Construction and Investigation of Metallochromic Dyes

Abstract

Novel fluorophores were synthesized, characterized, and examined with respect to their metal-binding properties. These compounds (Figure 1) consist of a heteroaromatic core substituted with two triazole rings, synthesized via copper-catalyzed azide-alkyne cycloaddition. Binding of a metal ion is achieved through coordination between two nitrogens (one in the triazole ring, and one in the heteroaromatic core). For practical purposes, these sensors must be soluble in water. This is accomplished through the use of a water-soluble side chain; in this case, one with a branched oligoethylene glycol substituent. This bulky side-chain decreases fluorescence quenching from intermolecular aggregation, resulting in metal ion sensors that are brightly fluorescent, even in water. Figure 1 Metal ion sensors. R represents tri(ethylene glycol) monomethyl ether, X represents H, F, or Cl, and Y represents O, S, or Se. These molecules are designed so that they serve as the binding receptor and the sensing element. We are then able to tune the structure of the core molecule, thereby adjusting the metal-binding activity, as well as the optical properties. In Figure 1, the series of molecules on the left is tunable through halogen substitution of the phenazine core. On the right, variation of the chalcogen heteroatom serves the same purpose. Increasing understanding of this kind of structure-property relationship is vital for the future construction of highly sensitive and selective fluorescent sensors. The results show that of the phenazine-containing compounds, those that are more electron-poor (halogen-substituted) are not able to efficiently bind metal ions in aqueous solution. A similar effect is seen with the benzochalcogendiazole compounds, with binding affinity increasing moving down the group, parallel to the decreasing electronegativity of the chalcogen atom. The heteroaromatic core also plays a significant role in the selectivity; the phenazine compound functions as a selective silver sensor, while the benzochalcogendiazole compounds respond to copper, silver, and nickel. The response to each metal is unique, and statistical analysis of the resulting data enables differentiation of these three metals with a single molecule.