Cation–Anion Dual Sensing of a Fluorescent Quinoxalinone Derivative
Using Lactam–Lactim Tautomerism
- Publication date
- Publisher
Abstract
A quinoxalinone derivative capable
of lactam–lactim tautomerization was designed as a new fluorescence
probe for sensing of cation (M<sup>+</sup> = Li<sup>+</sup> and Na<sup>+</sup>) and anion (X<sup>–</sup> = F<sup>–</sup>,
Cl<sup>–</sup>, Br<sup>–</sup>, and CH<sub>3</sub>COO<sup>–</sup>) in organic solvents. In THF, the minor lactam tautomer
exhibited a weak fluorescence band at 425 nm with a typical Stokes
shift of ∼4400 cm<sup>–1</sup>, whereas the major lactim
tautomer exhibited an intense fluorescence band at 520 nm with large
Stokes shift of ∼8900 cm<sup>–1</sup> due to excited-state
intramolecular proton transfer (ESIPT). The presence of either cations
or anions was found to promote lactim-to-lactam conversion, resulting
in the lowering of the ESIPT fluorescence. The lone pairs on the alkylamide
oxygen and the quinoxalinone ring nitrogen of the lactam were found
to bind Li<sup>+</sup> to form a 1:2 coordination complex, which was
confirmed by single crystal X-ray structural analysis and fluorescent
titrations. In addition, the N–H bond of the lactam was able
to recognize anions via N–H···X hydrogen bonding
interactions. Where X = F<sup>–</sup> or CH<sub>3</sub>COO<sup>–</sup>, further addition of these anions caused deprotonation
of the lactam to generate an anionic state, consistent with the crystal
structure of the anion prepared by mixing tetrabutylammonium fluoride
and the quinoxalinone derivative in THF. Dual cation–anion-sensing
responses were found to depend on the ion-recognition procedure. The
anionic quinoxalinone derivative and its Li<sup>+</sup> complex, which
are formed by the addition of CH<sub>3</sub>COO<sup>–</sup> and Li<sup>+</sup>, respectively, displayed different fluorescence
enhancement behavior due to the two anions exchanging with each other