Photochemistry light sources

Improved irradiation of the reaction

Historically, photochemistry uses high intensity mercury lamps to facilitate photocatalyzed reactions. These light sources come with significant difficulties in their operation. Mercury lamps produce a broad spectrum of wavelength with strong emissions in the UV spectrum. In medium pressure mercury-vapor lamps, the wavelength range emitted is from 200–600 nm. Therefore, to offer specific and selective photocatalyzed reactions, a suitable filter is require to block unwanted wavelengths.  These filters will typically block either shorter wavelengths (long-pass filters) or allow specific wavelengths to pass (band-pass filters). Undesired wavelengths can cause unwanted side-reactions and the potential decomposition of products.

Low and medium pressure mercury lamps produce a lot of heat. This requires external cooling to be implemented and to ensure that the heat the lamps produce is isolated from the photochemistry reactor to prevent this interfering with the desired reaction.

One of the most serious implications of using mercury lamps is the high level of ionising radiation they emit. Specific features need to be implemented to prevent the unwanted exposure of users to UV light which can be extremely harmful resulting in sun burn and extreme damage to the eyes.

The use of compact fluorescent light bulbs (CFL) has gained some use for visible light-induced reactions which can emit a broad range of wavelengths, but they have limitations when applied to flow photochemistry reactors due to the broad area of irradiation resulting in loss of light directed towards the reaction.

The continued development of light emitting diode (LED) technology is driving their use in flow photochemistry reactors. LED light sources typically have a narrow emission band (normally in 20nm ranges) so can be selected to match the specific photochemical application. They also have a small irradiation window allowing them to be directed towards the channels in flow reactors. LED light sources also have a low heat load enabling them to be used at high intensities with relatively low cooling required and are energy efficient. All of these benefits have seen the rise in popularity of their use in photochemistry reactors.

 

Figure 1 – Emission Spectra of Common light sources used for Photochemical Applications<sup>1</sup>
Figure 1 – Emission Spectra of Common light sources used for Photochemical Applications1

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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