Photochemistry is the area of science exploring the effects of light on organic and inorganic molecules. The wavelengths of interest in photochemistry are UV (wavelengths 100 – 400 nm), visible light (400 – 750 nm) and Infrared light (750 – 2500 nm).
Photochemistry is critical to life on earth. It is the mechanism by which plants and algae turn light from the sun into sugars. Through the same photochemical reaction carbon dioxide and water are converted into oxygen. This process is called photosynthesis.
Photochemistry, photons and photocatalysts
A photochemical reaction is initiated by the emission of a photon of light from a light source. The light source could be a polychromatic source such as the sun. In laboratories, it is more common to use a mercury lamp or LED array. The photon can be absorbed either, directly by the reactant, by a photosensitizer, or by a photocatalyst.
If the photon is absorbed directly by the reactant. The absorption of the photon provides the activation energy for the reaction to progress.
If the photon is absorbed by a sensitizer. The sensitizer absorbs the photon and transfers the energy to the reactant.
In photogenerated catalysis, the photocatalytic activity relies on the ability of the catalyst to create electron–hole pairs following the absorption of a photon. These electron-hole pairs generate free radicals that can undergo secondary reactions.
Photochemistry and photosynthesis
Photochemistry via the reaction of photosynthesis is the mechanism by which the oxygen content in the earth’s atmosphere is maintained. Therefore, via photosynthesis photochemistry supplies all the energy necessary for life on earth. In addition, all the organic compounds on earth exist through photochemistry.
Photosynthesis is a photocatalytic reaction. The photocatalyst employed is chlorophyll, the green pigments in the leaves and stem of plants. By way of example, the absorption spectra for Chlorophyll are shown below, the absorption maxima occur at 420 nm:
Light sources used in laboratory photo-reactions
The chart below shows the emission spectra of typical light sources used for reactions in the laboratory.
Combining continuous flow with photochemistry
Over the last couple of decades, the demand for sustainable and environmentally friendly technologies has led to an increased awareness of organic photochemical reactions as clean, reagent-less applications.[2] Parallel with this has been growing awareness and take-up of continuous flow chemistry. A flow chemistry approach to photochemistry only increases its productivity, flexibility, and potential over the traditional batch process. Flow photochemistry has many advantages over conventional batch applications, such as consistent light penetration, controlled exposure times, precise temperature control, easy scalability, and the continuous removal of photochemical products from the irradiated area. These features will typically result in higher conversions and yields, improved selectivity, enhanced energy efficiency, and a reduction in waste due to lower solvent volumes.
Examples of photo-reactions
References:
- Wayne, C. E.; Wayne, R. P. Photochemistry, 1st ed.; Oxford University Press: Oxford, United Kingdom, reprinted 2005. ISBN 0-19-855886-4.
- . J.P. Knowles, L.D. Elliott & K.I. Booker-Milburn Beilstein J. Org. Chem. 2012 8, 2025-2052
- Speciality Chemicals Magazine – November 2014
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