Photoinduced thermal reactions

The typical results reported in this chapter, clearly demonstrate how the lifetime of excited states and the low-spin/high-spin character of such states can be tuned by pressure. Furthermore, photochemical bond formation and cleavage processes are accelerated or decelerated by pressure, respectively, in a similar way as found for the corresponding thermal reactions. As a result of this, the associative or dissociative nature of such substitution reactions can be characterized. A further characterization of the intimate nature of the reaction mechanism can also be obtained for photochemical isomerization and electron-transfer reactions as reported in Sections V and VI, respectively. The same applies to photoinduced thermal reactions, where the interpretation of the pressure dependence is not complicated by photophysical relaxation processes. The results for the subsequent thermal reactions can be compared with a wealth of information available for such processes [1-6]. Especially the construction of reaction volume profiles has turned out to be a powerful tool in the elucidation of such reaction mechanisms. 

Although the most numerous investigations of dissociative electron transfer have concerned thermal reactions, photoinduced dissociative electron transfer has also attracted a great deal of recent theoretical and experimental attention. As discussed in Section 6, one of the key questions in the field is whether photoinduced dissociative electron transfers are necessarily endowed with a unity quantum yield as one would predict on purely intuitive grounds. Quantum yield expressions for the concerted and stepwise cases are established and experimental examples are discussed. 

The thermal Diels-Alder reactions of anthracene with electron-poor olefinic acceptors such as tetracyanoethylene, maleic anhydride, maleimides, etc. have been studied extensively. It is noteworthy that these reactions are often accelerated in the presence of light. Since photoinduced [4 + 2] cycloadditions are symmetry-forbidden according to the Woodward-Hoffman rules, an electron-transfer mechanism has been suggested to reconcile experiment and theory.212 For example, photocycloaddition of anthracene to maleic anhydride and various maleimides occurs in high yield (> 90%) under conditions in which the thermal reaction is completely suppressed (equation 75). 

Puranik and Fink found that the low-temperature photoinduced rearrangement of a sily-lene to a silacyclobutadiene is reversible in a thermal reaction at 160 K (equation 54)107. The silylene persists in fluid solution at 200 K ... 

The Diels-Alder reaction is a well-established synthetic method that allows the creation of two new carbon-carbon bonds and leads to the formation of six-membered rings. Eventually, the photochemical reaction can advantageously compete with the thermal process. For instance, anthracene undergoes thermal and photochemical Diels-Alder reactions with alkenes, but the photoinduced addition of maleic anhydride to the homochiral anthracene, as depicted in Scheme 9.28, is faster than the thermal reaction and occurs with excellent diastereoselectivity (only one diastereoisomer) [42]. 

The photochemical protocol may be the method of choice in cases where the thermal reaction fails. This is true for exo-alkylidene oxacyclopentylidene chromium complexes such as 62, which are inert under thermal conditions but undergo a photoinduced benzannulation with, for example, 3-hexyne to give benzofuran 63 (Scheme 24) [61]. 

There are three principal modes of ET, namely, thermal, optical and photoinduced ET, and these are shown schematically in Fig. 1. Optical ET differs from photoinduced ET in that ET in the former process results from direct electronic excitation into a charge transfer (CT) or intervalence band, whereas photoinduced ET takes place from an initially prepared locally excited state of either the donor or acceptor groups. Photoinduced ET is an extremely important process and it is widely studied because it provides a mechanism for converting photonic energy into useful electrical potential which may then be exploited in a number of ways. The most famous biological photoinduced ET reaction is, of course, that which drives... 

In neutral solution, in contrast, phenol nitration and nitrosation are pho-toinduced processes since no thermal reaction has been observed between phenol and nitrite ion. The pH value where the thermal and photoinduced processes have similar importance is around 5.5. Thermal processes prevail at lower pH and photoinduced ones at higher pH [55,62]. 

Aromatic substrates are by far the most commonly used substrates in the rapidly expanding area of photoinduced electron transfer [1,2]. This is obviously due to the favourable location of the frontier molecular orbitals in such compounds. The same factor facilitates the formation of electron transfer donor-acceptor (EDA) complexes both in the ground state (these possibly are intermediates in some thermal reactions, e.g. selected electrophilic substitutions), and in the excited state (exciplexes). 

