A ditopic fluorescent sensor for potassium fluoride.

The addition of potassium fluoride 'switches on' the fluorescence of sensors and while potassium chloride and bromide cause no fluorescence change; the fluorescence can be 'switched off' by removing the potassium cation from the benzocrown ether receptors of sensors and through the addition of [2.2.2]-cryptand and restored by the addition of the potassium cation as potassium chloride.