Michael Oelgemöller and Emma Coyle of Dublin City University, Ireland, discuss how microreactors may change synthetic organic photochemistry

At the International Congress of Applied Chemistry in New York in 1912,¹ Giacomo Ciamician, the father of organic photochemistry, presented his spectacular vision of 'The Photochemistry of the Future': 'On the arid lands there will spring up industrial colonies without smoke and without smokestacks; forests of glass tubes will extend over the plains, and glass buildings will rise everywhere; inside of these will take place the photochemical processes that hitherto have been the guarded secret of the plants, but that will have been mastered by human industry which will know how to make them even more abundant fruit than nature, for nature is not in a hurry and mankind is.'

Generally speaking, light can be used to efficiently and selectively induce chemical changes. It can be easily tuned and controlled, literally with a flick of a switch. Photochemistry also allows scientists to construct exotic, high-energy molecules with relative ease. Due to the specialised equipment and reaction conditions, however, chemical transformations with light have been widely ignored by the chemical industry. More than 90 years later, Ciamician's vision is yet to be realised.

Microreactors, otherwise known as microchannelled or microstructured reactors, have recently become widespread in research. Originally developed for analytical applications as the famous 'lab on a chip', these devices have also found promising uses in synthetic organic chemistry.

Since many commercially available microreactors are glass-based and transparent, they can be easily adopted for photochemical applications. Miniature light sources, such as light-emitting diodes, can be used and offer real advantages over conventional light sources - they are small, energy efficient, come in a range of wavelengths and produce very little heat, thus reducing the need to cool the reaction. Because the reaction channels in a microreactor are shallow, light can penetrate even concentrated solutions. Solution flow rate controls the exposure to light and can be easily varied to rapidly optimise photochemical reactions. Additionally, the reactions can be monitored on-line, for example by analysing the effluent using a UV spectrometer. Microreactors can also be used in parallel to scale-up reactions, a process known as numbering up.

There are a number of different microreactor types available today. As their name suggests, serpentine channel reactors have long, snaking reaction channels, which range from several centimetres to more than a metre in length. The dwell-reactor produced by mikroglas, for example, is the size of an external floppy drive but its reaction channel is 1.15 metres long. The reactor consists of a reaction channel and a second, cooling channel through which water flows. Another design is the falling film reactor, which generates a thin falling film of solution like a waterfall that passes by the light source. This device is especially advantageous for gas-liquid reactions, such as photooxidations or photohalogenations. Many researchers, however, continue to custom build their own reactors based on their needs and applications.

The photochemical transformations studied to date in microreactors include homogeneous reactions, such as photocyanation and photodecarboxylation; heterogeneous reactions between liquid and gaseous reagents, such as photooxygenations; and photocatalytic processes using semiconductors. In many cases, the selectivities and yields are better than those from large scale experiments, clearly demonstrating the feasibility and superiority of microphotochemistry.

Ciamician's vision may thus be realised in the form of a microchip, rather than the glass buildings he envisaged. By scaling down photochemical reactions using microreactors, photochemical reactions can be conveniently carried out in research laboratories, for example for finding and developing leads for drug discovery. In addition, numbering up, rather than scaling up, may enable photochemical products to be produced industrially.

¹ G Ciamician, Science, 1912, 36, 385

Read more in 'Micro-photochemistry: photochemistry in microstructured reactors. The new photochemistry of the future?' in issue 11 of  Photochemical & Photobiological Sciences

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