Squaraine Rotaxane as a Reversible Optical Chloride Sensor

Abstract

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 micros. 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 C(4)O(2) core of the squaraine station, which is in turn attacked by the ambient moisture to produce a bleached product.