text: One of the earliest ideas for the use of π-conjugated molecules in molecular computation was proposed by Ari Aviram from IBM in 1988. The motivation for this work was to develop theoretical models for mixed-valence type-σ-π-σ-bonded molecules and explore their stereochemical properties for potential applications in molecular devices. The system comprised two π-conjugated systems, 1 and 2, made of oligothiophenes joined by a spiro linkage, and connected to gold electrodes by thiol linkers. π-system 1 is non-conducting in its neutral state, whereas system 2 is electrically conducting in its radical cation form. The twisted structure of the system courtesy of the spiro linkage provides hindrance to the transport of electrons between the two π-systems. However, through electrodes directed toward the linkage, a strong electric field could facilitate electron transfer from system 1 to system 2, with the radical cation going the opposite way.
  The first practical realization of molecular logic was courtesy of de Silva and coworkers in their seminal work, in which they constructed a molecular photoionic AND gate with a fluorescent output. While a YES molecular logic gate, as described previously, can convert signals from their ionic to photonic forms (hence the term “photoionic”), they are singular-input-singular-output systems. To build more complex molecular logic architectures, two-input gates, namely AND and OR gates, are needed. Some early works made some progress in this direction, but could not realize a complete truth table as their ionic forms (protonated) could not bind to the substrate in all cases. De Silva and co. constructed an anthracene-based AND gate made up of tertiary amine and benzo-18-crown-6 units, both of which were known to show photoinduced electron transfer (PET) processes. In the system shown, they both acted as receptors, connected to the anthracene-based fluorophore by alkyl spacers. The PET is quenched upon coordination with protons and sodium ions, respectively, for the two receptors, and would lead the anthracene unit to fluoresce. The truth table for an AND gate was completely realized as the system would exhibit a fluorescence output only in the presence of both protons and sodium ions in the system.