Proton-controlled Action of an Imidazole as Electron Relay in a Photoredox Triad

Abstract

Electron relays play a crucial role for efficient light-induced activation by a photo-redox moiety of catalysts for multi-electronic transformations. Their insertion between the two units reduces detrimental energy transfer quenching while establishing at the same time unidirectional electron flow. This rectifying function allows charge accumulation necessary for catalysis. Mapping these events in photophysical studies is an important step towards the development of efficient molecular photocatalysts. Three modular complexes comprised of a Ru-chromophore, an imidazole electron relay function, and a terpyridine unit as coordination site for a metal ion were synthesized and the light-induced electron transfer events studied by laser flash photolysis. In all cases, formation of an imidazole radical by internal electron transfer to the oxidized chromophore was observed. The effect of added base evidenced that the reaction sequence depends strongly on the possibility for deprotonation of the imidazole function in a proton-coupled electron transfer process. In the complex with Mn II present as a proxy for a catalytic site, a strongly accelerated decay of the imidazole radical together with a decreased rate of back electron transfer from the external electron acceptor to the oxidized complex was observed. This transient formation of an imidazolyl radical is clear evidence for the function of the imidazole group as an electron relay. The implication of the imidazole proton and the external base for the kinetics and energetics of the electron trafficking is discussed.

Pushing the limits of flash photolysis to unravel the secrets of biological electron and proton transfer

“…We are happy to announce that today, this virtual volume unites 18 contributions by scientists from 17 countries (Canada, Czech republic, Finland, France, Germany, Hungary, India, Italy, Japan, Lebanon, Poland, Russia, Spain, Switzerland, Taiwan, UK, US) touching upon the wide variety of the subjects dealt with by Klaus at various times [25,[127][128][129][130][131][132][133][134][135][136][137][138][139][140][141][142][143].…”

“…Three more papers deal with chemical model systems for light-induced electron transfer [138][139][140]. Gotico [139].…”

Proton Domino Reactions at an Imidazole Relay Control the Oxidation of a TyrZ‐His190 Artificial Mimic of Photosystem II

“…However, the spectra for all Ru-Im-xMe-PhOH complexes are clearly distinct from the spectra induced under the same conditions in the reference compound Ru-Im-Ph, where no fast phase is observed (Figure 4C, S7) and where ligand oxidation leads to formation of an imidazolyl radical with spectral features very similar to those observed in a Ru-Imi-terpyridine complex. [21] These observations are taken as evidence for the involvement of the phenol group in all Ru-Im-xMe-PhOH compounds. It can thus be concluded that in the Ru-Im-xMe-PhOH compounds a radical is formed, characterized by relatively strong absorption around 850 nm and in the 350-550 nm region, and which involves both the phenol and the imidazole groups.…”

“…[20] The differences in lifetime and emission yield between the three complexes is tentatively explained by a weak, substitutiondependent photoacid behavior of the phenanthroline-imidazole ligand attributed to electrostatic interaction between the positive charge on the Ru ion and the imidazole group. [21]…”

Supramolecular Coordination Cages for Artificial Photosynthesis and Synthetic Photocatalysis

“…Reduced 10 then diffuses to the catalyst and transfers one electron, thus leading to unidirectional electron flow. It has been shown previously that electron relays can facilitate both charge separation and efficient electron transfer between PS and catalyst. , A TOF = 23 h –1 and TON = 58 after 200 min of irradiation is obtained, which is significantly higher than the reference system (complex without cage; TOF = 1 h –1 and TON = 6).…”