A Unidirectional Open-Close Mechanism of Metal-Ion-Driven Molecular Hinges with Adjustable Amplitude

Abstract

A basic requirement for each molecular system that is supposed to perform work is a synchronized and unidirectional movement. Unidirectionality can be achieved by a change of configuration or conformation that is controllable by external stimulation. Molecular hinges based on a bipyridine unit work unidirectionally and are able to reach an amplitude of motion that amounts to about 180 degrees. To analyze if it is possible to adjust the height of the unidirectional amplitude of motion, three planar chiral molecular hinge systems with a 2,2'-bipyridine unit as functional element were designed and stimulated with various divalent metal ions in different solvents. The configurations of the hinges were determined by DFT calculations using B3LYP and the 6-31G* basis set and experimentally verified by 2D NMR NOESY spectra. Circular dichroism (CD) and UV spectroscopy were used to study the properties of the hinges by the addition of metal ions (primarily Zn(2+) and Hg(2+)) in dichloromethane and methanol. The choice of metal ions and solvents determines whether or not and how far the hinges are closed. Furthermore, a drastic change in the height of the amplitude of motion can be reached by modifying the position of the bipyridine unit in the hinge. Amplitude values from 45 up to 190 degrees were obtained from quantum mechanical calculations. This control of the amplitude of motion can in the future be used for more complex switching processes of molecular machines.