A Novel Optical Sensor for Metal Ions Based on Ground-State Intermolecular Charge-Transfer Complexation

A derivative of dipyridylethylene <b>2</b> was synthesized in order to study intermolecular coordination between the 18-crown-6 ether units of bis-crown stilbene <b>1</b> and the NH<sub>3</sub><sup>+</sup> groups of <b>2</b>. In acetonitrile solution, the formation of a supramolecular complex is evidenced by a considerable upfield shift of the <sup>1</sup>H NMR signals from the aromatic and olefinic protons. Emission quenching and the appearance of a new absorption band in the visible spectral region indicate the formation of a charge-transfer complex. The addition of Ba<sup>2+</sup> to a solution of the complex leads to significant fluorescence enhancement and to the disappearance of the charge-transfer absorption band.