Dual fluorescent zwitterionic organogels of a quinoxalinone derivative using cation–anion detection keys

Abstract

An [18]crown-6 unit was introduced into a cation–anion dual-ion-sensing quinoxalinone derivative (1) as a new fluorescent molecule for successive cation (K⁺) and anion (F⁻ and CH₃COO⁻) sensing in CH₃CN. High anion-sensing abilities for F⁻ and AcO⁻ were observed at the hydrogen-bonded acidic N–H proton of the positively charged K⁺-capturing 1 at the [18]crown-6 site due to electrostatic cation–anion interactions. On the other hand, the acidic N–H proton of lactam tautomer 1 strongly recognized basic AcO⁻ or F⁻ anions via N–H⋯AcO⁻ or F⁻ hydrogen-bonding interactions, and further AcO⁻ or F⁻ additions facilitated the deprotonation reaction, forming anionic 1⁻. Anionic 1⁻ showed a much higher K⁺-sensing ability at the [18]crown-6 site than neutral 1 through effective cation–anion electrostatic interactions. Interestingly, the electrostatically stabilized zwitterionic K⁺·1⁻ formed fluorescent organogels in CH₃CN, acetone, and THF; the organogels underwent reversible transformation between a blue fluorescent organogel and green fluorescent sol by the addition of trifluoroacetic acid (gel → sol) and trimethylamine (sol → gel). Both K⁺ and AcO⁻ (or F⁻) ions acted as the key ions in the fluorescent organogel formation.