 In CHEQ, a reduction is observed in fluorescent emission of the chemosensor in comparison to that seen the originally for the 'free' sensor upon host–guest formation.As electron transfer is directional, such systems have also been described by the PET principle, being described as an enhancement in PET from the receptor to the fluorophore with enhanced degree of quenching.Such an effect has been demonstrated for the sensing of anions such as carboxylates and fluorides.