Retro-Diels-Alder-Triggered Supramolecular Polymerization of Tetrabenzoporphyrin into Pyramidal Aggregates

Supramolecular polymerization of π-extended compounds with hydrogen-bonding sites remains challenging, especially at high concentrations, due to their limited solubility in nonpolar solvents. Herein, we report a novel method incorporating a thermal precursor approach for the supramolecular polymerization of a poorly soluble tetrabenzoporphyrin (BP) derivative with multiple hydrogen-bonding sites and chiral cholesterol groups (BP_Chol). Polymerization in the nonpolar solvent tetralin is induced by the retro-Diels-Alder reaction of the highly soluble precursor (CP_Chol) to BP_Chol in the hot solution and subsequent natural cooling, as confirmed by monitoring absorption spectra. The emergence of circular dichroism signals corroborates the formation of aggregates with supramolecular chirality, as the first chiroptical characterization of BP aggregates. Detailed structural analysis using high-resolution microscopies reveals a unique pyramidal multilayer structure with a spiral nature. Additionally, faster cooling produces monolayer nanosheets, which differ from the multilayer structures formed upon natural cooling. This is due to a different nucleation process in the cooperative polymerization. By using the nanosheets as seeds, seed-induced polymerization is achieved, resulting in extended sheets. Notably, the indirect precursor method using CP_Chol enables outstanding polymerization at high concentrations (mM level) to afford micro-sized pyramids, which cannot be realized by a direct method using BP_Chol.