Supercharging Chemistry: A jump forward in light-driven chemistry
New collaborative research involving RASEI Fellow Niels Damrauer, addresses one of the ‘house of cards’ problems sometimes critical in photoredox catalysis.
Think about how you build a house of cards, every time you add a new card, there is a chance the whole thing will fall apart. This is a challenge often faced by chemists when they are trying to put together the components needed for a light-driven reaction. While this type of chemistry has huge potential in making the chemistry cleaner and more efficient, one of the features that can cause the whole thing to fall apart is a phenomenon called back electron transfer, where the desired chemical reaction is reversed, wasting energy and limiting the kinds of reaction that can be performed.
This collaborative team that includes RASEI Fellow Niels Damrauer from CU Boulder and the groups of Garret Miyake and Robert Paton from Colorado State University in Fort Collins, has developed a new catalyst system that overcomes this fundamental obstacle. Published in a recent issue of Science, this work introduces a ‘super-reducing’ organic photoredox catalyst that, through preventing this backward reaction, opens the door to powerful new redox chemistries.
To better understand this discovery it is useful to think of the process like filling a bucket with water. In typical photoredox reactions, the bucket has a leak. As water is poured into the bucket (adding energy from light), some of it immediately drains out. This ‘leak’ is back electron transfer (BET), and it is especially problematic for complex and difficult reactions that require a lot of energy – it is like trying to fill a very leaky bucket with a very slow faucet.
The research collaboration, part of the National Science Foundation (NSF) funded Center for Chemical Innovation (CCI) Center for Sustainable Photoredox Catalysis (SuPRCat) took inspiration from nature to develop a solution for this problem. In photosynthesis plants use a process called proton-coupled electron transfer (PCET) to efficiently capture and store energy from sunlight, preventing energy loss. The team used a combination of sophisticated computational modeling and experimental investigation to design a catalyst that incorporates a similar mechanism. When the catalyst is energized by light it simultaneously transfers an electron to the target molecule and releases a proton (a hydrogen atom without its electron). This prevents the reaction from going backwards. This small change has a huge impact on how the reaction proceeds, it is essentially like patching the leak in the bucket as you pour the water in, ensuring that all the energy is used for the desired reaction.
As is often the case with research, the path to this discovery was not a straight line. The investigations initially focused on changing an existing catalyst framework. During these experiments they noticed that one of the new catalysts (PC40Me) was unexpectedly effective. The reduction of benzene is known to be a difficult transformation, but reactions catalyzed with PC40Me were possible. They found that under the reaction conditions PC40Me was transforming into a new chemical structure, and it was this new system that was efficient for the historically difficult reduction of benzene. Armed with this knowledge the team built new catalyst designs around the new structure, creating a more efficient catalyst (named PC8). PC8 is not only a ‘super reducer’ capable of reducing a broad range of aromatic compounds under mild conditions, it also proved to be extremely robust.
The key features of this work lies in its potential to be a new tool in how we design and build everything from pharmaceuticals to plastics. By providing a way to perform these difficult reduction reactions more efficiently and sustainably, this catalyst system has the potential to reduce waste and energy consumption. By opening the door to a transformation that has typically been thought of as difficult and un-efficient, it could act as an enabling technology in the synthesis of new classes of molecules that were previously out of reach.
This work highlights the power of collaboration. The combination of different tools and approaches that were required to complete this work would have been prohibitive for a single research group. By combining expertise the team were able to unravel this complex chemical puzzle, not only demonstrating a new transformation, but providing some design rules that can be used by future photocatalysis practitioners in reducing BET.
