Electrostatic barriers in rotaxanes and pseudorotaxanes

Area of Science:

• Supramolecular Chemistry
• Materials Science
• Nanotechnology

Background:

• Controlling molecular motion is crucial for developing advanced molecular electronic devices.
• Mechanically interlocked molecules (MIMs) offer potential for precise molecular control.
• Understanding kinetic barriers is key to designing functional MIMs.

Purpose of the Study:

• To demonstrate the efficacy of bipyridinium (BIPY^2+) dications as electrostatic barriers in aqueous environments.
• To control the shuttling and threading behavior of rotaxanes and pseudorotaxanes.
• To investigate the thermodynamic and kinetic effects of BIPY^2+ on molecular assembly.

Main Methods:

• Synthesis of a degenerate [2]rotaxane using a threading-followed-by-stoppering approach.
• Variable-temperature 1H NMR spectroscopy to determine kinetic barriers.
• Spectrophotometric techniques to study threading and dethreading processes in related supramolecular systems.

Main Results:

• A kinetic barrier exceeding 17 kcal mol^-1 at 343 K was observed for CBPQT(4+) shuttling over a central BIPY^2+ unit.
• BIPY^2+ units significantly influence both thermodynamics and kinetics of pseudorotaxane formation.
• A free energy barrier as high as 21.7 kcal mol^-1 at room temperature was measured for threading involving BIPY^2+.
• BIPY^2+ acts as an electrostatic barrier through Coulombic repulsion and intramolecular self-folding with DNP units.

Conclusions:

• Bipyridinium dications are effective electrostatic barriers for controlling molecular motion in aqueous MIMs and supramolecular systems.
• This control is achieved through a combination of Coulombic repulsion and donor-acceptor interactions.
• The findings enable precise engineering of molecular components for applications in molecular electronics.