1
Introduction

1.1 A Brief History of Photochemistry

Photochemistry, which means chemical changes induced by absorption of light, constitutes the basis of human life. This is linked to the property of a green leaf to absorb the blue and red components of sunlight and generate carbohydrates and oxygen. Only water and carbon dioxide are necessary for that unique process of unprecedented selectivity, considering that only carbon dioxide is reduced even though the competitive and much more reactive oxygen molecule is present in about 600-fold excess. Thus, photosynthesis1 supports mankind with food to eat and oxygen to breathe (Equation 1.1). Therefore, it is not surprising that in the very earliest human cultures

the sun was worshiped as a god. A prominent example is Egypt, where in the fourteenth century BC pharaoh Ikhnaton rejected the many old gods and introduced a monotheistic religion based on the sun-god Aton. Also, in the Christian genesis God said, "let there be light," after he had created the earth and heaven (Genesis, verses 3-4). Besides photosynthesis, sunlight controls also the growth of plants through the protein phytochrome [1]. The complicated action mechanism can be broken down to an olefinic cis-trans isomerization. In the protein rhodopsin, the same molecular process forms the basis of human vision.

Light absorption by other eye proteins controls the concentration of hormones such as melatonin relevant for circadian rhythms, the immune system, and seasonal defective disorders such as the "winter blues." In the eyes of some migratory birds, another protein, cryptochrome, upon light absorption generates a short-lived triplet ion pair having a magnetic moment. Interaction with the terrestrial magnetic field seems to be the underlying mechanism of these birds' admirable navigation capability. A similar type of light-induced magnetic sensing is invoked also for the spawn-migration of some salmons (Oncorhynchus nerka) [2].2

The well-known synthesis of vitamin D in human skin is based on a sunlight-driven electrocyclic ring opening of a 1,3-cyclohexadienyl fragment. Sufficient supply of this vitamin seems to have also a positive influence on various types of cancer. Contrary to this direct chemical action of sunlight, which is localized in the skin, there is also an indirect one on the skin surface. Already, Egyptian physicians were curing skin cancer by smearing bergamot oil onto the tumor and exposing the patient to sunlight. This indirect effect is based on the oil-photosensitized formation of the very reactive singlet oxygen and is utilized nowadays under the name of photodynamic therapy (PDT) in cancer treatment. The use of artificial light sources such as optical fibers allows conducting PDT also on internal tumors. Scheme 1.1 summarizes the biological actions of sunlight.

Scheme 1.1

In addition to this unique relation with human life, photochemistry is important because of its distinct influence on natural and artificial matter.3 An early example is the photochromic effect induced by a cis-trans photoisomerization of olefins. It has been claimed that the Macedonian troops of Alexander the Great carried rag bands around their wrists that contained a photochromic dye. The color change observed after a specific time of sunlight exposure probably was the visual command for attack. This early device for "optical communication" was referred to as "Alexander's Rag Time Band." Until the end of the eighteenth century, the interaction of light with matter was limited to qualitative observations such as the darkening of colors and silver salts. Around 1790, J. Priestley observed a red colorization when he exposed nitric acid ("spirit of niter") to sunlight. He also reported that in photosynthesis a reaction of water is responsible for gas evolution and that a green compound is necessary for that. And in 1804, N.T. de Saussure observed the mandatory provision of water and carbon dioxide for the formation of oxygen. In early nineteenth century, the explosive action of light on hydrogen/chlorine mixtures (R. Bunsen, H. Roscoe, J. W. Drapers, and W.C. Wittwers) and the reduction of iron(III) to iron(II) upon exposing oxalic acid solutions to sunlight (J.W. Döbereiner) were reported. Performed with artificial light under well-defined conditions, the latter reaction became the basis for ferrioxalate actinometry4 (C.A. Parker). From the observation that mixtures of silver salts and chalk darken when left in daylight (J.H. Schultz), silver halide photography was developed (N. Niépce, L. Daguerre, and W.F. Talbot). In the second half of that century, organic photochemical syntheses became a central topic. The earliest example is a photorearrangement of santonin (Figure 1.1), an anthelmintic sesquiterpene lactone present Artemisia plants (F. Sestini, S. Cannizzaro).5 Other reactions are the photodimerizations of anthracene (C.J. Fritzsche) and thymoquinone (C.T. Liebermann). The latter constitutes the first solid-state (2 + 2)-cycloaddition. Further examples are geometric isomerizations of olefins such as cinnamic acids (W.H. Perkin, C.T. Liebermann). It was proposed that the absorption of light "causes a weakening of the double bond, so that the formerly doubly bound carbon atoms become temporarily trivalent. This leads to a migration of groups, then to rotation, and then to renewed bonding of the carbon atoms" (K. Wislicenus). Another noteworthy reaction is the addition of benzaldehyde to benzoquinone affording 2-benzoylhydroquinone, which is probably the first example for a synthetically useful photoreaction. This and analogous reactions were referred to as syntheses by sunlight and assumed to be similar to photosynthesis of green plants (H. Klinger). To probe this similarity, even wavelength-dependent irradiations were conducted as early as in 1888 using inorganic filter solutions. It was found that quinones reacted fastest with blue light, whereas the green plant preferred red light.

