UK researchers have demonstrated the first example of a molecular memory device that follows-up storage with logic processing. The development by Prasanna de Silva and his team at Queen's University Belfast takes molecular logic integration a step further. [ de Silva, A.P. et al., Nature Commun. (2019); DOI: 10.1038/s41467-018-07902-7]

Molecular-logic based computation has grown steadily over the last quarter of a century with many teams around the world looking to develop systems and devices that might ultimately usurp the semiconductor devices on which computing has relied for the last half a century. The prospect is that by shaking down electronics to the molecular level we would overcome the limitations of microlithography and other techniques and so extend Moore's Law well into the future. Moreover, molecular logic devices might open up new applications and new thinking in terms of what we might do with computation.

Many examples of combinational logic gates and a handful of examples of sequential variants have been developed since the 1990s. Some of these systems have found lateral applications in sensor technology. However, despite the great inspiration such systems have in biology where molecular logic is intrinsic to life, there are as yet almost no examples of molecular-logic based computation in chemistry laboratories where sequential and combinatorial operations have been integrated.

De Silva and his team explain how they hope to remedy this situation. They have devised a rather simple-seeming alcohol-ketone redox interconversion. They can then use this reaction to switch a macrocycle between a large or small cavity, with erect aromatic walls which create a deep hydrophobic space or with collapsed walls, respectively. De Silva explains how small aromatic guests can be captured within the raised walls or released in an all-or-none manner by a chemical command.

The team further explains that during this capture process, the fluorescence of the alcohol macrocycle is quenched through a PET (photoinduced electron transfer) process which means an external observer can determine the occupancy state of the system without additional probes. This chemical "flip-flop" process can be used as the input for an INHIBIT Boolean logical gate, a molecular device that the team built several years ago.

The coupling of the flip-flop with INHIBIT has its analog in biology where a package of neurotransmitters is injected into the cleft of the synapse and leads to progression of a neural signal. One can foresee the integration of this molecular logic system with an array of others to build up sophisticated computational devices that would work in solution or might be constructed on an appropriate membranous substrate in the future.

From the chemical perspective, de Silva points out how the system is something of a breakthrough in molecular logic research. "This is the first 'all-or-none' switching of guest capture/release in simple p-cyclophanes in both the solid- and aqueous solution-phases," de Silva told Materials Today. He adds that it is also the first 'all or none' molecule capture/release by phenylene wall-erection/collapse of a larger molecular host in water via a simple ketone/alcohol interconversion.