All-Photonic Molecular Keypad Lock

Herein we demonstrate a molecular triad (1, Figure 1) comprising two photochromic units and a fluorescent porphyrin reporter that functions as an all-photonic keypad lock. The lock is a molecular automaton that opens only when exposed to one out of 8 possible 2-digit codes, can be reset many times, and can be activated remotely. Investigations of molecular approaches to logic operations began with simple switches and logic gates, and is now focused on the design and the synthesis of more complex molecular devices including small electronic circuits. An example is the molecular keypad lock. Although molecular keypad locks and similar devices have been demonstrated before, the previous examples all require addition of chemical species as input signals. This imposes limitations on both the operational speed and the nature of the surrounding medium, as fluid solution is required. Moreover, the repeated operation of chemically driven devices leads to build-up of chemical waste and/or dilution. Moleculebased logic elements with optical inputs and outputs such as 1 and other molecules we have reported 8] do not require access for chemicals or wires, can in principle operate on a much faster time scale and in rigid media, and can be cycled numerous times with no buildup of byproducts. A special advantage of optical inputs for the keypad lock is that it is capable of being operated remotely by light beams. The functional principle of a keypad lock is that it gives an output signal (opens) only when given the correct inputs in the correct order. In logic terms, a 2-input keypad lock such as 1 corresponds to a 2-input priority AND (PAND) gate. The major difference between the PAND gate and the regular AND gate is that its output is sensitive not only to the application of the inputs but also to the sequence in which they are applied. The 2-input PAND gate has two two-valued (on, off) inputs, A and B, and an output. Any of the 6 ordered combinations of inputs in which at least one input is off leaves the output off. When both inputs are on, the only application sequence that yields an on output is A followed by B. In addition to the keypad lock, PAND gates have applications in fault analysis and elsewhere.