Triplet chain process

The latter activation barrier is much more compatible with the measured rate constant for photoracemization at room temperature (kr = 2.4x10 s-1 (46)) than the former which has been selected by Irie et al. (45). To complicate matters further, a triplet chain process dominates at higher concentrations of BN... 

In comparison to the conventional two-way isomerization (Z E) of stilbene and other aryl alkenes 12 (Scheme 5), a novel one-way isomerization (Z - E) of C=C double bonds was achieved upon replacing a phenyl group of stilbene by a 2-anthryl group9a,c. Tokumaru and coworkers found in isomerization studies on substituted anthracenes9 that substitution at the C=C bond resulted in complete isomerization of the Z-isomer to the corresponding /i -isomer upon irradiation, via a quantum chain process. Interestingly, the isomerization takes place as an adiabatic process in the triplet manifold on both direct and triplet sensitized irradiations. 

MgR, R = Et, iso-Pr, tert-Bu, were found to rapidly insert 2 into the Mg-C bond by a mechanism which is likely to involve initial (spin-allowed) SET, Mg-C bond cleavage and peroxymetal radical formation as initiation sequence and subsequent radical activation of 02 within a radical chain process (Eq. (6)) [11,47], The direct ( concerted ) insertion of ground-state 62 into M-C bonds is considered to be unlikely since the primary products would be formed in a high-energy triplet state [47]. 

Usually, photoinitiating systems based on aromatic ketone/tertiary amine combinations are more active in the polymerization of unsaturated monomers compared with ketones alone [37], particularly in the air, not only because the formation of exciplexes competes efficiently with the quenching of ketone triplet state by oxygen, but also because amines behave as oxygen scavengers through a chain process [34] (Scheme 8) ... 

Table 2 summarizes the substituent effects on the mode of cis-trans isomerization in the triplet state [33,41,52-74]. Roughly speaking, the triplet energy of the aryl substituent on the ethylenic carbon controls the mode of isomerization. Thus, olefin substituted by an aryl group with a very low triplet energy tends to undergo one-way cis trans isomerization in the excited triplet state with a quantum chain process. 

The quantum chain process is not limited to the one-way isomerizing olefins but could be observed in olefins two-way isomerizing in the triplet state where the energy minimum exists at t. 6a, 7b, and 8b undergo two-way isomerization in the triplet state [63-66], However, the quantum yields of cis trans isomerization of 6a, 7b, and 8b increase with increasing cis isomer concentration and exceed unity the value of 8b was reported to be 42 on benzU sensitization at [cis-8b] = 2.7 X 10 M [66], Here are mentioned 1.) porphyrin-sensitized isomerization of stilbene and 2.) effects of additives on the quantum chain process of styrylstilbene. 

When porphyrins with much higher triplet energies such as palladium octaethylporphyrin (17 Et = 44.8 kcal mol" ) were used as sensitizers, even the cis trans isomerization of stilbene took place as a quantum chain process = 1-6) [95]. The high quantum efficiencies were explained by a quantum chain process in which the metalloporphyrin serves as both an energy donor and an acceptor. Since the quantum yield of cis trans isomerization of 1,2-diphenylpropene = 0-37) remained as a normal value under the same experimental conditions as those of stilbene, the potential energy surface of the triplet state is an important factor for occurrence of the quantum chain cis-trans isomerization. That is, in 1,2-diphenylpropene the triplet state exists exclusively as a perpendicular conformation, where the triplet state and the ground state lay very close in energy and the deactivation can only take place thermally. 

In order to increase the quantum yield of cis -> trans isomerization, it is proposed to use aromatic hydrocarbons such as anthracene as a carrier of the quantum chain process. (Z,E)-(cis-18) and (E,E)-l,4-di(3,5-di-tert-butylstyryl) benzene (trans-18) underwent mutual isomerization on biacetyl sensitization with a quantum chain process to afford a 4>c >t value of 1.2-1.3 in a cis isomer concentration of 1 X 10 M [96]. The quantum chain process proceeds through the energy transfer from the trans triplet (H ) to the ground state cis isomer ( c). On addition of anthracene (AN) as a quencher or a carrier of the excited state, the quantum... 

The quantum chain process can also take place by quenching of the H by anthracene to produce the ground state trans and the triplet state anthracene... 

The quantum chain process has also been observed in benzophenone-sensitized isomerization of 2,6-dimethyl-2,4,6-octatriene (25) [120]. The effect of azulene on the photostationary state isomer composition suggests that both tt and tc are the stable conformers in the triplet state and are equilibrated. However, equilibration of all excited intermediates is not complete within the lifetime of the excited triplet state ( 50 ns). The triplet lifetime of 1,3,5-hexa-triene (26) is reported as 100 ns (Table 5) [121]. 

