Supramolecular Packing Controls H₂ Photocatalysis in Chromophore Amphiphile Hydrogels

Abstract

Light harvesting supramolecular assemblies are potentially useful structures as components of solar-to-fuel conversion materials. The development of these functional constructs requires an understanding of optimal packing modes for chromophores. We investigated here assembly in water and the photocatalytic function of perylene monoimide chromophore amphiphiles with different alkyl linker lengths separating their hydrophobic core and the hydrophilic carboxylate headgroup. We found that these chromophore amphiphiles (CAs) self-assemble into charged nanostructures of increasing aspect ratio as the linker length is increased. The addition of salt to screen the charged nanostructures induced the formation of hydrogels and led to internal crystallization within some of the nanostructures. For linker lengths up to seven methylenes, the CAs were found to pack into 2D crystalline unit cells within ribbon-shaped nanostructures, whereas the nine methylene CAs assembled into long nanofibers without crystalline molecular packing. At the same time, the different molecular packing arrangements after charge screening led to different absorbance spectra, despite the identical electronic properties of all PMI amphiphiles. While the crystalline CAs formed electronically coupled H-aggregates, only CAs with intermediate linker lengths showed evidence of high intermolecular orbital overlap. Photocatalytic hydrogen production using a nickel-based catalyst was observed in all hydrogels, with the highest turnovers observed for CA gels having intermediate linker lengths. We conclude that the improved photocatalytic performance of the hydrogels formed by supramolecular assemblies of the intermediate linker CA molecules likely arises from improved exciton splitting efficiencies due to their higher orbital overlap.

Extended-Charge-Transfer Excitons in Crystalline Supramolecular Photocatalytic Scaffolds

“…It was previously hypothesized that the difference in H 2 production between the two systems originates from the relative ability of the exciton to split and form the free charges necessary to participate in the electron transfer reactions required for H 2 production. 10 In section IV, it was shown that optically allowed excitons will separate quickly when there is strong mixing between the k = 0 FE and CT states. This requires both large, in-phase CT integrals as well as near-resonant conditions for the k = 0 FE and CT states, so that |t e + t h | ≫ |J k=0 − U|.…”

“…This ratio is in good agreement with the experimentally measured ratio of H 2 production between the two systems (2.4 ± 0.5). 10 …”

“…Interestingly, by modifying the alkyl spacer length between the chromophores and the solubilizing charge group of the molecule, we were able to alter the crystalline packing arrangement of the molecules within the nanostructures (Figure 1b,d,e). 10 This change in packing was accompanied by changes in intermolecular coupling among the chromophores, as evidenced by differences in the ground-state absorption spectra, including the energy of the absorption maximum and the energetic distribution of oscillator strength ( Figure 1c). Importantly, we observed a discontinuous jump in the rate of H 2 evolution that directly correlated with a change in molecular crystal packing, in particular an increase in b by about 1 Å in going from L1 to L5 (see Figure 1e).…”

“…Importantly, we observed a discontinuous jump in the rate of H 2 evolution that directly correlated with a change in molecular crystal packing, in particular an increase in b by about 1 Å in going from L1 to L5 (see Figure 1e). 10 Given that molecular packing has been shown to greatly impact device performance in organic photovoltaics, 11 we analyze here how intermolecular coupling in crystalline light-absorbing nanostructures affects the nature of their excited states and catalyst photosensitization abilities.…”

Multimodal Light-Harvesting Soft Hybrid Materials: Assisted Energy Transfer upon Thermally Reversible Gelation

“…[2][3][4][5][6][7] Several approaches have addressed the inclusion of photonic functionalities into supramolecular gels. 8 For example, gels with chromophore units have been used as photocatalysts 9,10 and excitation energy transfer has been studied in supramolecular gels formed by photoactive fibers which contained entrapped dyes. [11][12][13][14][15] Recently, we reported orthogonal fibrillization of two fluorescent supramolecular gelators.…”