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

Abstract

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) (CBPQT(4+)) 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 CBPQT(4+). 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 H-1 NMR spectroscopy in CD3CN. Moreover, we discovered an empirical selection rule forbidding CBPQT(4+) rings to occupy adjacent DNP sites, which elegantly explains both the product distributions and the H-1 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 degrees rotations of DNP units cause pairs of heterotopic BIPY2+ protons in the accompanying CBPQT(4+) 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.