Metal-Cation Recognition in Water by a Tetrapyrazinoporphyrazine-Based Tweezer Receptor.

A series of zinc azaphthalocyanines with two azacrowns in a rigid tweezer arrangement were prepared and the fluorescence sensing properties were investigated. The size-driven recognition of alkali and alkaline earth metal cations was significantly enhanced by the close cooperation of the two azacrown units, in which both donor nitrogen atoms need to be involved in analyte binding to switch the sensor on. The mono- or biphasic character of the binding isotherms, together with the binding stoichiometry and magnitude of association constants (KA), indicated specific complexation of particular analytes. Water solvation was shown to play an important role and resulted in a strong quenching of sensor fluorescence in the ON state. The lead compound was embedded into silica nanoparticles and advantageous sensing properties towards K+ were demonstrated in water (λF=671 nm, apparent KA=82 m−1, increase of 17×), even in the presence of (supra)physiological concentrations of Na+ and Ca2+.