Shape-assisted self-assembly

Abstract

Self-assembly and molecular recognition are critical processes both in life and material sciences. They usually depend on strong, directional non-covalent interactions to gain specificity and to make long-range organization possible. Most supramolecular constructs are also at least partially governed by topography, whose role is hard to disentangle. This makes it nearly impossible to discern the potential of shape and motion in the creation of complexity. Here, we demonstrate that long-range order in supramolecular constructs can be assisted by the topography of the individual units even in the absence of highly directional interactions. Molecular units of remarkable simplicity self-assemble in solution to give single-molecule thin two-dimensional supramolecular polymers of defined boundaries. This dramatic example spotlights the critical function that topography can have in molecular assembly and paves the path to rationally designed systems of increasing sophistication.

Fig. 1. Design principle, structural scheme, and assembly process.

Left, Shape as a design principle to restrict rotational and translational freedom. Right, Generic chemical structure of carpyridines with R group modifications to the core. Bottom, Multi-stage assembly process, where (i) units assemble into linear stacks guided by the shape of their core and driven by entropy before (ii) assembling into defined nanosheets.

Fig. 2. Variable temperature ¹H NMR of 2H-Car-C6 in toluene-d₈ and TCE-d₂.

Selected regions and temperatures only, see Supplementary Information for the entire datasets. In TCE broadening is only observed at low temperatures while in the aggregation-inducing solvent (toluene) distinct environments emerge at low and high temperatures. Gray signals in TCE are measured at 233 K.

Fig. 3. Microscopy images of 2H-Car-C6 self-assembled into 2D sheets.

a AFM images show sheets in contact with one another or isolated. b Sharp boundaries in multi-layer sheets allow distinction of individual units. c Individual columnar assemblies and their propagation direction can be distinguished in regions with defects, and by the step thickness of the edges (insert and Supplementary Fig. 50). d STEM image showing micrometer-long individual sheets and clusters. e and f AFM depth measurements of a hole indicate sheet thickness of 2 nm, consistent with that of a single molecule. g SAED pattern obtained from ensembles of 2D sheets. h and i POM images of bulk samples at different temperatures; 150 °C heating (h) and 25 °C after slow (<5 K min⁻¹) cooling from the isotropic melt (i).

Fig. 4. Structural considerations.

Left, DFT calculations of a four-unit stack of the carpyridine core (2H-Car-H) showing the calculated arrangement with retention of the principal axis of curvature. Right, Structural parameters derived from the X-ray single crystal structure of 2H-Car-C6 showing a slipped arrangement also predicted by DFT (see Supplementary Information).