, and the solvent-induced shifts in the wavelength of the fluorescence emission correlate well with the Lippert equation relating the stabilisation of the excited state to a function of solvent dielectric and refractive index. A poor correlation with the empirical ET(30) solvent polarity parameter is observed.