Two-Step Folding of Donor−Acceptor Foldamers

In a series of polymers containing alternately placed electron-rich dialkoxynaphthalene (DAN) donors and electron-deficient pyromellitic diimide (PDI) acceptors linked by hexa(oxyethylene) (OE-6) segments, the ability to form a folded D-A stack was intentionally disrupted by random inclusion of varying amounts of a comonomer that is devoid of DAN donor units. NMR spectroscopic studies of folding in these copolymers, induced by NH₄SCN that coordinates with the OE-6 segments and facilitates the charge-transfer (C-T) induced D-A stacking, clearly reveals the presence of PDI units that are isolated and those that are located at the ends of (D-A)_n stacks. Similar conclusions regarding the presence of stacked and unstacked regions along the polymer chain were also inferred from UV−vis spectroscopic studies that probe the evolution of charge-transfer band. One fascinating aspect of these copolymers was their ability to undergo a two-step folding: first, short (D-A)_n stacks are formed by the interaction of the NH₄⁺ ion with some specific regions of the polymer chain, and subsequently these stacks are further stacked via a two-point interaction with a suitably designed external folding agent that carries a DAN unit and an ammonium group. In the second step, the interaction first occurs by the coordination of the ammonium group of the folding agent with the OE-6 segment, which in turn facilitates the C-T interaction of the DAN unit with the adjacent uncomplexed PDI units along the polymer chain, leading to an increase in the stacking. Variations of several spectral features, during both UV−vis and NMR spectroscopic titrations, clearly reveal this novel two-step folding process.