Structural characterization of a fluorescein hydrazone molecular switch with application towards logic gates

A fluorescein acylhydrazone (2) and its metal complexes with Zn2+, Ni2+ and Cu2+ ions were synthesized and characterized via NMR (1H, 13C and 2D experiments), UV-Vis, fluorescence, and FT-IR spectroscopy and single crystal X-ray diffraction. Compound 2 crystallizes in the P triclinic space group with two fluorescein acylhydrazone (2) conformers and one water molecule per symmetric unit. The fluorescein acylhydrazone (2) and its metal derivates were analysed via cyclic voltammetry to determine the relationship between the shift in their redox potential upon the quenching of their fluorescence. Emission and UV-Vis absorption logic gates were fabricated based on the metal ion binding of 2 and its behaviour at different pH values. Compound 2 exhibited “on–off” behaviour in its fluorescence emission with the addition of triethylamine (TEA), and this effect was highly enhanced by the coaddition of Zn2+. Conversely, its fluorescence is highly quenched by the addition of trifluoroacetic acid (TFA) or Ni2+ ions, which allows the fabrication of an AND gate using TEA and Zn2+ as inputs and an INHIBIT gate with the addition of TFA or Ni2+, both using the fluorescence emission as outputs. The addition of Cu2+ results in a fluorescence emission at 525 nm, which is red-shifted with the addition of TEA. This behaviour allowed the construction of an XOR gate with the fluorescence emission at 525 nm as the output and the addition of Cu2+ and TEA as inputs. The absorption of 2 in the visible region was used to create AND gates with the addition of Cu2+ ions and TEA or Ni2+ and TEA as inputs. The INH, AND, XOR gates were manipulated by the controlled addition of different metal cations, TEA and TFA as inputs, and fluorescence emission and visible absorption as outputs, allowing the creation of three different combinatorial circuits, in which we highlight a new molecular switch “Half-Adder”.