Flow photochemistry has developed rapidly since the early reports just over 10 years ago. Initial studies focussed on the microflow regime, which itself was born out of the “lab-on-achip” arena. Since then there have been many reports of various well engineered microflow photochemical reactors. Most of these have shown that many photochemical reactions can be carried out with higher yields (space/time) and selectivities and with fewer side reactions than comparable batch reactors. On the whole, however, microreactors are uncompetitive with classic immersion-well batch reactors when it comes to the key issue of productivity. This is unsurprising given the very low reaction volumes and flow rates involved, and as such comparison of two such different reactor topologies is not useful. Microflow reactors are particularly well placed to make best use of the current developments in LED technology. As microflow reactors cannot make use of a large photon flux, much of the radiation from powerful UV lamps is wasted. Use of arrays of compact LEDs is much more suitable and efficient. At the moment LEDs of λ < 365 nm are expensive, prohibitively so at wavelengths of 300 nm and below where a single LED can cost as much €300. This price will come down in future, but it is likely that only a microflow reactor could benefit from this. With further developments photochemical microflow reactors are likely to find many applications, particularly if they can be coupled with automation: screening for new photoreactions, reaction and wavelength optimisation, drug discovery, microactinometers for quantum yield measurements, etc.
Since its introduction in 2005, the FEP macroflow reactor of Booker-Milburn and Berry has demonstrated that batch-locked reactions can be scaled up from sub-gram quantities to over 500 g per day in a single pass. A flagship example of this was recently reported by Seeberger and Lévesque in their continuous (>200 g per day) synthesis of artemisinin, the current front-line treatment for malaria. Related designs have very recently demonstrated that photocatalysis can be carried out in macroflow devices with high productivities. This is a very significant development as photocatalysis is a powerful emerging area for synthetic chemistry and promises to have wide application. The value of FEP and related tube designs lies in the simplicity of their construction: all the tubing, glassware, lamps and pumps are commercially available at a very economical price and a functioning reactor can be set up in a matter of hours in a standard fume hood.
With now easy access to flow photochemistry we hope that the synthetic community at large will make more use of photochemical bond-forming reactions and apply them to their general synthetic problems. As way of stimulus, the following provocative question can be asked: can your ground-state chemistry give you easy, clean and reagentless access to 100 g quantities of molecules with high structural complexity? Flow photochemistry can.
The full review article, written by Jonathan Knowles, Luke Elliott and Kevin Booker-Milburn from the University of Bristol, is available online in Beilstein Journal of Organic Chemistry and is free to access.