NMR Investigations into the Vase-Kite Conformational Switching of Resorcin[4]arene Cavitands



We report the detailed investigation of temperature- and pH-triggered conformational switching of resorcin[4]arene cavitands 110 (Figs. 1, 8, and 9). Depending on the experimental conditions, these macrocycles adopt a vase conformation, featuring a deep cavity for potential guest inclusion, or two kite conformations (kite 1 and kite 2) with flat, extended surfaces (Schemes 1 and 2). The thermodynamic and kinetic parameters for the interconversion between these structures were determined by variable-temperature NMR (VT-NMR) spectroscopy (Figs. 27 and 10, and Tables 1 and 2). It was discovered that vasekite switching of cavitands is strongly solvent-dependent: it is controlled not only by solvent polarity but also by solvent size. Conformational interconversions similar to those of the parent structure 1 with four quinoxaline flaps are also observed when the octol base skeleton is differentially or incompletely bridged. Only octanitro derivative 2 was found to exist exclusively in the kite conformation under all experimental conditions. The detailed insight into the vasekite conformational equilibrium gained in this investigation provides the basis for the design and construction of new, dynamic resorcin[4]arene cavitands that are switchable between bistable states featuring strongly different structures and functions.