Synthesis of Diphenyl Ethers for the Detection of Ions by Photophysical Methods.

Abstract

The selective recognition of cations and anions is vital due to their diverse application in chemistry, biology and the environment. Diphenyl ethers present an attractive architecture with the right balance between rigidity and flexibility due to aromatic planer rings, rotation around the ether linkage, and an inbuilt cavity. Also, the substituents on the phenyl rings can alter the electron density inside the void.

Synthesis of several diphenyl ethers has recently been reported, along with their applications to detect ions. However, the literature lacks a systematic study around the motif by varying substituents on phenyl rings. The present work, presented in three units, explored the selectivity, sensitivity, detection limit, solvent effect and other parameters for environmentally and biologically essential ions by changing the functional group on the molecules.

Section-I describes the importance, background, rationale, and recent literature about detecting essential ions. It also details the methodology, techniques and calculations used to conduct the experiments.

Section-II defines the detection of cations with initial studies on optimizing functional groups on diphenyl ethers for recognizing Hg(II) and Pb(II) ions in an aqueous medium. The molecules' narrow pH range (7.5 to 9.0) to respond to cations constrained their real-life application. Nonetheless, the studies based on pH, solvents and computational data added value to exploring the interaction mechanism. The detection limit of toxic and biologically important cations, Hg(II) and Cu(II) ions respectively, was refined using a thiocarbohydrazides-based dimeric diphenyl ether (Compound-5) in an aqueous medium. It could detect Hg (II) and Cu(II) ions with a minimum concentration limit of 27 nM and 160 µM and an association constant of 1.2 × 10^5 M^ -1L and 9.1 × 10^5 M^-1L. The work demonstrated and validated the detection of Hg(II) ions in non-invasive biological samples (urine) as a real-life application.

Section-III initiates studies with seven diphenyl ethers containing acidic protons to detect anions via a proton abstraction mechanism. Two compounds displayed selectivity for cyanide ion (Compound-2) and cyanide, fluoride and acetate ions (Compound-4) with detection limits in the nanomolar range using an acetonitrile medium. Further optimization under different pH and solvent conditions made compound-4 more selective in detecting the cyanide ions in the neutral pH range. The optimization studies not only explored the proposed proton abstraction mechanism but also detected CN - ions reasonably with a minimum detection limit of 100 nM and a binding constant equal to 0.5 x10^5 M^-1L .

Theoretical calculations, NMR titrations and IR spectra were used as additional tools to get an insight into the binding sites and interaction mechanism.