Microwaves in Photochemistry

Chemistry under extreme or nonclassical conditions is currently a dynamically developing issue in applied research and industry. Alternatives to conventional synthetic or waste treatment procedures might increase production efficiency or save the environment by reducing the use or generation of hazardous substances in chemical production. 

Microwave heating has already been used in combination with other unconventional activation processes. Such combinations might have a synergic effect on reaction efficiencies or, at least, enhance them by summing the individual effects. Application of MW radiation to ultrasound-assisted chemical processes has recently been described by some authors [19-21]. Mechanical activation has also been successfully combined with MW heating to increase the chemical yields of several reactions [22]. There have also been attempts to affect photochemical reactions by use of other sources of nonclassical activation, for example ultrasound [23, 24]. 

Combined chemical activation by use of two different types of electromagnetic radiation, microwave and ultraviolet-visible, is covered by the discipline described in this chapter. The energy of MW radiation is substantially lower than that of UV 

Microwaves in Organic Synthesis, Second edition. Edited by A. Loupy Copyright 2006 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 3-527-31452-0 

The theory of EDL operation, as it is currently understood, is shown in Figs. 19.2 [33] and 19.3 (an example of a mercury EDL, or Hg EDL). Free electrons in the fill (i.e. electrons that have become separated from the environment because of the ambient energy) accelerate as a result of the energy of the electromagnetic (EM) field. They collide with the gas atoms and ionize them to release more electrons. Repetition of this causes the number of electrons to increase significantly over a short period of time, an effect known as an avalanche . The electrons are gener- 

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In 1975, Leung and El-Sayed reported on a very interesting observation that the rate of the biphotonic photochemistry of pyrimidine in benzene at 1.6 K had been found to decrease when the system had been exposed to microwaves in resonance with its zero-field (ZF) transitions or to a static magnetic field [1], The effect of resonant microwaves and static magnetic field (1 T) on decreasing the rate of the photochemical disappearance of pyrimidine is shown in Fig. 10-1. We can see from this figure that these perturbations cause a decrease in the value of the rate constant of the observed photochemical change by a factor of 1/2 -1/3. 

The spectral characteristics of EDL are of general interest in microwave-assisted photochemistry experiments. The right choice of EDL envelope and fill material can be very useful in planning an efficient course of the photochemical process without the need to filter out the undesirable part of the UV radiation by use of other tools, for example glass or solution filters or monochromators [59, 60]. 

Cirkva and Hajek have proposed a simple application of a domestic microwave oven for microwave-assisted photochemistry experiments [105]. In this arrangement the EDL (the MW-powered lamp for this application was specified as a micro-wave 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 liquid sample. This arrangement provided the unique possibility of studying photochemical reactions under extreme thermal conditions [106]. 

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

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

From comparison of the data presented in Table 2.2 [8], it is obvious that the energy of the microwave photon at a frequency of 2.45 GHz (0.0016 eV) is too low to cleave molecular bonds and is also lower than Brownian motion. It is therefore clear that microwaves cannot induce chemical reactions by direct absorption of electromagnetic energy, as opposed to ultraviolet and visible radiation (photochemistry). 

It is well known that y or X photons have energies suitable for excitation of inner electrons. We can use ultraviolet and visible radiation to initiate chemical reactions (photochemistry). Infrared radiation excites bond vibrations only whereas hyperfrequencies excite molecular rotation. In Tab. 1.1 the energies associated with chemical bonds and Brownian motion are compared with the microwave photon corresponding to the frequency used in microwave heating systems such as domestic and industrial ovens (2.45 GHz, 12.22 cm). 

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. 

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... 

The microwave photochemical reactor is an essential tool for experimental work in this field. Such equipment enables simultaneous irradiation of the sample with both MW and UV/VIS radiation. The idea of using an electrodeless lamp (EDL), in which the discharge is powered by the MW field, for photochemistry was bom half a century ago [46, 68]. The lamp was originally proposed as a source of UV radiation only,... 

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. 14.6 Photochemistry in a microwave oven (the EDL floats on the liquid surface).

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