Industrial applications of photochemistry

Starting from the various primary photochemical processes (luminescence, non-radiative transition to the ground state, electron and energy transfer, isomerization, addition, hydrogen abstraction, and fragmentation), the most important technical applications of photochemistry are reviewed. The main applications of luminescence phenomena are optical bleaching of textiles and paper. Rapid radiationless transitions to the ground state, often brought about by means of quenchers or of thermoreversible light reactions, are necessary for light protection of plastics and human skin. Electron and energy transfer processes have found a wide range of applications in photography, preparative photochemistry, and light sources, respectively. Photofragmentations are used in reprography andin the photochemical synthesis of detergents, insecticides and monomers for polyamides. For the industrial chemist, photochemistry is just one of the many means of producing chemical compounds or bringing them into reaction. However, it has some advantages over therma~ catalytic and other methods that immediately fascinate him. These include: {1) Selective activation of individual reactants, (2) Specific reactivity of electronically excited molecules, (3) Low thermal Ioad on the reaction system, (4) Exact control of radiation in terms of space, time and energy. But photochemistry is not without its own specific problems, including: (1) Absorption characteristics-Only absorbed light can be exploited for chemical purposes. Forthis reason, many reaction systems are ruled out for photoreactions because of their unfavourable absorption characteristics. (2) Interna! and external light filters-Photoreactions may be rapidly terminated if products with competing absorptions are formed. (3) Investment costs-Photochemical production plants may incur high unit capital costs if the space-time yield is low as a result of limitations imposed by the power of the lamps. (4) Electricity costs-Light is moreexpensive than heat because considerable Iosses occur in the production of electrical energy and its conversion into usable light energy. 535 MARTIN PAPE With these points in mind, the industrial photoehernist concentrates on reaction systems that bring out the full advantages of photochemistry and minimize the difficulties inherent in the method. The following objects have proved to be of economic interest: (1) Use of light for synthesis, (2) Synthesis of photosensitive compounds, (3) Development of u.v.-stabilizers, (4) Synthesis of compounds with specific spectral properties, (5) Contributions to ecology. The main aim of preparative photochemistry is to reduce manufacturing costs for chemical products by introducing photochemical steps in the syntheses. Light-sensitive compounds have great technical significance in photography, reprography, and printing. Important applications have been also found in u.v.-curable paints, primers, and printing inks. Photostabilizers are primarily used in plastics and man-made fibres. Another interesting field of application is sunscreen cosmetics: In the synthesis of compounds with special spectral properties, attention is focused on lightfast dyes, optical brighteners, fluorescent dyes, and chemiluminescent systems. The main photochemical contributions to ecology are chemical storage of solar energy, investigations on the photodegradation of biologically active ingredients, the investigation of photochemical smog formation, and the development of photodegradable plastics. For all these tasks the theoretical and practical assistance of pure photochemists is extremely welcome. Difficulties for the joint discussion may sometimes arise due to the fact that the photochemists at the universities are usually less concerned about commercial products and their markets than about reactions and mechanisms. luminescence noncrädiative transition to the ground state electron transfer energy transfer isomerization addition hv hydrogen abstraction fragmentation A Figure I. Excited state reaction paths. 536 INDUSTRIAL APPLICATIONS OF PHOTOCHEMISTRY However, a close connection can hardly be recognized between the type of reaction and the technical application. Thus various mechanisms, e.g. luminescence, energy transfer, non-radiative deactivation, or thermally reversible isomerization are used for photostabilization. Likewise, the initiator radicals in photopolymerization and photocrosslinking reactions may be produced, e.g. by hydrogen abstraction or by fragmentation. The reason for this diversity is that optimization of technical products and systems often has to meet many mutually conflicting demands. An ultra-violet stabilizer may incorporate the most advanced scientific knowhow, but it would not sell if, say, it were too sparingly soluble in the polymer, if it were too volatile or readily extractable, if it did not have sufficient resistance to heat and chemieals to withstand processing of the plastics, or if it did not appear to be entirely physiologically harmless. Conversely, a stabilizer may be third-rate according to photochemical criteria but could turn out to be a tremendous sales success, simply because it is particularly cheap to produce. Coping with so many parameters is a great challenge to the technical knowledge and imagination of the industrial photoehernist He must recognize all possibl.! economic advantages and opportunities of photochemical reactionsand then put them into practice. Up-to-date knowledge of technolqgy is essential. Thus I shall begin my review with the primary processes starting from the electronically excited states and then introduce you to their most important technical applications realized so far. I have chosen this approach to attract also the interest of pure photochemists because I am sure that with their aid and cooperation we shall be in a better position to find further practical applications of photochemistry.