Abstract
We report the detailed investigation of temperature‐ and pH‐triggered conformational switching of resorcin[4]arene cavitands 1–10 (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. 2–7 and 10, and Tables 1 and 2). It was discovered that vase→kite 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 vase⇌kite 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.