Sequential, Multistep, and Cooperative Helicity Evolution in Supramolecular Polymers of Chlorophyll Rosettes

Helicity is a fundamental structural principle that imparts order and function to biological systems, with many biological assemblies exhibiting helical architectures that often emerge and mature dynamically over time, enabling adaptive structural transformations and functional regulation. Nonetheless, examples of synthetic supramolecular polymers in which helicity arises dynamically from kinetically trapped, nonhelical structures remain rare. Here we report a chlorophyll-based supramolecular polymer that undergoes a spontaneous, sequential, multistep conversion from nonhelical fibers to helical architectures. Chlorophyll derivatives bearing barbituric acid hydrogen-bonding sites first assemble into hexameric rosettes, which then kinetically stack to yield metastable, nonhelical fibers. Over time these fibers evolve through three discrete stages into helically twisted structures, each characterized by a progressive tightening of the helical pitch. Because the rosette comprises six highly aggregative chlorophyll chromophores─each bearing multiple stereocenters and peripheral side chains─its structural complexity prevents rapid adoption of a single, globally stable stacking conformation. Instead, the assembly traverses a rugged energy landscape, proceeding in a sequential, stepwise, discontinuous manner through a series of local minima toward more stable arrangements. This multistep transformation was elucidated by UV/vis absorption, circular dichroism (CD), atomic force microscopy (AFM), and small-angle scattering (SAS) techniques. Kinetic analysis with a cooperative model further revealed that cooperative structural transitions, in which adjacent helical units promote additional helicity, play a pivotal role in this process. Strikingly, in the tightest helical state the supramolecular chirality observed by AFM is opposite to that derived from chiral chlorophyll stacking inferred from CD spectroscopy. This discrepancy is consistent with the creeper-helix model, in which an offset in the chromophore stack inverts the CD response, as suggested by spectral calculations based on an exciton model. Taken together, these results indicate that the stepwise tightening of the helical pitch is driven by an increasing translational offset between stacked rosette units. Thus, even one-dimensional supramolecular polymers can undergo cooperative, crystal-phase-transition-like structural reorganizations.