Thermodynamic Insights on a Bistable Acid–Base Switchable Molecular Shuttle with Strongly Shifted Co‐conformational Equilibria

Abstract

Bistable [2]rotaxanes in which the affinities of the two stations can be reversed form the basis of molecular shuttles. Gaining quantitative information on such rotaxanes in which the ring distribution between the two stations is largely nonsymmetric has proven to be very challenging. Herein, we report on two independent experimental methodologies, based on luminescence lifetime measurements and acid-base titrations, to determine the relative populations of the two co-conformations of a [2]rotaxane. The assays yield convergent results and are sensitive enough to measure an equilibrium constant (K≈4000) out of reach for NMR spectroscopy. We also estimate the ring distribution constant in the switched (deprotonated) state (K'<10 ), and report the highest positional efficiency for stimuli-induced shuttling to date (>99.92 %). Finally, our results show that the pK of the pH-responsive station depends on the ring affinity of the pH-insensitive station, an observation that paves the way for the design of new artificial allosteric systems.

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

“…The presence of multiple emission decays for Rot 1s and Rot 2s was discussed in a previous work . The longer lifetime in the two‐station rotaxane Rot 2s was ascribed to the presence of the minor (ca.…”

“…The comparison of two dyads containing one primary ammonium station and either an innocent triazolyl moiety ( Rot 1s ) or a triazolium unit that can act as a secondary station for the ring ( Rot 2s ) rules out a significant role of the secondary station in the photophysical behavior, and confirms that the macrocycle is surrounding the amH + site in the vast majority of the Rot 2s species . In both cases, the multiexponential decay of the NI singlet excited state indicates the presence of different conformations in which the excited NI unit either generates the charge separated state crown + ‐NI − , which recombine with similar lifetimes (ca.…”

“…NI, NI-am, NI-amH + + , Rot 1s , Rot 2s and Rot' 2s were synthesized and fully characterized( 1 H, 13 C, COSY and ROESY NMR spectroscopy,h igh-resolutionm ass spectroscopy) in ap reviousw ork. [37] The synthesis andcharacterization ( 1 Ha nd 13 CNMR spectroscopy,h igh-resolution mass spectroscopy) of DB24C8-PTZ, Rot 1s -PTZ, Rot 2s -PTZ and Rot' 2s -PTZ are reported in the Supporting Information. The synthesis of the PTZ-containing rotaxanes follows the procedures used fort he PTZ-free equivalents.…”

“…The presence of multiple emission decays for Rot 1s and Rot 2s was discussed in ap revious work. [37] The longer lifetime in the two-station rotaxane Rot 2s was ascribed to the presence of the minor (ca. 0.03 %) co-conformationali somer in which the DB24C8 ring is encircling the triazolium station instead of the ammonium station.N ote that this longer component is also found for Rot 2s -PTZ.T he ultra-fast analysis confirmedt he multi-exponential nature of the fluorescenced ecay of the two rotaxanes and evidenced as hort component on the order of 2 and 6psf or Rot 1s and Rot 2s ,r espectively ( Figure S49).…”

Distinguishing Two Ammonium and Triazolium Sites of Interaction in a Three‐Station [2]Rotaxane Molecular Shuttle

“…[8] The system has become popular because of itse asy synthetic access and high efficacy in terms of molecular machinery. [9] In particular, we demonstrated that, in an interlocked [2]rotaxane, the readily obtained N-methyltriazolium station was an effi-cient molecular station for the DB24C8 macrocycle in the absence of any better site of interaction (such as ammonium and anilinium). Indeed,t he DB24C8 macrocycle provedt oi nteract selectively with the N-methyltriazolium residue at basic pH, whereas it shuttles towards the aniliniumo rt he ammonium moieties at acidic pH.…”