text: In compound B in Figure 10 B, the bottom section contains a tertiary amino group, also capable of binding to protons. In this system, fluorescence only takes place when both cations available. The presence of both cations hinders photoinduced electron transfer (PET) allowing compound B to fluoresce. In the absence of both or either ion, fluorescence is quenched by PET, which involves an electron transfer from either the nitrogen atom or the oxygen atoms, or both to the anthracenyl group. When both receptors are bound to calcium ions and protons respectively, both PET channels are shut off. The overall result of Compound B is AND logic, since an output of "1" (fluorescence) occurs only when both Ca and H are present in solution, that is, have values as "1". With both systems run in parallel and with monitoring of transmittance for system A and fluorescence for system B the result is a half-adder capable of reproducing the equation 1 + 1 = 2.
  In a modification of system B, not two, but three chemical inputs are simultaneously processed in an AND logic gate. An enhanced fluorescence signal is observed only in the presence of excess protons, zinc and sodium ions through interactions with their respective amine, phenyldiaminocarboxylate, and crown ether receptors. The processing mode operates similarly as discussed above – fluorescence is observed due to the prevention of competing photoinduced electron transfer reactions from the receptors to the excited anthracene fluorophore. The absence of one, two, or all three ion inputs results in a low fluorescence output. Each receptor is selective for its specific ion as an increase in the concentration of the other ions does not yield a high fluorescence. The specific concentration threshold of each input must be reached to achieve a fluorescent output in accordance with combinatorial AND logic.