Molecular tweezer

Molecular tweezers, and molecular clips, are noncyclic host molecules with open cavities capable of binding guest molecules. The term "molecular tweezers" was first used by Howard J. Whitlock, but the class of hosts was developed and popularized by Steven C. Zimmerman in the mid-1980s to early 1990s and later by Frank-Gerrit Klärner and colleagues. The open cavity of the molecular tweezers may bind guests using non-covalent bonding which includes hydrogen bonding, metal coordination, hydrophobic forces, van der Waals forces, π-π interactions, and/or electrostatic effects. These complexes are a subset of macrocyclic molecular receptors and their structure is that the two "arms" that bind the guest molecule between them are only connected at one end leading to a certain flexibility of these receptor molecules (induced fit model).

One example of molecular tweezers has been reported by Lehn and coworkers. This molecule is capable of binding aromatic guests. The molecular tweezers are composed of two anthracene arms held at a distance that allows aromatic guests to gain π-π interactions from both (Figure 1).

Another class of molecular tweezers is composed of two substituted porphyrin macrocycles tethered by an amide linker with a variable length. This example of a molecular tweezer shows the potential mobility of this class of molecules, as the orientation of the porphyrin planes which comprise the tweezer can be altered by the guest which is bound

Yet another structure for molecular tweezers which specifically bind fullerenes is called a buckycatcher and has been reported. This molecular tweezer is composed of two concaved corannulene pincers that complement the surface of the fullerene guest (Figure 2). An association constant (Ka) of 8600 M⁻¹ between the host buckycatcher and a C60 fullerene was calculated using ¹H NMR spectroscopy.

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