Coordination Chemical Scaffolds for Dual Proton Coupled Photoelectron Transfer

• Applicant: Professor Dr. Wolfram W. Seidel
• Subject Area: Inorganic Molecular Chemistry - Synthesis and Characterisation
• Term: from 2018 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 404422233

Photocatalysts are potentially susceptible to decomposition because chemical bonds in excited states are generally weakened. This immanent problem of photocatalysis is even increased by long-living excited states, which are actually sought for in order to reach high conversion rates. Most of the successful strategies to solve this dilemma make use of charge separation (CS) by intra- or intermolecular electron transfer, which leads in turn to the formation of radicals. In photocatalysis almost always radical chemistry is involved, which can cause undesired side reactions. An alternative approach to avoid the occurrence of radicals or at least to avoid intermolecular radical reactions is based on a consecutive dual electron transfer. However, the generally low probability of a second excitation even for long life times of the first excited state turn this idea to an ambitious objective. However, with respect to the chinone/hydrochinone pair in natural photosynthesis and the NAD+/NADH pair as versatile biochemical redox agent, two-electron processes in combination with proton-coupled electron transfers seem to be biomimetic in converting light into usable chemical energy.The assembly of classical photo-active centres like a Ir(III) or Ru(II) polypyridine complex units with (i) two redox-active, potentially electron donating metalloligands and (ii) with a Brønstedt-basic two-electron acceptor as NAD/NADH model in an Y-shaped arrangement is the essential concept of the project. Within this frame, novel redox-active N,C-κ2-donating ligands including an alkyne complex moiety, which bears terminal donor functionalities, are the central invention. The redox activity of the electron donating ligands is based on the redox activity of the side-on alkyne coordinated metal. The inherent problem of unproductive charge recombination (back electron transfer) of a comparatively compact CS triad is addressed by chemical excited state trapping by protons and subsequent radical pair formation. A 2,3-5’,6’-fused pyridylphenanthroline derivative was chosen as an appropriate NAD/NADH model ligand.The analytical proof of a dual proton coupled photoelectron transfer represents the specific incentive of the project. In addition, potential photo-NAD/NADH systems could be thought as catalytically active, powerful synthetic tools, which transfer formal H atom equivalents from weak reducing agents in the presence of weak proton donors to organic substrates like ketones. The original light excitation energy is than transformed in chemical bond energy by increasing the hydride donor or reducing potential of the starting material by the intermediate catalyst. However, a profound understanding of the mutual interaction of photodynamic behaviour and elementary proton transfer kinetics are paramount.

• DFG Programme: Priority Programmes
• Subproject of: SPP 2102: Light Controlled Reactivity of Metal Complexes