Photochemical reactions general principles

The photochemical reactions of organic compounds attracted great interest in the 1960s. As a result, many useful and fascinating reactions were uncovered, and photochemistry is now an important synthetic tool in organic chemistry. A firm basis for mechanistic description of many photochemical reactions has been developed. Some of the more general types of photochemical reactions will be discussed in this chapter. In Section 13.2, the relationship of photochemical reactions to the principles of orbital symmetry will be considered. In later sections, characteristic photochemical reactions of alkenes, dienes, carbonyl compounds, and aromatic rings will be introduced. 

How can these photochemical and electrochemical data be reconciled With the benzylic molecules under discussion, electron transfer may involve the n or the cr orbital, giving rise to stepwise and concerted mechanisms, respectively. This is a typical case where the mechanism is a function of the driving force of the reaction, as evoked earlier. Since the photochemical reactions are strongly down-hill whereas the electrochemical reaction is slightly up-hill at low scan rate, the mechanism may change from stepwise in the first case to concerted in the second. However, regardless of the validity of this interpretation, it is important to address a more fundamental question, namely, whether it is true, from first principles, that a purely dissociative photoinduced electron transfer is necessarily endowed with a unity quantum yield and, more generally, to establish what are the expressions of the quantum yields for concerted and stepwise reactions. 

We have emphasized that the Diels-Alder reaction generally takes place rapidly and conveniently. In sharp contrast, the apparently similar dimerization of olefins to cyclobutanes (5-49) gives very poor results in most cases, except when photochemically induced. Fukui, Woodward, and Hoffmann have shown that these contrasting results can be explained by the principle of conservation of orbital symmetry,895 which predicts that certain reactions are allowed and others forbidden. The orbital-symmetry rules (also called the Woodward-Hoffmann rules) apply only to concerted reactions, e.g., mechanism a, and are based on the principle that reactions take place in such a way as to maintain maximum bonding throughout the course of the reaction. There are several ways of applying the orbital-symmetry principle to cycloaddition reactions, three of which are used more frequently than others.896 Of these three we will discuss two the frontier-orbital method and the Mobius-Huckel method. The third, called the correlation diagram method,897 is less convenient to apply than the other two. 

In a thermal reaction R—>TS—>P, as shown in Figure 4.4, the transition state TS is reached through thermal activation, so that the general observation is that the rates of thermal reactions increase with temperature. The same is in fact true of many photochemical reactions when they are essentially adiabatic, for the primary photochemical process is then a thermally activated reaction of the excited reactant R. A non-adiabatic reaction such as R - (TS) —> P is in principle temperature independent and can be considered as a type of non-radiative transition from a state R to a state P of lower energy, for example in some reactions of isomerization (see section 4.4.2). 

These considerations illustrate the general tendency of S surfaces to have minima at unsymmetrical geometries. The T surface has a similar tendency, for different reasons, and it is probably fair to say that in photochemical reactions, the unsymmetrical reaction paths are the ones normally followed. In spite of this, most illustrations of potential energy curves in the literature and in this book are for symmetrical paths, since these are much easier to calculate or guess. It is up to the reader of any of the theoretical photochemical literature to keep this in mind and to correct for it the best he or she can, using the principles outlined in this chapter. 

Based on the fundamental considerations in Sections 1.3 and 1.4, the principle of quantum yield was introduced in Section 2.1.2 to allow a treatment of photochemical reactions in a way comparable to thermal reactions. The difference from thermal reactions has been demonstrated by taking account of the photophysical steps in Sections 2.1.3.3 and 2.1.4.3. These have to be considered in detail to find out whether the partial photochemical quantum yield depends on the intensity of the irradiation source or even on concentrations. Furthermore the definitions derived in Chapter 2.1 and summarised in Table 2.2 are used. In particular the definitions for the degrees of advancement for partial steps in general x, for photophysical steps in special x-, and for linearly independent steps x of the reaction procedure have to be remembered. 

In general absorbance varies during a photochemical reaction. Therefore within the rate laws either the concentrations have to be substituted by the absorbances or the absorbance at the wavelength of irradiation by the concentrations to be able to calculate the integrals, eqs. (3.43) and (3.44), respectively. In principle the relationship between absorbance and concentration is given by the Bouguer-Lambert-Beer law as derived in Section 1.4.3 by Fig. 1.2. For uniform reactions, absorbance at the wavelength of irradiation is defined in Section 1.4.4 by eq. (1.38) to... 

In principle spectroscopic methods have turned out to be preferable in kinetic analysis. Nowadays a large variety of spectroscopic methods exist which allow the progress of the photochemical reaction to be monitored. However, none of them perfectly satisfies under all conditions the requirements stated above. Even though it is difficult to state a general procedure, some generalised ideas are discussed next to find arguments for the selection of certain analytical tools. 

Two lines of inquiry will be important in future work in photochemistry. First, both the traditional and the new methods for studying photochemical processes will continue to be used to obtain information about the subtle ways in which the character of the excited state and the molecular dynamics defines the course of a reaction. Second, there will be extension and elaboration of recent work that has provided a first stage in the development of methods to control, at the level of the molecular dynamics, the ratio of products formed in a branching chemical reaction. These control methods are based on exploitation of quantum interference effects. One scheme achieves control over the ratio of products by manipulating the phase difference between two excitation pathways between the same initial and final states. Another scheme achieves control over the ratio of products by manipulating the time interval between two pulses that connect various states of the molecule. These schemes are special cases of a general methodology that determines the pulse duration and spectral content that maximizes the yield of a desired product. Experimental verifications of the first two schemes mentioned have been reported. Consequently, it is appropriate to state that control of quantum many-body dynamics is both in principle possible and is... 

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