Coordination Chemical Scaffolds for Dual Proton Coupled Photoelectron Transfer

光催化剂可能容易分解，因为激发态的化学键通常被削弱。长期存在的激发态甚至增加了光催化的这个内在问题，实际上，人们寻求这种激发态是为了达到高转化率。大多数解决这一困境的成功策略是利用分子内或分子间电子转移的电荷分离（CS），这反过来导致自由基的形成。在光催化中几乎总是涉及到自由基化学，这可能引起不希望的副反应。避免自由基发生或至少避免分子间自由基反应的另一种方法是基于连续双电子转移。然而，即使第一次激发态的寿命很长，第二次激发的可能性也很低，这使得这个想法成为一个雄心勃勃的目标。然而，就天然光合作用中的chinone/hydrochinone对和作为多功能生化氧化还原剂的NAD+/NADH对而言，双电子过程结合质子耦合电子转移将光转化为可用的化学能似乎是仿生的。经典的光活性中心，如Ir（III）或Ru（II）多吡啶配合物单元，与(i)两个氧化还原活性的潜在供电子金属配体和（II） br ønstedt-碱性双电子受体（作为NAD/NADH模型）以y形排列组装是该项目的基本概念。在此框架内，新型的氧化还原活性N，C-κ2供体配体，包括具有末端供体功能的炔配合物片段，是核心发明。供电子配体的氧化还原活性取决于侧对炔配位金属的氧化还原活性。通过质子的化学激发态捕获和随后的自由基对形成，解决了相对紧凑的CS三元组的非生产性电荷重组（反向电子转移）的固有问题。选择2,3-5 ',6 ' -熔融吡啶吩啉衍生物作为合适的NAD/NADH模型配体。双质子耦合光电子转移的分析证明是该项目的具体激励。此外，潜在的光- nad /NADH系统可以被认为是具有催化活性的强大合成工具，它在弱质子供体存在的情况下将形式H原子当量从弱还原剂转移到有机底物，如酮。通过中间催化剂增加氢化物供体或起始物质的还原电位，将原始光激发能转化为化学键能。然而，深刻理解光动力学行为和基本质子转移动力学的相互作用是至关重要的。

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.