Squaraine rotaxane as a reversible optical chloride sensor.

Area of Science:

• Supramolecular Chemistry
• Materials Science
• Chemical Sensing

Background:

• Mechanically interlocked molecules offer unique responsive properties.
• Squaraine dyes are known for their deep-red fluorescence.
• Developing sensitive and visual detection methods for anions is crucial.

Purpose of the Study:

• To synthesize and characterize a squaraine rotaxane with anion-responsive optical properties.
• To investigate the mechanism of anion-induced molecular motion and its effect on fluorescence.
• To develop a visual sensing platform for chloride ions.

Main Methods:

• Nuclear Magnetic Resonance (NMR) spectroscopy for structural and binding studies.
• Steady-state and time-resolved fluorescence spectroscopy (TCSPC, transient absorption, nanosecond laser flash photolysis) for optical property analysis.
• Fabrication and testing of dipstick sensors using immobilized rotaxane.

Main Results:

• Chloride binding induces reversible macrocycle translocation in the rotaxane.
• This motion leads to a significant increase in deep-red fluorescence emission.
• The rotaxane-based dipsticks show an 18-fold fluorescence enhancement for chloride detection, visible to the naked eye.
• Chloride concentration correlates with the rate of hydrolytic bleaching of the sensor.

Conclusions:

• The squaraine rotaxane functions as a molecular machine with tunable optical output.
• The developed dipsticks provide a simple, visual method for qualitative and potentially quantitative chloride sensing.
• The sensing mechanism involves anion-triggered molecular motion and subsequent chromophore bleaching.