20 KJEMI 2 2024 Light as a clean energy source It is no secret; climate change is upon us. In the last thirty years, chemists all over the world have been doing their part for a better future by developing more sustainable production processes and reducing the use of toxic compounds. A major strategy in the green shift of chemical synthesis is catalysis, as it drastically reduces the molar equivalents of reagents needed in reaction mixtures.1 Many catalytic systems require energy – in various forms – to initiate chemical reactions. When we think of «natural» or «clean» energy in the wide sense, sunlight stands out as a seemingly perfect energy source. Infinite (the sun isn’t about to explode just yet), safe, free, available (if we’re lucky), it has all the advantages imaginable. However, lining up your reaction flasks in the sunshine and waiting for chemistry to do the rest (Fig. 1) seems a little utopic and, for obvious reasons here in the Arctic town of Tromsø, unpractical. Luckily, lamps were invented a long time ago, and even these weaker light sources can provide enough energy to initiate many reactions. Nowadays, strong, reliable, and wavelength-specific LED lamps are easily available, making it more attractive than ever to use light in the lab. Radical chemistry: dark history and great revival Radical chemistry suffers from a bad – and historically well deserved – reputation. While it is a highly interesting field that enables the formation of exotic molecular structures unreachable by traditional ionic chemistry, radical reactions can be non-selective and hard to control. It doesn’t help that traditional radical chemistry typically relies on the use of toxic tin reagents or explosive peroxides as radical initiators. UV light has been used in numerous examples as an alternative strategy, but this brings its own health hazards and requires special equipment.2 Therefore, chemists tend to put radical chemistry in the «dangerous, unpredictable and not worth my time and equipment investment» category. But not all hope for this field is lost! In the last twenty years, photoredox catalysis and electrochemistry have emerged as mild and safe radical initiation techniques.3, 4 This is a turning point in the history of radical chemistry as it marks the beginning of an era of renewed global interest in the field. What’s light got to do with it? Most organic molecules are colorless, meaning they do not absorb light in the visible light range. By contrast, colored molecules absorb light by increasing the energy of an electron within the molecule. This process, known as excitation, requires a wavelength of light that matches the energy gap between the different permitted energy levels of the electron. The consequence of this is that not just any wavelength can excite any molecule. Light, radicals and photoredox catalysis Have you ever walked by an organic chemistry lab filled with strong colorful lights and multicolored vials and wondered what kind of research could possibly be going on there? Or overheard a confusing conversation containing the words «wavelength», «photocatalyst» or «electron acceptor»? Chances are, you are looking at the everyday life of chemists working with photoredox catalysis. Floriane Baussière, PhD fellow, UiT, The Arctic University of Norway Figure 1. How I would run my reactions in an ideal world – picture taken during an uncharacteristically sunny week.
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