Flow Photochemistry in Organic synthesis

Continuous or flow processes have long been used in the heavy chemicals and petrochemical industries. However, development of the use of continuous flow chemistry by synthetic chemists for the preparation of small, multifunctional molecules has been slower. It is only in the last 10 years that growing interest has been shown in structured fluidic reactor technology for organic synthesis, thanks to the advantages in terms of mixing, heat transfer and safety associated with the use of these devices over conventional reactors. Photochemistry offers synthetic chemists an excellent opportunity to introduce a high level of molecular complexity in a "reagent-free" environment. However, photochemistry in conventional reactors has often been under-exploited as being too difficult to implement. As a result, many processes with no equivalent in conventional activation methods have been largely under-utilized. Interestingly, photochemists were relatively early adopters of fluidic technology, recognizing the advantages of microsystems in terms of light penetration, improved process safety, selectivities and reproducibility, as well as easier scaling-up in combination with modern, powerful light sources. A wide range of fluidic photochemical reactors is available, from reactors offering very precise control of the flows involved to compact reactors enabling the synthesis of quantities of materials with a production rate of a few kilograms per day. The availability of a wide variety of photochemical fluidic devices that are easy to set up on a laboratory scale, and the availability of reliable, robust ready-to-use systems, have led to a revival of photochemistry in both industry and academia. The aim of this article is to introduce the fundamental concepts of photochemistry and to present the various fluidic technologies available for photochemistry. Finally, examples of fluidic photochemical devices applied to organic synthesis are discussed to demonstrate their interest and contribution to the synthesis of complex molecules.