The role of the size of aza-crown recognition moiety in azaphthalocyanine fluorescence sensors for alkali and alkaline earth metal cations

Abstract

A series of fluorescence sensors bearing one 1-aza-12-crown-4, 1-aza-15-crown-5, 1-aza-18-crown-6 or 1-aza-21-crown-7 as a recognition moiety and an aza-analogue of phthalocyanine as a fluorophore was prepared. All compounds absorbed and emitted light in the red region. Sensing properties based on intramolecular charge transfer were studied via absorption and fluorescence titration experiments with alkali metal cations and alkaline earth metal cations. Important relationships between aza-crown size and binding affinity were observed in the group of alkali metal cations. Affinity for lithium decreased in series from the smallest crown to the largest, 1-aza-15-crown-5 bound sodium and potassium similarly, and 1-aza-18-crown-6 had the highest affinity to potassium. Alkaline earth metal cations were bound more tightly, which was obvious from more pronounced changes in the absorption spectra, and from the higher increase of fluorescence upon cation addition. A limited size preference was observed in the group of alkaline earth metal cations.