A sandwich-type zinc complex from a rhodamine dye based ligand: a potential fluorescent chemosensor for acetate in human blood plasma and a molecular logic gate with INHIBIT function

A new fluorescent zinc complex Zn-R1 based on rhodamine-tris(hydroxymethyl)aminomethane spirolactam ligand R has been synthesised and characterised. Single crystal X-ray diffraction analyses revealed that in this complex R exists in its ring opened spirolactam form R1 and the complex Zn-R1 has a distinctive geometry in which the Zn2+ ion is sandwiched between the dye motifs of two R1 ligands. Zn-R1 exhibits very strong orange fluorescence and pink colour in acetonitrile : H2O (9.5 : 0.5%, v/v) solutions and the addition of acetate ions causes quenching of the fluorescence accompanied by a colour change from pink to colourless. The mode of interaction between Zn-R1 and acetate ions has been confirmed by absorption, fluorescence, NMR and HR-MS techniques. Further, Zn-R1 is highly selective towards acetate ions and exhibits potential as a sensor for acetate in solid state and in solutions including human blood plasma-like systems. The reversibility of acetate binding by Zn-R1 is demonstrated by adding an excess of acetate and Zn2+ ions alternately to the solution of Zn-R1 for multiple times. The fluorescence output shown by Zn-R1 on addition/non-addition of Zn2+ and acetate ions as sequential inputs can be used to construct an INHIBIT type molecular logic gate.