Crystal-Phase Transitions and Photocatalysis in Supramolecular Scaffolds

Abstract

The energy landscape of a supramolecular material can include different molecular packing configurations that differ in stability and function. We report here on a thermally driven crystalline order transition in the landscape of supramolecular nanostructures formed by charged chromophore amphiphiles in salt-containing aqueous solutions. An irreversible transition was observed from a metastable to a stable crystal phase within the nanostructures. In the stable crystalline phase, the molecules end up organized in a short scroll morphology at high ionic strengths and as long helical ribbons at lower salt content. This is interpreted as the result of the competition between electrostatic repulsive forces and attractive molecular interactions. Only the stable phase forms charge-transfer excitons upon exposure to visible light as indicated by absorbance and fluorescence features, second-order harmonic generation microscopy, and femtosecond transient absorbance spectroscopy. Interestingly, the supramolecular reconfiguration to the stable crystalline phase nanostructures enhances photosensitization of a proton reduction catalyst for hydrogen production.

Molecular Control of Internal Crystallization and Photocatalytic Function in Supramolecular Nanostructures

“…The scattering patterns for these three CAs were remarkably similar, indicating the formation of closely related unit cells. In contrast to previously reported CAs, 7,31,35 the addition of salt to screen electrostatic repulsion was not required to achieve crystallization within the nanostructures. We hypothesize this is due to enhanced intermolecular attraction as a result of stronger molecular dipole moments in amine substituted PMIs.…”

“…We also note that each molecule is declined 28.5° with respect to the normal of the ribbon. This particular packing arrangement is distinct from that of other CAs designed in our laboratory previously 7,34–36 suggesting there is an extensive library of structures which could exist through further modification of the PMI monomers and the pathways used to produce supramolecular structures. A single crystal of 3-pentyl CA was grown and found to organize into an antiparallel packing arrangement, as observed in water above, indicating its low energy nature in both solution and solid states (Fig.…”

“…4D, see SI for kinetic model details). 7,53 This barrier represents the energy needed for nucleus formation and subsequent growth into crystalline ribbons. We also used differential scanning calorimetry (DSC) to measure the ΔH value associated with crystallization at 70 °C and found it to be equal to –6.5 kJ/mol (see Fig.…”

“…26 The strong propensity of rylene-based chromophores such as perylene diimide (PDI) and perylene monoimide (PMI) to π-π stack makes them ideal targets to investigate how molecular packing affects electronic properties in supramolecular systems. 7,20,27–29 One example from Adams et al . revealed photoconductivity of PDI thin films could be dramatically altered by controlling the extent of H and J aggregation of chromphores.…”

“…29,33 In our previous work we found that crystalline packing of chromophore amphiphiles based on PMIs can be altered through side-chain modification at the imide 33 or 9-position. 7,34 The latter is of particular interest because it affords the opportunity to investigate different crystal packing arrangements through small modular changes without affecting the imide linkage that affords solubility in water. This should allow for the discovery of an expanding library of PMI nanostructures with unique packing arrangements and properties.…”