Improved irradiation to the reactor
Photochemistry reactions are activated by the adsorption of photons (discrete “packets” of energy) by a reagent in a reaction. To put it more simply, photochemical reactions occur when light provides energy to trigger a reaction. The problem with photons is the attenuation effect of photon transport, that is they lose their energy very quickly. This is described by the well-known Bouguer-Lambert-Beer law which explains that due to absorption effects the radiation distribution is not uniform. For a photocatalyzed reaction, a homogeneous condition is ideal for better yields and selectivity.
There are several factors illustrated in the equation. The concentration of the reaction (c) plays a role in the light absorption with the pathlength of the light propagation (l), or the distance of the light source from the reaction mixture also an important factor.
Even the reaction mixture can effect attenuation. Starting materials, products, photosensitizers, and photocatalysts can all act as filters reducing the light intensity.
An example of how the attenuation of light affects a reaction is shown below. Here the % transmittance of a common photocatalyst, tris(bipyridine)- ruthenium(II) chloride [Ru(bpy)3]2+, is plotted against the path length for different concentrations (Figure 2). For a typical catalyst concentration (e.g. 2.5 mM), Figure 2 shows that less than 0.1% of light is transmitted past a length of 0.1 cm from the point of incident light and that even in a 1 cm vial, the majority of the reaction mixture is not being irradiated. Even after reducing the catalyst concentration 10-fold (0.25 mM), there would be ∼1% of the incident light transmitted to the centre of the reaction vessel. Unfortunately, by lowering the concentration of the catalyst, we also lower the rate of reaction.
When we look at performing photochemical reactions in microchannels (with an internal diameter of < 1 mm), we can see that full transmission of light is allowed through to the reaction solution. This results in a higher and more homogeneous irradiation of the reaction giving shorter reaction times and consequently less side-products caused by over-irradiation, often seen in equivalent batch conditions. This has been the major driving force behind the development of flow photochemistry reactors. By using well established flow chemistry reactors, the transmission of light is greatly increased. There are some important factors to consider when selecting the material of the flow photochemistry reactor. It is well known that certain materials can act as a wavelength filter, much like the reactants. Therefore, it is essential that the walls of the reactor are transparent to the frequency of light that is being used to irradiate the process.
Glass is an excellent example of a light transparent material. However, some types of glass can be incompatible with strong acid conditions. Photochemistry reactors have been increasingly made from polymer materials such as perfluoroalkoxyalkane (PFA) or polytetrafluoroethylene (PTFE). These materials have excellent light transmission, are highly chemically inert and have good thermal properties. Both PFA and FEP tubing have very good light transmission in the ultraviolet and visible light regions and allow a flexible configuration to flow photochemistry reactors. Figure 3 shows the cut-off wavelengths for a number of different materials that can be used within photochemical reactors.
|Reactor material||Wavelength cut-off|
|Quartz glass||170 nm|
|Vycor glass||220 nm|
|Corex glass||260 nm|
|Pyrex glass||275 nm|
|Polymethylmethacryate (PMMA)||248 nm|
|Polydimethylsiloxane (PDMS)||±255 nm|
|Polytetrafluoroethylene (PTFE)||±200 nm|
|Perfluoroalkoxyalkane (PFA)||±180 nm|
|Perfluoroethylenepropylene (FEP)||±180 nm|
Figure 3 – Wavelength cut-off of materials used in flow photochemistry reactors1
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1 Cambié, D.; Bottecchia, C.; Straathof, N. J. W.; Hessel, V.; Noël, T. Applications of Continuous-Flow Photochemistry in Organic Synthesis, Material Science, and Water Treatment. Chem. Rev., 2016, 116 (17), 10276–10341. https://doi.org/10.1021/acs.chemrev.5b00707
2 Knowles, J. P.; Elliott, L. D.; Booker-Milburn, K. I. Flow Photochemistry: Old Light through New Windows. Beilstein J. Org. Chem., 2012, 8, 2025–2052. https://doi.org/10.3762/bjoc.8.229
4 Sambiagio, C.; Noël, T. Flow Photochemistry: Shine Some Light on Those Tubes! Trends Chem., 2020, 2 (2), 92–106. https://doi.org/10.1016/j.trechm.2019.09.003
5 Plutschack, M. B.; Pieber, B.; Gilmore, K.; Seeberger, P. H. The Hitchhiker’s Guide to Flow Chemistry. Chem. Rev., 2017, 117 (18), 11796–11893. https://doi.org/10.1021/acs.chemrev.7b00183