Photocatalysis in coordination cages using supramolecular arrays of chromophores

利用光引起化学反应是公认的，从可再生能源的角度来看，具有根本的重要性。最近开发的一种方法是通过光氧化还原催化实现这一点。金属络合物催化剂或有机催化剂吸收光进入高能激发态，持续数百/数千纳秒；然后这可以向底物提供电子(或接受来自底物的电子)，产生自由基阴离子(或阳离子)，然后进行所需的反应。在过去的10年里，利用具有适当激发态的简单光吸收物种的光物理性质已经成为合成化学中公认的工具。在这个项目中，我们希望通过使用配位笼作为多组分催化剂将这一原理提高到更高的水平。配位笼是具有大的中心空腔的金属/配体组合，具有大的中心空腔，可以容纳小分子的客体。笼子包含大量的金属和配体组件，它们的上层建筑以规则的阵列围绕着中央空腔。它们可以这样一种方式制备，即它们在上层建筑中含有大量的金属络合催化剂或有机催化剂单元。在我们将要准备的最大的笼子中，24个单独的芳香族发光单元可以被结合到一个笼状组件中，该组件围绕着一个中心空腔，客体分子将结合该空腔。在单一反应物种周围有24个潜在的光氧化还原催化剂几乎是不可能通过任何其他方式实现的。目的是看看当一个潜在的底物(反应物)结合在其中一个笼子的中心空腔内时，它是否比它在溶液中自由时更有效地经历光氧化还原催化转化，在催化剂激发态存在的短时间内，它必须与催化剂随机碰撞。将底物结合在笼腔中，通过使客体非常接近催化剂单元的高局部浓度，消除了溶液中不同物种之间偶然碰撞的要求，因此电子转移将非常快，因此催化应该更快和更有效。此外，由于笼型腔对它们所结合的客体表现出尺寸和形状的选择性，笼型催化剂对特定底物应表现出更高的选择性，允许从混合物中选择、结合、转化和喷射出腔外的一种底物，而其他底物不受影响。这方面的成功将导致基于超分子主客体原理的新一代光氧化还原催化剂，其效率远远高于目前的催化剂。此外，存在一种令人兴奋的可能性--假设底物的单个分子被大量潜在的电子供体包围--两个电子基本上可以同时转移到单个客体，从而得到双还原产物。这在正常情况下是极难实现的，因为一个底物分子与两个单电子催化剂分子在它们都处于短暂的激发状态时不太可能发生碰撞；打个比方，就像试图用两颗步枪子弹同时击中一个飞行的粘土目标。然而，每个束缚客体周围大量发色团的极高局部浓度使得这在统计上更有可能，因此双电子光催化可能在广泛的笼状/客体系统中成为现实。这对太阳能的收集至关重要，因为无论是分解水以产生氢燃料，还是固定二氧化碳以产生甲醇作为燃料，涉及的许多重要反应都需要同时转移两个电子：使用配位笼作为多电子光氧化还原催化剂可以使这一点成为现实。

The use of light to cause chemical reactions is well established and, from a renewable energy perspective, of fundamental importance. A recently-developed way in which this can be made to happen is via 'photo-redox catalysis'. A metal complex catalyst, or an organic catalyst, absorbs light to enter an high-energy excited state which persists for hundreds / thousands of nanoseconds; this can then donate an electron to (or accept an electron from) a substrate, generating a radical anion (or cation) which then undergoes the desired reaction. In the last 10 years this use of the photophysical properties of simple light-absorbing species with appropriate excited states has become a well-established tool in synthetic chemistry.In this project we wish to take this principle to a much higher level by using coordination cages - hollow, pseudo-spherical metal/ligand assemblies with large central cavities that can accommodate small molecule 'guests' - as multi-component catalysts. The cages contain large numbers of metal and ligand components built into their superstructure in a regular array surrounding the central cavity. They can be prepared in such a way that they contain large numbers of metal complex catalysts or organic catalyst units in the superstructure. In the largest cages of the type that we will prepare, 24 individual aromatic luminescent units can be incorporated into a single cage-like assembly surrounding a central cavity which a 'guest' molecule will bind. Having 24 potential photo-redox catalysts surrounding a single reactive species could would be almost impossible to achieve in any other way.The aim is to see if, when a potential substrate (reactant) is bound inside the central cavity of one of the cages, it undergoes a photo-redox catalytic transformation far more effectively than when it is free in solution where it has to collide randomly with the catalyst in the short space of time that the catalyst excited state exists. Binding the substrate in the cage cavity removes the requirement for chance collisions of separate species in solution by holding the guest very close to a high local concentration of catalyst units, such that electron transfer will be very fast and hence the catalysis should be much faster and more efficient. In addition, because the cage cavities show size- and shape-selectivity for the guests that they bind, the cage-based catalysts should show much higher selectivity for specific substrates allowing one substrate from a mixture to be selected, bound, transformed and ejected form the cavity whilst others are unaffected. Success here will result in a new generation of photo-redox catalysts, based on supramolecular host/guest principles, that are far more effective than the current ones.In addition, the exciting possibility exists that - given a single molecule of a substrate surrounded by a large number of potential electron-donors - two electrons could be transferred essentially simultaneously to a single guest to give a doubly-reduced product. This is extremely difficult to achieve normally because of the unlikelihood of one substrate molecule colliding with two one-electron catalyst molecules while they are both in their short-lived excited state; an analogy would be like trying to hit a flying clay target with two rifle bullets simultaneously. However the very high local concentration of large numbers of chromophores around each bound guest makes this much more statistically likely, such that two-electron photocatalysis may become a reality in a wide range of cage/guest systems. This is of fundamental importance for solar energy harvesting as many of the important reactions involved in either water splitting to generate H2 fuel, or fixation of CO2 to generate methanol as a fuel, require simultaneous transfer of two electrons: use of coordination cages as multi-electron photo-redox catalysts could make this a reality.