Self-Assembled, Crystalline Organic Nanostructures for Photocatalysis

The goal of this thesis was to integrate light-absorbing supramolecular materials into a photocatalytic system for solar-to-fuel conversion. Toward this end, a series of perylene-based chromophore amphiphiles was synthesized and their self-assembly properties explored. Characterization of these materials by electron microscopy and x-ray scattering techniques revealed molecular assembly into 1D ribbon nanostructures. Surprisingly, these ribbons were observed to spontaneously crystallize in solution, as observed by wide-angle and grazing incidence X-ray scattering. These crystalline nanostructures could be gelled with oppositely charged electrolytes, forming a 3D light-absorbing scaffold. By designing and synthesizing oppositely charged proton reduction catalysts to electrostatically bind to the light-absorbing scaffold, hydrogen gas was detected by gas chromatography after white light illumination of the scaffold / catalyst system. As a direct result of their crystalline nature, the exciton properties of these materials and the photocatalytic properties of the system could be tuned by slight modification in their molecular packing arrangement. These changes were achieved by creating a library of chromophores with small functional groups directly attached to the PMI core. Some amphiphiles in this library were observed to undergo a crystalline phase transition between two unique packing arrangements as evidenced by variable temperature absorbance and x-ray scattering experiments. This transition involved a substantial change in the exciton properties of the material. Surprisingly, some crystalline phases carried the distinct spectral signature of charge-transfer (CT) excitons, an excitation that is shared among multiple chromophores. Characterization of this CT state was accomplished by ground state and transient absorption spectroscopy, transient electron paramagnetic resonance spectroscopy, and second-order harmonic generation microscopy. The crystalline nanostructures of the library that yielded evidence for CT-excitons were the most photocatalytically active. This observation is consistent with established theories developed elsewhere that connect CT-exciton formation with an enhancement in exciton mobility