Synthesis and emission properties of 1,1,4,4-tetracyanobutadienes derived from ynamides bearing fluorophores

A series of 1,1,4,4-tetracyanobutadienes (TCBD) bearing a large diversity of fluorophores was prepared following a multi-step synthesis. In a crucial last step, all compounds were obtained from the corresponding ynamides which were particularly suitable for the formation of the TCBDs in the presence of tetracyanoethylene via a [2+2] cycloaddition/retroelectrocyclization step (CA-RE). Fluorenyl derivatives including several variations on position 7 and 9, in addition to phenanthrenyl and terphenyl derivatives provided ynamide-based TCBD affording remarkable emission properties covering a large range of wavelengths. Those compounds emit both in solid state and in solution from the visible region to the NIR range, depending on the molecular structures. Quantum yields in cyclohexane reached unforeseen values for such derivatives, up to 7.8%. A huge sensitivity to the environment of the TCBDs has also been unraveled for most of the compounds since we observed a dramatic fall of the quantum yields when changing the solvent from cyclohexane to toluene, while they are almost non-emissive in dichloromethane.