Inside Front Cover: Resorcin[4]arene Cavitand-Based Molecular Switches (Adv. Funct. Mater. 2/2006)



Miniaturized molecular grippers for supramolecular construction are described by François Diederich and co-workers on p. 147. These grippers are based on resorcin[4]arene cavitands with four quinoxaline bridges; these macrocycles can be switched between a closed (left) and open (right) configuration via temperature changes, pH changes, or stoichiometric metal-ion complexation. The background is a scanning tunneling microscopy image of a self-assembled monolayer of the closed cavitands. Individual gripper “fingers” are seen as lighter spots, four per gripper, which in turn pack into a well-ordered array.

Resorcin[4]arene cavitands with four quinoxaline bridges are a family of macrocycles that adopt, at elevated temperature, a contracted, vase-type conformation, capable of guest inclusion, whereas at low temperature they switch to an expanded, kite-type conformation with a large flat surface. The present investigations lay the foundation for the use of such dynamic cavitands as miniaturized mechanical grippers for supramolecular construction at the single-molecule level. New vase–kite switching modes, stimulated by pH changes or stoichiometric metal-ion complexation, have been discovered and monitored by 1H NMR and optical absorption spectroscopy. The solid-state geometries of the two states have been revealed by X-ray crystallography, and the kinetics and thermodynamics of the switching processes in solution as well as their solvent dependency has been investigated in great detail. Monolayers of the cavitand in the vase form have been studied by scanning tunneling microscopy at molecular resolution; conformational switching is also observed in Langmuir monolayers at the air/water interface. Synthetic protocols have been developed for preparation of partially and asymmetrically bridged resorcin[4]arene cavitands, which are also shown to undergo conformational switching. These synthetic advances pave the way to new, dynamic molecular receptors for steroids, tetrathiofulvalene-bridged grippers with the potential to undergo electrochemically induced conformational switching, and systems with greatly extended, rigid cavity walls functionalized at the termini by dipyrrometheneboron difluoride dyes. The latter cavitands are shown by fluorescence resonance energy transfer to undergo geometrically precisely defined motions between a contracted (≈ 7 Å linear extension) and a strongly expanded (≈ 7 nm linear extension) state.