Fluorescence Regeneration as a Signaling Principle for Choline and Carnitine Binding: A Refined Supramolecular Sensor System Based on a Fluorescent Azoalkane

Abstract The fluorescent azoalkane, 2,3‐diazabicyclo[2.2.2]oct‐2‐ene (DBO), forms inclusion complexes with p‐sulfonatocalix[4]arene (CX4). The binding constants are on the order of 10³ M^–1 in water. The addition of CX4 to DBO solutions results in an efficient fluorescence quenching (up to 90 %). This supramolecular system can be used as a truly water‐soluble sensor system to signal the binding of organic ammonium ions over a large pH range. Addition of choline and carnitine derivatives and tetraalkylammonium ions results in regeneration of this fluorescence, from which the binding constants (K_C = 10³–10⁵ M^–1) are calculated by means of a competitive complexation model. Electrostatic effects are observed, namely, a more‐than‐one order of magnitude weaker binding of the carnitines in neutral solution.