Synthesis and solution-state dynamics of donor–acceptor oligorotaxane foldamers

We describe in detail a strategy for creating foldamers in which interactions between mechanically interlocked components dictate the single-molecule assembly of a folded secondary structure. This unique folding motif is based on a flexible polyether dumbbell bearing 1,5-dioxynaphthalene (DNP) donors, which folds its way through a series of cyclobis(paraquat-p-phenylene) (CBPQT⁴⁺) acceptor rings in a serpentine fashion to enable extended donor–acceptor (D–A) stacking between DNP and the electron-poor 4,4′-bipyridinium (BIPY²⁺) units in CBPQT⁴⁺. These oligorotaxanes can be prepared in a wide range of sizes, with molecular weights up to >15 000 Da, on account of novel one-pot reactions we developed to generate the necessary oligo-DNP precursors. The product distributions from the final kinetically controlled stoppering reactions are highly biased towards oligorotaxanes in which approximately half of the DNP units are encircled by rings, a fact which can be rationalized if the dominant solution-state structures of the pseudorotaxane precursors reflect the solid-state superstructures of analogous compounds, which express 50% recognition site occupancy because of their proclivity to pack into continuous D–A–D–A stacks. The presence of well-defined folded structures in solution have been confirmed by ¹H NMR spectroscopy in CD₃CN. Moreover, we discovered an empirical selection rule forbidding CBPQT⁴⁺ rings to occupy adjacent DNP sites, which elegantly explains both the product distributions and the ¹H NMR spectra. Depending on their adherence to this selection rule, all of the oligorotaxanes belong to one of three families: whereas “Confused” oligorotaxanes adopt multiple translational isomers that satisfy the rule and “Frustrated” species cannot obey it at all, members of the “Happy” family each express only one rule-compliant “Goldilocks” isomer. The NMR spectra of these oligorotaxanes also shed light on their dynamics; rapid 180° rotations of DNP units cause pairs of heterotopic BIPY²⁺ protons in the accompanying CBPQT⁴⁺ rings to exchange sites, giving rise to time-averaged signals. This process, which we term “superrotation”, will apply much more generally to other mechanically interlocked systems.