All-photonic molecular XOR and NOR logic gates based on photochemical control of fluorescence in a fulgimide-porphyrin-dithienylethene triad

A molecular triad consisting of a porphyrin linked to two photochromes, a fulgimide, and a dithienylethene, is synthesized and studied. When both photochromes are in their visible-light-absorbing forms, excitation of the fulgimide at 470 nm initiates a two-step singlet energy-transfer relay wherein excitation migrates first to the porphyrin and then to the dithienylethene. Photoisomerization of the dithienylethene to the open form using visible light prevents the second step, and excitation ultimately resides on the porphyrin, which fluoresces. Photoisomerization of the fulgimide eliminates significant absorption by the molecule at 470 nm, and consequently porphyrin excitation by energy transfer. Photoisomerization of each photochrome may be preferentially achieved, allowing access to all four isomeric states of the molecule. These states correspond to the outputs of logic gates, allowing solutions of the triad to perform either NOT-OR (NOR) or exclusive OR (XOR) functions using only optical inputs and outputs.