A dual state emission luminogen based on the 1,3,3-trimethylindoline and chroman-2,4-dione conjugate for highly selective dual channel detection of cyanide ions†

Dual state emission luminogens (DSEgens) with strong fluorescence in both solution and solid states have extensive potential for numerous applications. Herein, a chroman-2,4-dione and indoline conjugate, 2, was synthesized for highly selective and sensitive turn-on fluorescent detection of cyanide ions. Compound 2 behaves as a molecular rotor and shows the dual state emission (DSE) phenomenon and multicolour emission. It displays bright fluorescence in both the concentrated solution and the solid-state. The compound is nonfluorescent in dilute solution, and with increasing concentration, it shows aggregation caused red-shifted emission. With increasing concentration, the emission colour changes from green to yellow to orange-red. The CC bond attached to the indoline moiety is a compelling target for nucleophilic addition. Cyanide ions reacted with the probe which remarkably changed the spectroscopic properties. With the gradual addition of cyanide, the colour of the probe solution was changed from yellow to colorless. The very weakly emissive probe 2 rapidly reacted with CN⁻ and emitted strongly due to the inhibition of internal charge transfer (ICT) from indoline to chroman-2,4-dione. The DSEgen properties and CN sensing were thoroughly investigated and supported using spectroscopic studies, TDDFT, and single-crystal X-ray diffraction.