J.R. Bolton

PHOTOCHEMISTRY OF THE SOLID STATE

Publisher Summary This chapter focuses on photochemistry of the solid state. The study of photochemistry of solids is still at the phenomenological stage, and interpretations of experimental results are largely speculative. In contrast, the theoretical study of the solid state is well advanced and is beginning to provide quantitative explanations for many of the properties of solids, such as energy states, absorption spectra, and luminescence, which, even if not photochemical, are at least plainly related to photochemical events. Some simple photochemical processes such as dimerizations are governed by the arrangement of the molecules in the crystal, but when the geometrically favored product cannot form for steric reasons, the reaction occurs in defective regions of the crystal where the reverse of the normal arrangement of the molecules obtains.

CHAPTER 5 – EXPERIMENTAL TECHNIQUES

Publisher Summary This chapter provides an overview of the techniques for measuring light intensities and quantum yields, and the methods used to detect transient photochemical products. In the determination of a quantum yield, two kinds of measurements are required. The first is a determination of the amount of a product; the second is the determination of the amount of 1ight absorbed. The product of photo excitation need not necessarily be a chemical product, that is, the term quantum yield may be applied to any measurable phenomenon and includes phosphorescence or fluorescence. Absolute measurements of light intensity of emission are amongst the most difficult to make. The apparatus used for chemical quantum yields may be of varying degree of complexity. More sophisticated devices have been evolved for multiple irradiations.

INDUSTRIAL APPLICATIONS OF PHOTOCHEMISTRY

Publisher Summary This chapter reviews the industrial applications of photochemistry. It presents some of the chemical and economic factors that are likely to influence the future. Industrial applications of photochemical synthetic methods can be divided into two parts according to the size or type of the operation. Bulk chemical products require large equipment and a flow system. Here the low price of the product will make the process cost a significant fraction of the overall cost. It is best if the process results in a large increase in molecular weight from starting material to product, otherwise all the profit must come from the increased value of the product relative to starting material. Fine chemical products may well use smaller apparatus, similar to the type used for experimental studies.

PHOTOCHEMICAL PROCESSES IN THE GAS PHASE

Publisher Summary This chapter reviews some features of typical photochemical processes conducted in the vapor state. It also presents a survey of the most challenging and technically complex of all gaseous photochemical systems, the reactions of the urban atmosphere. Photochemical isomerization can be brought about by direct optical excitation to the π, π* singlet or triplet states of the olefin, though the practical difficulties are severe. In both of these states, the relaxed configuration is 90°-twisted and thus reverts to a mixture of cis- and trans-forms of the ground state. Because the absorption band corresponding to the singlet → triplet excitation is exceedingly weak, isomerization through the triplet state is usually carried out by photosensitization. In populated areas, the lower atmosphere is a reaction vessel charged with the by-products of combustion, its chemistry stimulated by sunlight. In describing the reactions of this atmosphere, it is useful to distinguish the primary pollutants, directly involved in the atmospheric photochemical cycle, from the secondary pollutants that interact with events in the cycle and unbalance it to the extent that steady-state conditions do not prevail.

MECHANISMS OF ORGANIC PHOTOCHEMICAL REACTIONS

Publisher Summary This chapter presents a few mechanisms of organic photochemical reactions. It presents five general and typical reactions: (1) hydrogen abstraction, (2) bond cleavage, (3) cycloaddition, (4) valence isomerization, and (5) rearrangements. The task of understanding a photochemical reaction is more demanding than that for a ground-state reaction for several reasons. It is clear that the same type of product can arise from different types of excited states and also from one state, but by more than one mechanism. An additional feature, not paralleled in ground-state chemistry, is that knowledge of the competing deactivation processes is also required. Cyclic conjugated enones and cross-conjugated dienones undergo a variety of interesting complex transformations upon irradiation. These reactions have been extensively studied, and it is now clear that much of the complexity is because of partitioning of the first-formed intermediate into different pathways leading to a variety of products by ground-state reactions. The intermediate may be stable at low temperature and may then be detected and characterized. The bicyclohexenones undergo a variety of rearrangements, which involve partitioning in both the excited state and the subsequent thermal reactions.

