 In chemosensing, the use of fluorophore connected to the receptor via a covalent spacer is now commonly referred to as fluorophores-spacer-receptor principle.In such systems, the sensing event is normally described as being due to changes in the photophysical properties of the chemosensor systems due to chelation induced enhanced fluorescence (CHEF), and photoinduced electron transfer (PET), mechanisms.In principle the two mechanisms are based on the same idea; the communication pathway is in the form of a through-space electron transfer from the electron rich receptors to the electron deficient fluorophores (through space).