Luminescent Sensing of Specific Analytes by Redox Active Anthraquinone Amide Macrocycles
Fluorescence based sensors, which are prized for their sensitivity and selectivity, have been employed in specific detection of heavy metal ions. This study utilizes a ‘fluorophore-spacer-receptor’ model to synthesize two fluorescent sensors. The sensors are made of an amide substituted macrocyclic ring receptor and either an anthracene or a rhodamine-B derived fluorophore. The macrocyclic ring structures are attached to an anthraquinone moiety. Synthesis of the sensors was effected by a simple ester hydrolysis from 1,8-diester anthraquinone and tris(2-aminoethyl)amine derivatives in toluene/methanol. The oxidized form of anthraquinone effectively quenches the fluorescence by energy transfer from the fluorophore to the anthraquinone moiety. Reduction of the anthraquinone restores the fluorescence of anthracene and rhodamine because the reduced quinone is not as effective as an energy acceptor. This mechanism causes the redox sensitivity of the sensors. The reduced anthracene azamacrocycle demonstrates pH sensitivity due to a photo-induced electron transfer (PET) mechanism. The rhodamine azamacrocycle demonstrates selective turn-on fluorescence in the presence of Hg2+. Competition studies of alkali, alkaline earth, and transition metal ions confirm selectivity. Both an electrochemical mechanism, the oxidation state of the quinone, and a chemical mechanism, the binding of a proton or mercury ion, control total fluorescence emission.