text: De Silva and McClenaghan designed a proof-of-principle arithmetic device based on molecular logic gates. As depicted in Figure 10 A, Compound A is a push-pull olefin with the top receptor containing four carboxylic acid anion groups (and non-disclosed counter cations) capable of binding to calcium. The bottom part is a quinoline molecule which is a receptor for hydrogen ions. The logic gate operates as follows. Without any chemical input of Ca or H, the chromophore shows a maximum absorbance in UV/VIS spectroscopy at 390 nm. When calcium is introduced a hypsochromic shift (blue shift) takes place and the absorbance at 390 nm decreases. Likewise, addition of protons causes a bathochromic shift (red shift), and when both cations are in the water, the net result is absorption at the original 390 nm wavelength. This system represents an XNOR logic gate in absorption and an XOR logic gate in transmittance.
  In another XOR logic gate system, the chemistry is based on the pseudorotaxane depicted in Figure 11. In organic solution the electron-deficient diazapyrenium salt (rod) and the electron-rich 2,3-dioxynaphthalene units of the crown ether (ring) self-assemble by formation of a charge transfer complex. An added tertiary amine like tributylamine forms a 1:2 adduct with the diazapyrene and the complex gets dethreaded. This process is accompanied by an increase in emission intensity at 343 nm resulting from freed crown ether. Added trifluoromethanesulfonic acid reacts with the amine and the process is reverted. Excess acid locks the crown ether by protonation and again the complex is de-threaded.