However, very efficient deracemization was reported in tris(4,4 -dimethyl-2,2 -bipyridine)iron(II), [Fe(4,4 -Me2bpy)3]2+, with tris(trichlorobenzenedi-olato)phosphate (trisphat), where trisphat is shown in Scheme 16 [42]. Though this is not a photoreaction but a thermal reaction, we briefly discuss this result because this deracemization is very interesting and provides an interesting idea of photoinduced deracemization. 

Recendy, photopolymer systems have aroused increased interest because of their manifold applications in several high technologies [1-3]. Among such systems, those derived from photoinduced polymerization play an important role. The fundamental principles of these systems are based on the production of species X by photoreactions, which then initiates thermal reactions of low-molecular products leading to polymer or network formation see Eq. (1). In general, these thermal reactions are associated with low activation energies (about 60 kJ mol 1 for free radical chain polymerization). Therefore, such processes can also occur suffidentiy fast at room temperature. 

The time scale of this delay may be very different depending on the nature of the photoinduced phenomenon. For example, it may be nanoseconds for fluorescence emission, hours for a secondary thermal reaction. 

Although thermal and photoinduced SET reaction products are frequently identical due to converging reaction pathways [12], there are cases where synthetically useful differences can be observed [38] which can be related to different spin states of the intermediate radical pair. 

Inoue and Endicott 135) have studied the photochemical properties of [Rh(C2H5)(NH3)5] in aqueous solution. The rra/w-[Rh(C2H5)(OH2) (NH3)4] complex was examined in greater detail because of complicating thermal reactions with [Rh(C2H5)(NH3)5]. The experimental evidence was consistent with photoinduced homolytic cleavage of the Rh—C2H5... 

The electronic transition to the MLCT and nn excited states are optically allowed transitions, and they have relatively large transition moments. They do not involve the population of an orbital that is antibonding with regard to the M—L bonds, in contrast to the forbidden transitions to the LF excited states. This is one of the reasons for photostability of Red) complexes. However, as discussed in the following sections, photoinduced chemical reactions have been reported in some cases, where transitions to reactive higher-energy states arise from photoexcitation with shorter wavelength irradiation or thermal activation from lowest excited state. 

Figure 22 includes the temperature dependent polymerization rates (1), (2) and (3). The thermal polymerization kinetics (1), they — (2), and the UV photopolymerization kinetics (3) have been investigated by the method of diffuse reflection spectroscopy and other methods The activation energy of the thermal reactions (2) and (3) following the photoinduced dimerization processes, (150 + 30) meV, is appreciable lower than those of the dimer DR intermediates. However, the processes which dominate the polymerization reaction are determined not by the short diradicals with n 6 but by the long chains with n 7, which all have a carbenoid DC or AC structure. The discrepancy of the activation energies therefore may be due to the different reactivities of the diradical and carbenoid chain ends. The activation energies of the thermal addition reactions of the AC and DC intermediates at low temperatures have not been determined and therefore a direct comparison with those of the diradicals is not possible. 

This review illustrates the above delineated characteristics of electron-transfer activated reactions by analyzing some representative thermal and photoinduced organometallic reactions. Kinetic studies of thermal reactions, time-resolved spectroscopic studies of photoinduced reactions, and free-energy correlations are presented to underscore the unifying role of ion-radical intermediates [29] in—at first glance—unrelated reactions such as additions, insertions, eliminations, redox reactions, etc. (Photoinduced electron-transfer reactions of metal porphyrin and polypyridine complexes are not included here since they are reviewed separately in Chapters 2.2.16 and 2.2.17, respectively.)... 

Photochemical reactions involving photo-excited states can also be catalyzed as well as the thermal reactions of ground states. However, the lifetimes of excited states are usually very short, particularly for the singlet excited states, and accordingly reactions of the excited state should be fast enough to compete with the decay of the excited state to the ground state (typically the lifetime is 10" -10 " s). Hence there seems to be little chance of catalysis to accelerate the reactions of excited states, which are already fast. There are many cases, however, such that photochemical reactions can be accelerated by some added substances which act as catalysts in the photochemical reactions [62-65]. Photoinduced electron transfer reactions can also be accelerated by the presence of an appropriate catalyst [52]. 

Other photoinduced cyclization reactions can occur by conrotatory bond formation to give the 9 PjIO P-antiisomers, isopyrocalciferol2 [474-70-4] (23) or isopyrocalciferol [10346-44-8] (24) (Fig. 5), whereas thermal cyclization at >100 C leads to the two 9,10-syn isomers, (9a,10 a)-pyrocalciferol (27)... 

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