Experimental Investigations of Complexes of Some Small Ring Fluorescent Benzo Oxa-Crown Ethers with Alkali Metal Ions in Solution Phase: A Review

This review paper focuses on the past and recent investigations carried out in solution state to determine the complexation chemistry, characterization, stability and photophysical properties of complexes of small ring benzo oxa-crown ethers namely benzo-15-crown-5, dibenzo-15-crown-5 and benzo-12-crown-4 with alkali metal ions. These crown ethers form stable complexes with physiologically important ions like Li+, Na+ and K+ and mimic natural ionophores in this property. The experimental observations from UV-visible spectroscopy, IR spectroscopy, NMR spectroscopy, conductometry and calorimetry are useful in confirming the formation of these complexes. In addition to monitoring the complexation process, techniques like IR and NMR spectroscopy have helped to designate the conformers of the complexes in solution. These crown ethers have been used in developing functional materials such as ion sensors, fluorescent logic gates, oil/water emulsion separating agents and extractors of toxic ions like cesium from nuclear waste. The benzene moiety in these macrocycles enables them to act as fluorescent sensors for alkali metal ions. They make ideal candidates to anchor on nanoparticles for sensing alkali metal ions. The ring current in the benzene ring of benzo-crown ethers makes the carbon and protons nonequivalent and hence easily distinguishable by methods like 1H and 13C NMR spectroscopy as compared to 12-crown-4 and 15-crown-5. The solvent systems including binary and ternary mixtures of solvents employed in the study of these complexes have been reported in this review. Important aspects of complexation thermodynamics, fluorescence intensity altering processes and energy transfer mechanisms are also discussed in this paper. The dielectrics and donicity of the medium form an additional aspect of the review. These crown ethers form supramolecular assemblies with alkali metal halides and can therefore serve as good models to study and explore suitability of characterization methods for noncovalent interactions.