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

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.