Fluorescein hydrazone-based supramolecular architectures, molecular recognition, sequential logic operation and cell imaging

Abstract

A fluorescein hydrazone (FDNS) is prepared by the coupling of fluorescein hydrazide with 3,5-dinitrosalicylaldehyde. It is well characterized using spectroscopic (IR, UV-visible, ¹H, ¹³C NMR, ESI-MS) techniques and X-ray crystallography. FDNS is embedded with several H-bonding domains which provide interesting intra and inter molecular H-bonded networks. Its crystal packing along the b crystallographic axis using H-bonding interactions provides a fascinating helical structure. It detects Cu²⁺ ions selectively over many relevant ions and displays a novel peak at λ_max = 495 nm. The significant enhancement in its fluorescence is observed with a peak at λ_em = 517 nm on addition of Hg²⁺ ions, which is quenched upon the addition of S²⁻ ions. The sensing of Hg²⁺ ions by FDNS follow a hydrolysis pathway whereas the binding of Cu²⁺ ions with FDNS provides a colour change. The addition of a solution of tetrabutylammoniumcyanide in methanol to a corresponding solution of FDNS caused a turn to a green colour immediately. But on keeping the solution at room temperature for 72 h, red coloured crystals are obtained. The crystals were authenticated by X-ray crystallography. It was found to be a new compound FKCN in which a tetrabutylammonium cation is co-crystallized with deprotonated FDNS. Its supramolecular assembly via H-bonding provides an interesting ladder type architecture. FDNS displays chronological logic gate-based detection of several ions (Cu²⁺, Hg²⁺, EDTA, and S²⁻) at ppm levels. The real sample analysis, live cell imaging and portable paper strip based detection of Cu²⁺ and Hg²⁺ ions via an obvious colour change endows FDNS with great economic significance in recognition processes.