Synthesis and investigation of photo-physical properties of fluorescent dyes obtained by the Knoevenagel condensation of mono-, di- or tri-formyl aromatic aldehydes and various CH-acid derivatives

Fluorescent dyes were constructed using the Knoevenagel condensation of the appropriate aldehydes and CH-acid derivatives. Aromatic aldehydes containing one, two or three formyl groups were used as substrates. Cyanoacetamide, ethyl cyanoacetate, benzoylacetonitrile and indan-1,3-dione play the role of CH-acid derivative. The appropriate aldehydes were obtained by the Duff formylation reaction. The dyes synthesis procedure was very simple. The condensation was carried out in water in the presence of NaOH, as the basic catalyst at room temperature, and the dyes were obtained in yields between 58 and 93 %. The work-up procedure was very simple, and the products do not require any further purification. The great advantages of this synthetic procedure are its simplicity, cheapness, compliance with the rules of green chemistry and the fact that it efficiently leads to structurally complex fluorescent dyes. Twenty-three new compounds were obtained, eleven of which exhibited fluorescence in solution and four in the solid state. Taking into account the fluorescence quantum yield and the emission maximum value close to the NIR range, the dye constructed by condensation of indan-1,3-dione and methyl 3,5-diformyl-4-hydroxybenzoate meets the above-mentioned requirements (Φ = 16 %, λ_em = 660 nm) best. The dye with the highest value of Φ = 25 % in solid is a derivative of cyanoacetamide and 3,4,5-trimethoxybenzaldehyde.