X-ray guided H NMR analysis of pinched cone calix[4]arenes

The analysis of structural parameters of azobenzene- and stilbene-bridged calix[4]arene obtained from AM1 calculation are in good agreement with those obtained from X-ray crystallography. The bridge longer than 9.0 Å such as p,p-trans-azobenzene and p,p-trans-stilbene cannot be constructed over the narrow rim of calix[4]arene through two ethylene oxide linkers. The m,m-stilbene bridge is the most promising photo switch because its shorter cis stereoisomer (5.85 Å) allows calix[4]arene to assume the perfect cone conformation, whilst its longer trans stereoisomer (8.00 Å) forces calix[4]arene to adapt a pinched cone conformation. The pinched cone conformation has longer distances between the neighbouring phenoxyl groups causing the weaker intramolecular hydrogen bonding and the upfield shifts of the phenolic proton signals to below 7.00 ppm. This upfield shift is useful for quick identification of pinched cone conformation of new calix[4]arene compounds.