Supramolecular Polymorphism of an Amphiphilic Boron-Dipyrromethene Dye Mediated by Carbonyl-Aromatic Interactions

Supramolecular polymorphism is attracting great interest due to its potential for generating versatile functional materials based on the same molecule. Despite the great efforts devoted to manipulating polymorphic aggregation systems, accessible design strategies for such systems are still limited. Herein, we describe the supramolecular polymorphism of an amphiphilic boron-dipyrromethene dye (BODIPY 1) through sophisticated manipulation of noncovalent interactions. In mixed solvents of DMF/H₂O, it self-assembles into two different aggregates with distinct structures and optical properties, i.e., kinetically controlled spherical micelles with bathochromically shifted spectra (J-agg) and thermodynamically controlled nanofibers with hypsochromically shifted spectra (H-agg). Notably, these aggregates of BODIPY 1 retain strong fluorescence. Systematic experimental and theoretical studies demonstrate that J-agg is driven simply by hydrophobic interactions, while the formation of H-agg is mediated by a carbonyl-aromatic interaction. The important role of the carbonyl-aromatic interaction in modulating supramolecular polymorphism is demonstrated by various structural characterizations and theoretical calculations. The transformation from J-agg to H-agg can be accelerated by modifying the terminal benzaldehyde group or eliminated by removing the aldehyde C═O. This work can inspire a new strategy for constructing supramolecular polymorphism systems with the desired optical properties.