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

Area of Science:

• Supramolecular Chemistry
• Materials Science
• Organic Chemistry

Background:

• Supramolecular polymerization of π-extended compounds is difficult, especially at high concentrations, due to poor solubility in nonpolar solvents.
• Tetrabenzoporphyrin (BP) derivatives with hydrogen-bonding sites and chiral cholesterol groups (BP_Chol) exhibit limited solubility.

Purpose of the Study:

• To develop a novel method for supramolecular polymerization of poorly soluble BP_Chol derivatives.
• To achieve polymerization at high concentrations in nonpolar solvents.
• To investigate the formation of chiral supramolecular structures.

Main Methods:

• Utilized a thermal precursor approach involving a retro-Diels-Alder reaction of a soluble precursor (CP_Chol) to generate BP_Chol in situ.
• Induction of polymerization via heating and subsequent natural or fast cooling in tetralin.
• Characterization using absorption spectra, circular dichroism (CD) spectroscopy, and high-resolution microscopy.

Main Results:

• Successfully achieved supramolecular polymerization of BP_Chol at high concentrations (mM level) using the precursor method.
• Formation of unique pyramidal multilayer structures with spiral chirality, confirmed by CD spectroscopy and microscopy.
• Demonstrated control over aggregate morphology, yielding monolayer nanosheets upon faster cooling and extended sheets via seed-induced polymerization.

Conclusions:

• The thermal precursor method overcomes solubility limitations, enabling efficient supramolecular polymerization of π-extended compounds at high concentrations.
• The study presents the first chiroptical characterization of tetrabenzoporphyrin aggregates, revealing their supramolecular chirality.
• Controlled cooling and seed-induced polymerization offer pathways to diverse supramolecular architectures, including micro-sized pyramids and nanosheets.