Photoionophores derived from crown ether polycarboxylic acids: synthesis, ion binding, and spectroscopic characterization

Abstract

Three types of potential photoionophores based on polycarboxylic acid crown ethers were prepared, and their cation complexation behaviours and spectroscopic properties were surveyed. The first type were neutral macropolycyclic hosts prepared by capping across the faces of the crown ether with aromatic diamine chromophores. The second were bis-crown ether carboxylates bearing a bridging aromatic chromophore. The third type appended an additional chromophore-donor site on the crown ether carboxylic acid framework. Cation complexation was examined by potentiometric titration. The neutral ligands were rather poor hosts for alkali metal cations. The other two types of crown ether carboxylates showed a combination of size selectivity and electrostatic stabilization, leading to significant and selective ion binding in water. Ligands of the third type also exhibited cation-dependent absorption spectra in neutral and basic aqueous solution. No significant alkali metal or alkaline earth cation-induced perturbation of the emission spectra was uncovered, but a sodium-and cesium-dependent long wavelength emission enhancement was observed in one of the neutral ligand systems. Photoionophores are selective complexing agents for specific phores investigated. Each case builds on the tendency of the ionic species in which the complexed guest species induces a carboxylic acid groups to occupy axial positions on the macroperturbation in the electronic state of the host molecule. Perturcyclic periphery (22). In case of type A this would lead to capbations can be detected by absorption or emission spectroscopy ping across the face of the macrocycle; hence, this type is to define chromoionophores (1, 2) or fluoroionophores (2), respectively. In principle, ion-selective complexing agents could be modified by addition of a suitable chromophore to yield a functional photoionophore with the selectivity of the unmodified ion-binding host. For alkali metal and alkaline earth cations several examples of this strategy are reported for crown ether (2-12), cryptand (13,14), hemispherand (15), calixarene (16,17), and cyclophane hosts (18). Phenols ( l , 4 , 12), polycyclic aromatic (3, 13, 18), and heteroaromatic chromophores (5, 8,9, 11) have all been exploited. Despite the successes, there is still a need for photoionophores for alkali metal cations that could be used in aqueous solution over a range of pH values (10). The majority of the known photoionophores are either insoluble in water, insensitive to alkali metal cations, or pH sensitive.This paper evaluates several potential photoionophores referred to as captand. The additional arms above and below the macrocyclic plane place the chromophore in proximity to a bound cation and define a three-dimensional cavity (23, 24).Type B is akin to other bis-crown ethers (25), with the additional feature that the chromophore lies between the rings. The crown ethers are expected to act as independent carboxylate crown ethers. Type C unites several themes in crown ether chemistry: lariat ether structu...

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