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Pharmaceutical Technology APIs, EXCIPIENTS, AND MANUFACTURING eBOOK 2021 53 window of reactors of this type and the ability to perform multiphase chemistry. Flow photochemistry reactors Photochemistry reactors increasingly use single wave- length LED light sources to control reaction selectiv- ity, but this is not the only benefit. LEDs are energy efficient and have a low heat load, enabling them to be used at high intensities with relatively low cooling required. Their small irradiation window also allows them to be directed toward the channel's in-flow re- actors. However, even with more efficient LED light sources, the problem with light irradiation still persists; photons lose energy quickly over very short distances from their source. This light attenuation, described by the Bouguer-Lambert-Beer law equation, is a problem with round bottom flasks used in batch processes as only the outer region of the reactor is irradiated. In contrast, the microcapillaries used in flow reactors en- able transmission of much more light to the center of the reaction, giving a more homogeneous irradiation. Reaction scalability. It could be argued that the popu- larity of photochemistry among synthetic organic chemists in industry has suffered from its limited scale-up potential. This can be attributed to the at- tenuation effect of photon transport, which increases with reactor volume, limiting the scalability of pho- tochemistry even on a laboratory scale under batch conditions. It also limits the classical method of flow chemistry scale-up—increasing reactor volumes and flow rates. Reactor volumes are often increased in a dimension-enlarging fashion, and this too is restricted by the attenuation of light. Flow photochemistry offers some strategies that can help in the scale-up of photocatalytic reactions. Throughput can be enhanced by running the reactions for longer—a well-known benefit of traditional flow chemistry—or by increasing the number of reactors, either by linearly expanding the number of reactors in series, or by external or internal numbering, which can be a more costly approach. The photon flux into the reaction can also be increased. In photochemistry, the photon is considered a reagent. In a photo-medi- ated reaction, increasing the light intensity increases the number of photons, allowing the use of the same photochemistry reactor for process scale-up. Applications Photochemistry has already been successfully used across a broad range of chemistry applications, in- cluding organic synthesis in drug discovery, fine chemicals and agrochemicals, polymer synthesis in materials science, water treatment, and degrada- tion studies. From the point of view of a synthetic organic chemist, it is only in more recent years that key transformations have been developed in f low photochemistry. These include photocycloadditions, photoisomerizations, photooxygenations, cyanation and halogenation reactions, light-induced cross- coupling reactions, and late-stage C–H activation. Halogenations. The development of flow photochem- istry for halogenations such as bromination and fluo- rination is of great value to the pharmaceutical and agrochemical industries. Brominated compounds are common building blocks for organic synthe- sis, whereas fluorinated compounds—containing a single fluorine substituent or a trifluoromethyl (CF 3 ) group—provide improved chemical stability, modu- late lipophilicity, and binding selectivity due to acid/ base characteristics. The need to access aryl bromides is widespread in fine chemicals synthesis, such as, in the synthesis of antibacterial, antifungal, and antiviral compounds. Flow photo-bromination can be achieved using N-bromosuccinimide, which requires light acti-