Figure 1.1 Photorearrangement of santonin to photosantonic acid.

At about the same time, the photochemical reduction of nitrobenzene to aniline by ethanol and the hydrodimerization of aldehydes and ketones to pinacols in alcohols were reported (Equation 1.2; G.D. Ciamician, P. Silber).

Long before the first oil crisis in 1973, the splitting of water into hydrogen and oxygen, the "holy grail" of photochemistry, was discussed as an inexhaustible energy source. About 100 years earlier, Jules Verne wrote in his book The Mysterious Island: "Yes, my friends, I believe that water will one day be employed as fuel, that hydrogen and oxygen which constitute it will furnish an inexhaustible source of heat and light. Water will be the coal of the future. " And Ostwald wrote in 1911 that mankind should cover a maximum part of its energy demand from solar energy [5]. The use of sunlight for organic syntheses was visionary predicted in 1912 by G.D. Ciamician stating that "On the arid lands there will spring up industrial colonies without smoke and without smokestacks; forests of glass tubes will extend over the plants and glass buildings will rise everywhere; inside of these will take place the photochemical processes that hitherto have been the guarded secret of the plants, but that will have been mastered by human industry which will know how to make them bear even more abundant fruit than nature, for nature is not in a hurry and mankind is" [6]. However, this vision still awaits fulfillment.6 But an organic reaction became the basis of an industrial process conducted with artificial light. That is the photolithographic process for the manufacture of positive photoresists employed in the integrated circuit industry. It consists of the photorelease of dinitrogen, subsequent Wolff rearrangement of the generated carbene to a ketene, and the addition of water (Scheme 1.2).

Scheme 1.2

In the second half of the twentieth century, mechanistic aspects prevailed, especially in organic chemistry. Because of the development of time-resolved emission and absorption spectroscopy, the direct observation of excited states, that is, molecular states generated by absorption of light in the wavelength range from approximately 300 to 700 nm, became feasible. The initially obtained time resolution of nanoseconds (Norrish and G. Porter) was gradually improved to pico-, femto-, and attoseconds through the replacement of conventional flash lamps by gradually improving laser systems [4, 8, 9]. Together with theoretical calculations, the chemical properties of excited organic compounds could be explored in great detail including physical primary processes such as energy transfer.7 This applies also for the fast developing areas of atmospheric photochemistry and photosynthesis, which are major topics in physical chemistry. Mechanistic photochemistry of organometallic and inorganic compounds developed much more slowly because of the more complicated electronic structure of metal complexes. It started with the analysis of absorption and emission spectra through the application of crystal and ligand field theory and molecular orbital calculations. These theoretical concepts were primarily used to understand the stereoselectivity and...