Lanthanide luminescent switches: modulation of the luminescence of bis-macrocyclic based Tb(iii) conjugates in water by H+, Na+ and K+

Abstract

The synthesis of four bis-macrocyclic conjugates made from the coupling of either diaza-15-crown-5 ethers (1 and 3) and diaza-18-crown-6 ethers (2 and 4) to either amide or carboxylate functionalized cyclen (1,4,7,10-tetraazacyclododecane), and their corresponding cationic Tb(III) complexes, Tb·1, Tb·2, and neutral complexes Tb·3 and Tb·4 are described. The effect on the ground, singlet excited states and the Tb(III) emission, was investigated either as a function of pH or the concentration of several Group I and II cations, upon excitation at 300 nm. The ground state and singlet excited states of the Tb(III) complexes were found to be modulated by ions such as H⁺, Na⁺ or K⁺, signifying the recognition of these ions by the crown ether receptors. In acidic media, below pH 4, the Tb(III) emission was highly pH sensitive, gradually increasing with large orders of magnitude of luminescence enhancements. For Tb·1 and Tb·2 complexes, the Tb(III) emission was also ‘switched on’ in alkaline media above pH 8. At pH 7.4, the recognition of Na⁺ or K⁺ also gave rise to a significant change in the Tb(III) emission due to the modulation of the antenna-receptor moieties by these ions. For Tb·1 and Tb·3 the largest changes were seen for Na⁺, whereas for Tb·2 and Tb·4 the largest changes were seen for K⁺.