Squaraine Rotaxane as a Reversible Optical Chloride Sensor

• Published in: Chemistry : a European journal 2010-03, Vol.16 (9), p.2916-2921
• Main Authors: Gassensmith, Jeremiah J., Matthys, Sarah, Lee, Jung-Jae, Wojcik, Aleksandra, Kamat, Prashant V., Smith, Bradley D.
• Format: Article
• Language: English
• Subjects: anions; Aqueous solutions; Binding; Chemistry; Chlorides - chemistry; Colorimetry; Cyclobutanes - chemistry; Excitation; Eyes; Fluorescence; Fluorescent Dyes - chemistry; Lasers; Magnetic Resonance Spectroscopy; molecular recognition; Phenols - chemistry; Photolysis; Product life cycle; Rotaxanes; Rotaxanes - chemistry; Stations; supramolecular chemistry; Time Factors
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.200902547

A mechanically interlocked squaraine rotaxane is comprised of a deep‐red fluorescent squaraine dye inside a tetralactam macrocycle. NMR studies show that Cl− binding to the rotaxane induces macrocycle translocation away from the central squaraine station, a process that is completely reversed when the Cl− is removed from the solution. Steady‐state fluorescence and excited‐state lifetime measurements show that this reversible machine‐like motion modulates several technically useful optical properties, including a three‐fold increase in deep‐red fluorescence emission that is observable to the naked eye. The excited states were characterized quantitatively by time‐correlated single photon counting, femtosecond transient absorption spectroscopy, and nanosecond laser flash photolysis. Cl− binding to the rotaxane increases the squaraine excited singlet state lifetime from 1.5 to 3.1 ns, and decreases the excited triplet state lifetime from >200 to 44 μs. Apparently, the surrounding macrocycle quenches the excited singlet state of the encapsulated squaraine dye and stabilizes the excited triplet state. Prototype dipsticks were prepared by adsorbing the lipophilic rotaxane onto the ends of narrow, C18‐coated, reverse‐phase silica gel plates. The fluorescence intensity of a dipstick increased eighteen‐fold upon dipping in an aqueous solution of tetrabutylammonium chloride (300 mM) and was subsequently reversed by washing with pure water. It is possible to develop the dipsticks for colorimetric determination of Cl− levels by the naked eye. After dipping into aqueous tetrabutylammonium chloride, a dipstick’s color slowly fades at a rate that depends on the amount of Cl− in the aqueous solution. The fading process is due primarily to hydrolytic bleaching of the squaraine chromophore within the rotaxane. That is, association of Cl− to immobilized rotaxane induces macrocycle translocation and exposure of the electrophilic C4O2 core of the squaraine station, which is in turn attacked by the ambient moisture to produce a bleached product. Seeing is believing: Cl− binding to the interlocked [2]rotaxane induces macrocycle translocation away from the squaraine station and a reversible increase in red fluorescence emission (such as that depicted). Immobilization of the rotaxane sensor creates a prototype dipstick system for fluorometric and naked‐eye colorimetric detection of Cl− in aqueous solution.