Molecular logic gate

A molecular logic gate is a molecule that performs a logical operation based on one or more physical or chemical inputs and a single output. The field has advanced from simple logic systems based on a single chemical or physical input to molecules capable of combinatorial and sequential operations such as arithmetic operations i.e. moleculators and memory storage algorithms.

For logic gates with a single input, there are four possible output patterns. When the input is 0, the output can be either a 0 or 1. When the input is 1, the output can again be 0 or 1. The four output bit patterns that can arise corresponds to a specific logic type: PASS 0, YES, NOT and PASS 1. PASS 0 always outputs 0, whatever the input. PASS 1 always outputs 1, whatever the input. YES outputs a 1 when the input is 1 and NOT is the inverse YES - it outputs a 0 when the input is 1. An example of a YES logic gate is the molecular structure shown below. A ‘1’ output is given only when sodium ions are present in solution (‘1’ input).

Molecular logic gates work with input signals based on chemical processes and with output signals based on spectroscopy. One of the earlier water solution-based systems exploits the chemical behavior of compounds A and B in scheme 1 .

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 Ca²⁺ or H⁺, the chromophore shows a maximum absorbance in UV/VIS spectroscopy at 390 nm. When calcium is introduced a blue shift takes place and the absorbance at 390 nm decreases. Likewise addition of protons causes a red shift and when both cations are in the water the net result is absorption at the original 390 nm. This system represents a XNOR logic gate in absorption and a XOR logic gate in transmittance.

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