ELECTRONIC SPECTRA OF EXCITED STATES

Publisher Summary This chapter illustrates the electronic spectra of excited states. Each band of the spectrum has, in principle, a substructure of vibrational quantum changes and a sub-substructure of rotational quantum changes, though these details are fully developed only in vapor phase spectra, that is, under conditions where individual molecules are relatively isolated from one another. The separate bands are characterized by intensity and position, or by the probability of the electronic transition and by its location on a wavelength or frequency scale. For a transition to occur by light absorption, there must be some form of interaction between the radiation and the molecule. The greater the electric moment change accompanying the transition, the greater the absorption intensity; therefore, one reason underlying differences in intensity among the various bands, probably the most obvious feature of any spectrum, is a variation in the dipole moment change. In some transitions, the electric moment change is small or zero, and the transition may then pass unseen in the spectrum.

TRANSIENTS AND THEIR BEHAVIOR

Publisher Summary This chapter focuses on transients and their behavior. Most photochemical reactions are complex in their mechanisms and involve a number of reactive intermediates or transients. The first type of transient in any photochemical reaction must be the electronic excited states of the absorbing molecule. These photoexcited states are usually detected by monitoring the emission from the singlet state or the triplet state. They may also be detected by direct absorption, an experiment that is more difficult with singlets than triplets. In a transient photolysis situation, it is convenient to assume that all the light from the flash source is emitted in a negligibly short period of time. In general, the absorption spectra of the two isomers will be different; hence, at any given wavelength, one or the other isomer will have a higher extinction coefficient. Either impurity quenchers or triplet–triplet annihilation are effective in removing T1, and phosphorescence is rarely observed in liquid solutions. In crystals or in low-temperature glasses, diffusion is very slow and quenching is not effective.

CHAPTER 10 – PHOTOCHROMISM

Publisher Summary This chapter discusses the photochromism mechanism. In photochromic substances, absorption of radiation leads to a quantum mechanically stable but thermodynamically metastable state B from which it may revert to the original state A by absorption either of light of a different wavelength or of thermal energy. Three conditions are, thus, necessary for a material to be photochromic: (1) the state B should be thermally stable, (2) it should absorb radiation in a different region of the spectrum to A, and (3) the radiation or thermally induced change B → A should occur. Despite these rather restrictive conditions, a wide range of organic and inorganic materials have been found to be photochromic either in the solid or glassy states or in solution. Photochromic changes that depend on heterolytic cleavage occur in the spirans, triarylmethane dyes, polymethine dyes, indenone oxides, and nitrones. A photochromic redox system consists of donor and acceptor components each of which must exist in at least two stable oxidation states. One of these must be colored, and the other must be uncolored. A characteristic quantity in photoconductivity is the mean range of the photoelectrons.

THEORY AND THE EXCITED STATE

Publisher Summary This chapter describes the theory and applications of quantum-mechanical methods to illustrate the properties for molecules in their electronically excited states. Although both the valence-bond and molecular-orbital approximations are commonly employed in dealing with the ground electronic state of molecules, most theoretical investigations of excited states employ the latter theory. According to molecular orbital theory, the low-lying excited states of such molecules correspond to electron configurations in which an electron in one of the least stable BMOs in area II is promoted to one of the more stable antibonding MOs. Thus, the photon energy necessary to induce the lowest energy excitation is directly proportional to the distance ΔE between the top of areas II and III and the bottom of the ABMO area IV. In most organic compounds containing no multiple bonds, the distance ΔE between the highest occupied BMO and the lowest unoccupied ABMO is very large, and no transitions are seen in the near-ultraviolet and visible wavelength region. The exact photon energy required depends upon the number of double bonds conjugated together and upon the manner in which this conjugation occurs.

SYNTHETIC APPLICATIONS OF ORGANIC PHOTOCHEMISTRY

Publisher Summary This chapter describes the synthetic applications of organic photochemistry. There are two general types of reaction—the allylic substitution and the addition—where the diene usually forms part of a system in which it is fixed in the cisoid form. Virtually any type of chemical reaction may appear at some time in a chemical synthesis, and photochemical reactions are no different. It is hoped that the foregoing selection will give some idea of the variety and scope available to chemists prepared to use the technique of photochemistry—a range that is expanding at the present, perhaps even faster than the already rapidly growing chemical field.