A Selective, Sensitive, Colorimetric, and Fluorescence Probe for Relay Recognition of Fluoride and Cu(II) Ions with “Off–On–Off” Switching in Ethanol–Water Solution

Anion to cation relay recognition was designed and realized for the first time with sequence specificity (F–→Cu2+) via a fluorescence “off–on–off” mechanism. Probe 1 was a highly selective, sensitive, and turn-on chemodosimeter for F– through a specific cyclization reaction triggered by the strong affinity of fluoride toward silicon with a significant change of fluorescence color in both ethanol and ethanol–water (1:1, v/v) solution. Fluorescence enhancement factors were dramatic: 833-fold in ethanol and 164-fold in ethanol–water (1:1, v/v) solution, respectively. The in situ system generated from the sensing of F– showed good relay recognition ability for Cu2+ via fast fluorescence quenching by the formation of a 1:1 complex in ethanol–water (1:1, v/v) solution. The isolated pure compound 2 also exhibited high selectivity toward Cu2+ in PBS buffer (pH = 7.0) solution. The origin of this sequence specificity of fluorescence recognition was disclosed through the crystal or optimized structures and DFT calculations of corresponding compounds.