Coordination Chemical Scaffolds for Dual Proton Coupled Photoelectron Transfer

光催化剂很容易分解，因为激发态的化学键通常会被削弱。光催化的这一内在问题甚至被长寿命激发态所增加，这些激发态实际上是为了达到高转化率而寻找的。大多数成功的解决这一困境的策略是利用分子内或分子间电子转移的电荷分离(CS)，这反过来又导致自由基的形成。在光催化中，几乎总是涉及到自由基化学，这可能会引起不受欢迎的副反应。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的双电子转移。然而，第二次激发的可能性通常很低，即使在第一激发态的长寿命内也是如此，这使得这个想法成为一个雄心勃勃的目标。然而，对于自然光合作用中的对苯二酚/对苯二酚和作为多功能生化氧化还原试剂的NAD+/NADH对，结合质子耦合电子转移的双电子过程似乎在将光转化为可用化学能方面具有仿生性。经典的光活性中心如Ir(III)或Ru(II)多吡啶配合物单元的组装(I)两个氧化还原活性的潜在电子供体金属寡体和(Ii)以Y型排列的Brnstedt-碱性两电子受体作为NAD/NADH模型是该项目的基本概念。在这一框架内，具有氧化还原活性的新型N，C-κ2-给体配体是核心发明，其中包括具有末端给体功能的炔类络合物部分。给电子体的氧化还原活性是基于炔侧配位金属的氧化还原活性。通过质子俘获化学激发态和随后的自由基对的形成，解决了相对紧密的CS三联体的非生产性电荷复合(反向电子转移)的固有问题。2，3-5‘，6’-吡啶基二氮杂菲衍生物被选为合适的NAD/NADH模型配体，双质子耦合光电子转移的分析证明代表了该项目的具体动机。此外，潜在的光-NAD/NADH系统可以被认为是催化活性强的合成工具，在弱质子供体存在的情况下，将形式氢原子当量从弱还原剂转移到酮等有机底物上。通过增加氢化物给体或通过中间催化剂降低起始物质的电势，将原始的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之间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.