Bistable or Oscillating State Depending on Station and Temperature in Three‐Station Glycorotaxane Molecular Machines

Abstract

High-yield, straightforward synthesis of two- and three-station [2]rotaxane molecular machines based on an anilinium, a triazolium, and a mono- or disubstituted pyridinium amide station is reported. In the case of the pH-sensitive two-station molecular machines, large-amplitude movement of the macrocycle occurred. However, the presence of an intermediate third station led, after deprotonation of the anilinium station, and depending on the substitution of the pyridinium amide, either to exclusive localization of the macrocycle around the triazolium station or to oscillatory shuttling of the macrocycle between the triazolium and monosubstituted pyridinium amide station. Variable-temperature (1)H NMR investigation of the oscillating system was performed in CD(2)Cl(2). The exchange between the two stations proved to be fast on the NMR timescale for all considered temperatures (298-193 K). Interestingly, decreasing the temperature displaced the equilibrium between the two translational isomers until a unique location of the macrocycle around the monosubstituted pyridinium amide station was reached. Thermodynamic constants K were evaluated at each temperature: the thermodynamic parameters DeltaH and DeltaS were extracted from a Van't Hoff plot, and provided the Gibbs energy DeltaG. Arrhenius and Eyring plots afforded kinetic parameters, namely, energies of activation E(a), enthalpies of activation DeltaH( not equal), and entropies of activation DeltaS( not equal). The DeltaG values deduced from kinetic parameters match very well with the DeltaG values determined from thermodynamic parameters. In addition, whereas signal coalescence of pyridinium hydrogen atoms located next to the amide bond was observed at 205 K in the oscillating rotaxane and at 203 K in the two-station rotaxane with a unique location of the macrocycle around the pyridinium amide, no separation of (1)H NMR signals of the considered hydrogen atoms was seen in the corresponding nonencapsulated thread. It is suggested that the macrocycle acts as a molecular brake for the rotation of the pyridinium-amide bond when it interacts by hydrogen bonding with both the amide NH and the pyridinium hydrogen atoms at the same time.

Photoinduced Electron Transfer Involving a Naphthalimide Chromophore in Switchable and Flexible [2]Rotaxanes

“…Rotaxanes based on a DB24C8 macrocycle surrounding an axle composed of a dialkyl‐ammonium (amH + ) and a triazolium (tria + ) station have recently been proposed as acid‐base switchable molecular devices . These rotaxanes can be readily synthesized and easily functionalized either at the ring or at the axle.…”

A pH-Sensitive Peptide-Containing Lasso Molecular Switch

“…The hermaphrodite unthreaded molecule 4u, consisting of the BMP25C8 macrocycle linked to the anilinium molecular axle, was prepared in a two-step sequence from the already synthesized alkyne 1 [55]. This alkyne compound possesses a carbamoylated di-tert-butylaniline moiety, so that no template effect can occur at this time.…”

Porphyrin‐Based Switchable Molecular Turnstiles

“…Similar behaviour was observed previously for the closed state of turnstile 1 upon binding of Ag + cations. [26,27] Here, the only possibility that affords the closed state is the formation of hydrogen bonds between the two acidic amide protons H r of the handle and the pyridine moiety of the stator. This hypothesis is supported by a comparison of the 1 H NMR spectra of the free handle 18 and compound 3 in CD 2 Cl 2 .…”

“…The design and synthesis of molecular motors and machines still remain a challenge. [17][18][19][20][21][22][23][24][25][26] Herein, we report on two new molecular turnstiles, 2 and 3, based on a Sn-porphyrin (stator) combining a peripheral monodentate coordinating site on the porphyrin core and a tridentate chelate on the handle (rotor) (Scheme 1).…”