Metal‐Cation Recognition in Water by a Tetrapyrazinoporphyrazine‐Based Tweezer Receptor

Abstract

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 Ca(2+) .

Red-Emitting Fluorescence Sensors for Metal Cations: The Role of Counteranions and Sensing of SCN– in Biological Materials

“…This fact is supported by the work of Junk et al, who described the role of water in cation coordination of 12-crown-4 complexes in the presence of different counteranions and by our observation that addition of water into anhydrous organic solution of some TPyzPz-aza-crown sensors led to the loss of their sensing activity …”

“…To enable clear characterization of the role of spatial arrangement and rigidity, TPyzPzs with monoazacrown substituents (type A, TPyzPzs 1 A and 2 A ), combinations of aza-crown and lariat ether (type B, TPyzPzs 3 B and 4 B ), combinations of aza-crown and crown with flexible linker (type C, TPyzPzs 5 C and 6 C ), or so-called tweezer arrangement (type D, TPyzPzs 7 D and 8 D ) were employed in this study. TPyzPzs with monoazacrown ( 1 A , 2 A ) , and the tweezer arrangement ( 7 D , 8 D ) have previously been investigated, but under different experimental conditions that do not allow direct comparison. According to the published data, TPyzPzs 1 A and 2 A exhibit high brightness in the ON state with worse selectivity .…”

“…The employing a tweezer arrangement led to substantial improvement in the selectivity and sensitivity for cation recognition. However, these sensors suffered from low fluorescence intensities in the ON state due to the presence of two donors for ICT . To overcome these drawbacks, we have designed new recognition moieties B and C (in 3 B , 4 B , 5 C , 6 C ) with one donor for ICT only.…”

“…In addition, the periphery of TPyzPzs can be easily decorated with suitable substituents that alter the photophysical properties. These fluorophores have become a fundamental part of fluorescence sensors for pH or metal cation − recognition, using intramolecular charge transfer (ICT) for switching on and off. A TPyzPz sensing device for CO 2 monitoring has also been developed …”

“…However, aza-crown recognition moieties still have many drawbacks. Despite numerous studies on the role of the size of the aza-crown, , its type , and rigidity, , our understanding of the structure–activity relationships remained limited due to the use of different fluorophore types, different solvents, and different salts in experiments. Based on data from the literature, − counteranions play an important role in cation recognition, but this role has not been systematically studied.…”