Photoionic devices with receptor-functionalized fluorophores

Fluorophores can be connected to ion receptors so as to permit extensive overlap, be geometrically orthogonal or be separated by a spacer. Each of these three formats 1-3 has led to applications as ion sensors or switches. We focus on 'fluorophore-spacer-receptor' systems whose modular structure allows semiquantitative design according to the principle of photoinduced electron transfer (PET). The fluorescence of these systems can be switched 'on' or 'off' when confronted with ions. Several new examples 15-17 responding to Na⁺, H₃N⁺CH₂CH₂CH₂CO ₂ and H⁺ are described. 17 and 18 are cases where kinetic factors become important in addition to the usual thermodynamic design criteria. Expansion of the basic three-module system by the addition of a separate receptor with a different ion binding profile as in 19 gives rise to molecular AND logic gates. On the other hand, OR logic gates such as 20 can be obtained from the basic system provided that the receptor is deliberately chosen to be nonselective under the conditions of use.