Tuning Halide Induced Metastability in a [2]Rotaxane by Varying Axle, Crown Ether, and Employing Bistability.

4-isopropylphenyl slippage stopper has been used for developing a series of [23]crown ether incorporated metastable [2]rotaxanes via click reaction. Two [23]crown ethers that is, 23C7 and X23C7 are employed. The crown ether stationed at the ammonium site exhibited extrusion selectively in the presence of fluoride anion (TBAF) over TBAX (X = chloride, bromide, and iodide) due to TBAX exhibiting H-bonding with the neutral triazole moiety of the axle. Replacing 23C7 by X23C7 has enhanced the overall stability of the [2]rotaxane by reducing the impact of metastability. Imparting bistability to the metastable [2]rotaxanes by transforming triazole to methyl triazolium (MTA+) moiety, offers the crown ether to choose between shuttling and extrusion, touting this as a competition between the two phenomena switching and metastability. Surprisingly, TBAF addition reveals preferential extrusion in case of 23C7 incorporated bistable [2]rotaxane while preferential shuttling was observed with X23C7 incorporated bistable [2]rotaxane. Micromolar to millimolar concentration variation has a telling effect on the mode of responsiveness in a bistable [2]rotaxane with a slippage stopper. On the contrary, in polar medium (DMSO-d6), extrusion is the only mode of responsiveness observed by NMR spectroscopy.