Photocatalysis in coordination cages using supramolecular arrays of chromophores

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.

Once the work has reached a degree of academic maturity beyond the immediate interest amongst the academic community (see 'academic beneficiaries), in the medium to long term the ability to develop photo-redox catalysts that are

(i) substantially more efficient that the current state-of-the-art due to elimination of the limiting solution collision step
(ii) size and shape selective due to the constraints of the cage cavity size and shape
(iii) tailored for a wide variety of specific substrates by controlling the chormophores in the cage array and the cavity size/shape.

will be of substantial interest to synthetic chemists for low-volume, high-value transformations. The ability to catalyse specific organic transformations is central to the entire fine chemicals and pharmaceutical chemistry sectors which make a huge contribution to the economy and people's quality of life. Whether in the form of medicines, fertilisers, pesticides or new materials there can be nobody in the developed world who does not benefit from our ability to make new organic compounds, and a new method for achieving efficient photoredox reactions that exploits supramolecular recognition between catalyst and substrate would contribute substantially to this field. As well as catalysing a wide range of redox-induced transformations using light, the effects of cavity size / shape limitations on host/guest recognition will provide very high selectivity. The fact that water is used as the solvent makes the reactions environmentally benign.

In summary, outside of academia, the work will be of benefit to the fine chemicals and pharmaceutical industries, allowing them to make more compounds more cheaply or to improve existing processes; and beyond that the consumers of these compounds - more or less everyone - will be the ultimate beneficiaries.