Solvent Chain Length Directed Supramolecular Polymorphism

Although solvents are central to self-assembly, their chain length has not been recognized as a factor controlling supramolecular polymerization. Herein, we introduce solvent chain length as a key parameter dictating the outcome of supramolecular polymerization. We investigated this phenomenon using a boron dipyrromethene (BODIPY) derivative bearing a phenyl group at the meso-position, amide groups, and solubilizing dodecyloxy side chains (1). Remarkably, while the shortest liquid alkanes, n-pentane and n-hexane, promote face-to-face (H-type) stacking interactions leading to one-dimensional fiber-like morphologies (AggB), the longest liquid linear alkane, n-hexadecane, induces a molecular rearrangement into slipped stacks that cannot efficiently grow into extended one-dimensional assemblies, resulting in discrete morphologies (AggA). In alkanes of intermediate length (C₇-C₁₅), both assembled states coexist to varying degrees, leading to concomitant supramolecular polymorphs, with the slipped packing becoming progressively favored as the chain length increases. Molecular dynamics (MD) simulations reveal that AggA is favored in longer-chain solvents owing to enhanced solvent interactions and conformational freedom of the side chains, while the proximity of aliphatic groups to the aromatic cores attenuates stack elongation. Our findings broaden the role of solvents as design elements in self-assembly and offer new insights into the development of functional supramolecular polymorphs.