Molecular Control of Internal Crystallization and Photocatalytic Function in Supramolecular Nanostructures

Supramolecular light-absorbing nanostructures are useful building blocks for the design of next-generation artificial photosynthetic systems. Development of such systems requires a detailed understanding of how molecular packing influences the material's optoelectronic properties. We describe a series of crystalline supramolecular nanostructures in which the substituents on their monomeric units strongly affect morphology, ordering kinetics, and exciton behavior. By designing constitutionally isomeric perylene monoimide (PMI) amphiphiles, we studied the effect of side chain sterics on nanostructure crystallization. Molecules with short amine-linked alkyl tails rapidly crystallize upon dissolution in water, whereas bulkier tails require the addition of salt to screen electrostatic repulsion and annealing to drive crystallization. A PMI monomer bearing a 3-pentylamine tail was found to possess a unique structure that results in strongly red-shifted absorbance, indicative of charge-transfer exciton formation. This particular supramolecular structure was found to have an enhanced ability to photosensitize a thiomolybdate catalyst ((NH₄)₂Mo₃S₁₃) to generate hydrogen gas. Supramolecular chemistry, which focuses in part on our understanding of many molecule systems, can be a useful tool in the development of novel strategies for renewable and clean energy technologies. In plants, assemblies of light-absorbing molecules arrange themselves in configurations that optimize their ability to efficiently collect light from the sun and utilize that energy for the production of useful chemicals. The function of these systems is critically dependent on the interactions of molecules within nanoscale structures present in green leaves. The long-term objective of this work is to learn how to design soft materials containing light-harvesting molecules and catalysts to create biomimetic systems that create fuels and other useful products by using sunlight as the energy source. Here, we have synthesized molecules and developed methods to manipulate their assembly into nanostructures that are highly effective at collecting light and participate in its conversion to chemical energy. The development of clean fuels is an exciting area of research with the goal of reducing the world's reliance on fossil fuels. In our system, nanoscale ribbons are used to collect light and transfer the energy to a catalyst. This catalyst can be used to convert protons into hydrogen gas. Hydrogen gas represents a high-energy and clean-burning fuel that can be used in place of fossil fuels in many applications.