Microwave photochemistry

UV radiation, certainly not sufficient to disrupt the bonds of common organic molecules. We therefore assume that, essentially, photoinitiation is responsible for a chemical change and MW radiation subsequently affects the course of the reaction. The objective of microwave photochemistry is frequently, but not necessarily, connected to the electrodeless discharge lamp (EDL) which generates UV radiation when placed in the MW field. 

This chapter presents a complete picture of current knowledge about microwave photochemistry. It provides the necessary theoretical background and some details about synthetic and other applications, the technique itself, and safety precautions. Although microwave photochemistry is a newly developing discipline of chemistry, recent advances suggest it has a promising future. 

The spectral characteristics of EDL are of a general interest in microwave photochemistry. The right choice of filling material can provide a desirable ultraviolet radiation. Atomic fills usually furnish line emission spectra (e.g. that of an Hg-EDL is... 

Cirkva and Hajek have proposed a simple application of a domestic microwave oven for microwave photochemistry experiments [86]. In this arrangement, the EDL (the MW-powered lamp for this application was specified as a microwave lamp or MWL) was placed in a reaction vessel located in the cavity of an oven. The MW field generated a UV discharge inside the lamp that resulted in simultaneous UV and MW irradiation of the sample. This arrangement provided the unique possibility of studying photochemical reactions under extreme thermal conditions (e.g. Ref. [87]). 

Fig. 14.5 A modified MW oven for microwave photochemistry experiments. A. magnetron, B. reaction mixture with the EDL and a magnetic stir bar, C. aluminum plate, D. magnetic stirrer, E. infrared pyrometer, F. circulating water in a glass tube, G. dummy load inside the oven cavity [88]. With permission from Elsevier Science.

Fig. 14.7 A reactor for microwave photochemistry derived from the Synthewave 402 (Prolabo). A. medium-pressure Hg lamp, B. window hermetic to MW radiation, C. reaction mixture, D. magnetron, E. regulator, F. IR sensor. Adapted from Ref. [92],...

Simultaneous application of UV and MW irradiation has found widespread use in industry. The techniques are based on the conventional UV lamps and MW-powered electrodeless lamps and MW devices [28], The following paragraphs discuss several patents and papers that describe industrial microwave photochemistry, such as treatment of waste water, sterilization, or industrial photo induced organic synthesis. 

In this review we have discussed how the concept of microwave photochemistry has already become an important issue in chemistry. Although still in the beginning, detailed analysis of past and present literature confirms explicitly the usefulness of this method of chemical activation. The field has been already established in industry and we hope it will also find its way into conventional chemical laboratories. 

Cirkva, V. and H4jek M., Microwave photochemistry photoinitiated radical addition of tetrahydrofuran to perfluorohexylethene under microwave irradiation, /. Photochem. Photobiol, A Chem., 1999,123, 21. 

Another approach to the production of UV photons includes the development of electrode-less discharge lamps driven by microwave excitation (e.g. Fassler et al., 2001, Ametepe et al., 1999, He et al., 1998). This type of lamp is shown in Fig. 4-17. In this case, the excitation of mercury vapor within the discharge gap is achieved by coupling in the energy with a water-cooled high-frequency spool. This concept may be a very convenient tool for microwave photochemistry experiments by simultaneous combination of microwave and VUV/UV irradiation of aqueous systems (c.f Klan et al., 2001, 1999). 

Klan, P. Hajek, M., Cirkva, V., The Electrodeless Discharge Lamp a Prospective Tool for Photochemistry. Part 3. The Microwave Photochemistry Reactor, J. Photochem. Photobiol. A 2001, 140, 185 189. 

In recent years, some working groups have demonstrated experimentally that stimulation of selected reactions by microwave assistance in combination with photochemistry is (partly) advantageous. Special investigations with combined microwave-photochemistry experiments in Brno (Klan), Prague (Hajek), and Jena (Ondruschka) should be mentioned here. One setup of the working group in Jena is shown in Fig. 2.27, cf Refs [98-101]. 

P. Kian, M. Hajek, Microwave Photochemistry, in [10], chapter 14, 463-486 and Chapter 19 in this book. 

P. Kean, V. Ciekva, Microwave Photochemistry, in Microwaves in Organic Synthesis, A. Loupy, (Ed.), Wiley-VCH, Weinheim, 2002,... 

Klan R Hajek M, Cirkva V (2001) The electrodeless discharge lamp a prospective tool for photochemistry part 3. The microwave photochemistry reactor. J Rhotochem Rhotobiol A Chem 140 185-189... 

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