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

Abstract

Dedicated to Professor K. N. Houk at the occasion of his 60th birthdayWe 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 3 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.1. Introduction. ± Resorcin[4]arene cavitands with four quinoxaline bridges were originally introduced and studied by Cram and co-workers [1]. A particulary interesting feature of these systems is their ability to switch between two states, a vase conformation with a deep cavity well suitable for guest inclusion [2] (for a review on molecular encapsulation, see [3]) and a kite conformation featuring a large flat surface. This vase > kite equilibrium is controllable by both temperature-[1] or pHvariation [4], and resembles the movement of a molecular gripper. According to Cram and co-workers, the temperature dependence of the vase > kite equilibrium is caused by solvation effects: at low temperatures, solvation of the larger, solvent-exposed surface favors the kite conformer, whereas, at higher temperature, the entropic term TDS solv for solvation of the larger kite surface becomes unfavorable, and the vase conformation predominates.We are interested in the potential utilization of modified resorcin[4]arene cavitands as molecular grippers, capable of guest hosting and releasing on demand, in device construction at the single-molecule level. This objective requires a deeper mechanistic understanding of the vase > kite conformational equilibration. In this context, the discovery of pH-triggered vase 3 kite switching and the use of UV/VIS spectroscopy to monitor the interconversion had been reported earlier [4] (for a first scanning tunneling microscopy (STM) study, imaging the vase conformation at molecular

Introverted Phosphorus‐Au Cavitands for Catalytic Use

“…4-Walled quinoxaline cavitands are known to fluctuate between vase (closed) and kite (open) conformations and reports on the effects of solvent and acid are known. [23] Typically, methine protons around δ = 5.5 ppm are indicative of vase conformers, whereas δ = 3.7 ppm indicates the kite form. For 2 in Table 1 strate the vase is preferred.…”

Clear evidence of fluorescence resonance energy transfer in gas-phase ions

“…It belongs to the family of BODIPY dyes and was of interest to us because its derivative can be used for labelingÅ aÅ dynamicÅ molecularÅ cavitandÅ ("gripper")Å [26,27].ÅStudiesÅofÅtheÅsinglyÅandÅdoublyÅdyeÅlabeled cavitands in the gas phase by fluorescence spectroscopy can provide valuable insight into the conformational dynamics of these flexible structures in a nonsolvated state and can be compared with the results obtained for theÅ solutionÅ byÅ dynamicÅ NMRÅ spectroscopyÅ [28].Å Furthermore,Å ParksÅ andÅ coworkersÅ [19,Å 20]Å previouslyÅ used BODIPY labeled biomolecules for FRET measurements in the gas phase, which provided an additional choice. Before starting to measure fluorescence in the gas phase, the dye 10 was characterized in solution.…”

Quinone‐Based, Redox‐Active Resorcin[4]arene Cavitands

“…We therefore prepared a series of quinoid cavitands with different wall sizes (ox-1 a-c, Figure 2) and investigated their conformational preferences in one member of each solvent class (for the synthesis of all reported compounds, see the Supporting Information, Section 2). [7] for the kite1-kite2 interconversion. The preference of cavitand ox-1 b for the kite conformation was Figure 1.…”