Optical Distinction between “Slow” and “Fast” Translational Motion in Degenerate Molecular Shuttles

• Published in: Angewandte Chemie International Edition 2017-05, Vol.56 (22), p.6136-6141
• Main Authors: Vukotic, V. Nicholas, Zhu, Kelong, Baggi, Giorgio, Loeb, Stephen J.
• Format: Article
• Language: English
• Subjects: co-conformations; Color; Crown ethers; Imidazole; molecular shuttles; Optical properties; optical switches; Rotaxanes; Shafts (machine elements); supramolecular chemistry; Translation; Translational motion
• ISSN: 1433-7851; 1521-3773
• DOI: 10.1002/anie.201612549

A series of six [2]rotaxane molecular shuttles was designed which contain an axle with a benzo‐bis(imidazole) core (in either a neutral or dicationic form) and a single 24‐membered, crown ether wheel (24C6, B24C6, or DMB24C6), and the shuttling rates of the ring along the axle were determined. The charged versions showed much slower shuttling rates as a result of the increase in noncovalent interactions between the axle and wheel. The [2]rotaxane with a B24C6 wheel shows a difference in fluorescence between the charged and neutral species, while the [2]rotaxane with a DMB24C6 wheel exhibits a difference in color between the charged and neutral compounds. These changes in optical properties can be attributed to the structural differences in the co‐conformations of the [2]rotaxane as they adapt to the changes in acid/base chemistry. This allowed the relative rate of the translational motion of a molecular shuttle to be determined by observation of a simple optical probe. Set the wheels in motion: The dicationic and neutral forms of a degenerate molecular shuttle containing an axle with a benzo‐bis(imidazole) core show very different shuttling rates as a result of differences in the noncovalent interactions between the axle and wheel in the two forms. Acid/base chemistry can be used to control the shuttling, which is signaled by an optical output: a change in color or fluorescence.