The compound class of 3-arylpropynylidene indolones, key intermediates in consecutive three-component syntheses of various indole-based chromophores with peculiar emission characteristics, are readily synthesized by a domino insertion-coupling synthesis with an electronically diverse substitution pattern in moderate to excellent yields. The title compounds are formed in E/Z-ratios from 100:0 to 0:100. Besides structure elucidation by NMR-spectroscopy and X-ray structure analysis, DFT calculations have been employed to rationalize the observed stereoselectivity. The photophysical properties of 3-arylpropynylidene indolones are characterized by intense, tunable, solid-state emission of N-substituted derivatives as quantified for drop-cast films. The electronic ground state structure was corroborated by DFT and TD-DFT calculations, showing that gradient-corrected exchange and correlation PBE (Perdew-Burke-Ernzerhof) functionals can be successfully employed to correctly reproduce the observed absorption characteristics of merocyanine derivatives. The huge Stokes shifts strongly depend on the electronic substitution pattern as supported by Hammett-Taft correlations.
Keywords: Sonogashira coupling; alkynes; density functional calculations; domino reactions; fluorescence; push-pull molecules.
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