Natural photosynthetic systems precisely position molecular light-harvesting and catalytic modules into complex, hierarchical protein host frameworks which create directional electron transfer pathways, stabilize long-lived charge separated states, and present a structural model for abiotic solar energy conversion. This paper will describe our recent efforts to implement biological design principles to develop and discover new photocatalysts towards the goal of artificial photosynthesis. Cobaloximes are among the most promising molecular compounds for H2 electro- and photocatalysis known, and here we will present the structural evolution of supramolecular cobaloxime photocatalyst assemblies in our group. An entirely new approach to linked cobaloxime-based photocatalysts uses Co(II)-templated formation of Ru(II)polypyridyl-decorated macrocycles. The supramolecular assemblies have been probed by high-resolution structural and physical characterization including synchrotron-based X-ray scattering, X-ray absorption spectroscopy, and ultrafast and nanosecond transient optical spectroscopies. Visible excitation of an equatorially-coordinated cobaloxime photocatalyst provides the first example of instantaneous photoinduced charge separation to yield the catalytically-active Co(I) oxidation state in cobaloxime-based assemblies. Interestingly, EPR analysis reveals that the ground state Co(II) center is high-spin, a unique aspect among previously described cobaloxime photocatalysts, and likely plays a significant role in the kinetics following visible excitation. The homogeneous photocatalyst designs will be juxtaposed with studies of heterogeneous metal-organic frameworks which integrate donor-photosensitizer-acceptor triads and sustain long-lived charge separation following visible excitation. The synthesis and high-resolution physical characterization of new architectures which contain identical photocatalytic modules in complementary environments will contribute to knowledge of how spatial organization impacts the fundamental donor-acceptor photochemistry and potentially lead to the development of new materials for efficient solar energy conversion.

Synthesis and structure-function analysis of photosensitizer-catalyst solar fuels assemblies

247th National Meeting (2014)

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