Synthesis of biologically important, fluorescence active 5-hydroxy benzo[g]indoles through four-component domino condensations and their fluorescence “Turn-off” sensing of Fe(iii) ions

Abstract

Several highly substituted 2-pyrrolyl-2-cyanoacetamides were prepared through four-component domino condensation of various easily available 1,3-dicarbonyl compounds, amines, arylglyoxals and malononitrile. Subsequently these cyanoacetamide derivatives were converted into biologically important 5-hydroxy benzo[g]indoles through thermal cyclization under metal-free conditions. The synthesized 5-hydroxy benzo[g]indoles are fluorescence active with good quantum yields (Φ_F = ∼0.50). They also show excellent fluorescence “Turn-off” sensing of Fe³⁺ ions (detection limit = ∼1.2 × 10⁻⁶ M). The interaction of 5-hydroxy benzo[g]indoles with Fe³⁺ ions can also be monitored through UV-Vis spectral change and naked-eye colour change in the presence and absence of UV radiation. The ¹H NMR titration unambiguously proves the formation of a complex between 5-hydroxy benzo[g]indoles and Fe³⁺ ion through the coordination of –OH groups with the metal. The binding constant of the complex (metal : ligand = 1 : 1) has been measured using Benesi–Hildebrand equation and found to be ∼7.97 × 10³ M⁻¹.