Chemical approaches to nanometre-scale logic gates

External stimulus-induced changes in molecular read-out give rise to bistable nanoscopic switches. Beyond simple bistable species, multistate (multi-input/output) systems lead to the idea of molecular logic gates. Since the appearance of a prototypical molecular logic gate in 1993, different strategies have been described for developing these molecular information processors. Notably, various photophysical processes have been employed to effect the requisite changes in photonic output as a function of the input. Equally, different questions have been addressed with varying degrees of success concerning the connectivity/integration of logic gates and the superposition/multiplexing of optical logic gates. After introducing pertinent processes available in the chemist's toolbox for designing functional nanometric devices, illustrated with a few simple examples, more sophisticated combinatorial logic arrays are described, including molecular systems capable of numeracy.