Detection of a charge-separated catalyst precursor state in a linked photosensitizer-catalyst assembly

Abstract

We have designed two new supramolecular assemblies based on Co(ii)-templated coordination of Ru(bpy)3(2+) (bpy = 2,2'-bipyridyl) analogues as photosensitizers and electron donors to a cobaloxime macrocycle, which are of interest as proton reduction catalysts. The self-assembled photocatalyst precursors were structurally characterized by Co K-edge X-ray absorption spectroscopy and solution-phase X-ray scattering. Visible light excitation of one of the assemblies has yielded instantaneous electron transfer and charge separation to form a transient Co(i) state which persists for 26 ps. The development of a linked photosensitizer-cobaloxime architecture supporting efficient Co(i) charge transfer is significant since it is mechanistically critical as the first photo-induced electron transfer step for hydrogen production, and has not been detected in previous photosensitizer-cobaloxime linked dyad assemblies. X-band EPR spectroscopy has revealed that the Co(ii) centres of both assemblies are high spin, in contrast to most previously described cobaloximes, and likely plays an important role in facilitating photoinduced charge separation. Based on the results obtained from ultrafast and nanosecond transient absorption optical spectroscopies, we propose that charge recombination occurs through multiple ligand states present within the photosensitizer modules. The studies presented here will enhance our understanding of supramolecular photocatalyst assembly and direct new designs for artificial photosynthesis.

Ru–protein–Co biohybrids designed for solar hydrogen production: understanding electron transfer pathways related to photocatalytic function

“…Axially-linked or equatorially-linked Ru–Co supramolecular complexes form Co( i ) species that exhibit short lifetimes of 20–70 ps due to rapid charge-recombination in small synthetic architectures that cannot provide opportunities to capture or accumulate electrons needed for catalysis. 5,6,50 In multimolecular systems, the Co( i ) state of Co–polypyridyl catalysts with Ru PSs have reported lifetimes of 30–60 μs, 30,49 while the Co( i ) state of another Co–polypyridyl complex with a Re PS has been proposed to last from 2–200 ms. 48 Time-resolved X-ray absorption techniques using the CoBF 2 catalyst, [Ru(bpy) 3 ] 2+ as a PS, and methyl viologen as an electron relay show transient formation of a Co( i ) species over the time regime of 0.5–100 μs, 51 which is consistent with the time regime for the Co( i ) species in our protein hybrids.…”

“…3 Supramolecular complexes for hydrogen production have proven to be quite beneficial due to their amenability to spectroscopic resolution of both ground and excited state species, 3 but are often hampered by limited stability in solution 4,5 or short excited state lifetimes and rapid charge recombination. 6 Nanoparticle systems offer an opportunity to improve catalyst photostability and collect electrons or holes on particles until needed for catalysis. 3 For example, extensive studies of the RuP–TiO 2 –cobaloxime system by the Reisner group have shown the ability of nanoparticle complexes to extend charge separation lifetimes to outcompete charge recombination for high H 2 production activity.…”

Co@NH2-MIL-125(Ti): cobaloxime-derived metal–organic framework-based composite for light-driven H2 production

“…We should emphasize that reduction of Co III centers is generally accepted to be one of the rst reaction steps in cobaloxime-catalyzed hydrogen evolution, while the presence of high-spin Co II species is likely to play an important role in photocatalytic activity of the whole system. 58 Catalysis taking place at the cobalt metal centres is beyond the scope of this research and thus shown as a dashed arrow in Scheme 3.…”

Time-resolved X-ray absorption spectroscopy for the study of molecular systems relevant for artificial photosynthesis

“…This implies that the excited state lifetime of the photosensitizer for such a system can be significantly shorter than in a multicomponent system. The main challenge with a supramolecular assembly is the back electron transfer that can be also fast [23,24] and significantly decrease the photocatalytic performance of the molecular device. The supramolecular approach, however, does not exclude completely the effects of diffusion limited processes.…”