Hie isomerization of 31 proceeds by a quantum chain process for several directions [125,126,130]. For example, all-trans retinal undergoes isomerization giving the 9-cis and 13-ds isomers in the excited triplet state the quantum yield of isomerization of the all-trans isomer increased from 0.13 at 1.1 X 10 M to... 

The cis-trans isomerization of various olefins in the triplet state was reviewed. The mode of cis-trans isomerization is correlated to the triplet energy of the aryl substituent (ArH), since only the planar triplet energy decreases with decreasing t of ArH. In addition, the energy minima as well as the stable conformations in the triplet state were discussed based on the triplet lifetime, the efficiency of isomerization, and the quenching experiments. Several examples of quantum chain process in olefins and polyenes were also described. 

Table 6 summarizes the features of one-way and two-way isomerization of arylethenes depending on the aryl substituents. Thus, the typical features of one-way and two-way isomerizations shown in Table 1 are modified, but oneway isomerization does not necessarily accompany the quantum chain process and two-way isomerization does not necessarily involve the diabatic process. For some compounds two-way isomerization can take place as an adiabatic process by mutual conversion of c and H followed by deactivation to the ground state. The short triplet lifetime due to the heavy atom effect may cause a very inefficient one-way isomerization. Furthermore, the quantum chain processes can be observed in many compounds when proper experimental conditions can be established. 

Olson [8-10] was first to postulate that photochemical trans-cis isomerization in olefins may be an adiabatic reaction. Hammond and co-workers [121] and others [122-124] reported the quantum chain cis-to-trans isomerization process from the triplet excited state of olefin. They postulated the energy transfer process from the trans isomer adiabatically formed to cis isomer in the ground state. Later Arai and Tokumaru [94] with their extensive investigations showed that cis-to-trans adiabatic photoisomerization resulted in quantum chain process from the triplet excited state in a variety of olefins. Nevertheless, the adiabatic photoisomerization in olefins originating from the singlet excited state is less studied and the same is highlighted. 

Similarly, photogeneration of other reducing radicals results in reduction of diaryliodonium salts. Hydrogen abstraction from ethers and alcohols by ketone triplets [97], formation of 1,4-biradical by reaction of ketones with unsaturated compounds such as acrylates [97,103], all yield radicals which reduce diaryliodonium salts. Hydrogen atom abstraction from ethers and amines provides a chain process for iodonium salt decomposition (Scheme 6), wherein quantum yields as high as 5 have been reported [60a]. 

It is to be noted that the chain carrier in the quantum chain process is the excited triplet state of the product of the reaction, t. ... 

The quantum yields of the cis trans isomerization of 8b in cyclohexane increased from 0.22 to 0.87 with increased cis-9b concentration from 1.7 X 10 to 4.9 X 10 M. Accordingly, the one-way cis -> trans isomerization also proceeded in the triplet manifold after intersystem crossing. At infinite dilution, where the quantum chain process did not work in the excited triplet state, 0c- t was estimated as 0.20. This value gave the ratio of the one-way isomerization via excited singlet and triplet states in dilute solution as 0.03-0.17. Figure 6 depicts the potential energy surfaces of the one-way isomerization of 8b in the excited singlet and triplet states. 

As described earlier, on benzil sensitization, 1-styrylpyrene (5b) undergoes two-way photoisomerization in the excited triplet state with a quantum chain process in the cis trans direction. However, on direct excitation, one-way cis trans isomerization takes place on the excited singlet surface as an adiabatic conversion from c to t as reported by Mazzucato et al. [77a]. 

Similar to 9-styrylanthracenes (17), introduction of polar substituents on the benzene nucleus of 1-styrylpyrene as well as use of polar solvents alters the isomerization mode. Thus rrans-l-[2(4-nitrophenyl)ethenyl]pyrene(18a) is difficult to isomerize from the trans to the cis isomer in acetronitrile on direct irradiation (0t-.c < 10 ) [7 ], although 18a, 18b, and 18c undergo mutual isomerization with a quantum chain process in the cis- trans direction on triplet sensitization [79]. 

As the origin of the occurrence of the quantum chain process in the isomerization of the dienes and trienes above, energy transfer from their excited triplet states to their ground states of the s-cis form was proposed. Compared to s-trans rotamers, s-cis rotamers have lower triplet energies and are more readily excited, although less populated in the ground state. For example, the following reaction may proceed [82] ... 

DiphenyIbutadiene (19) undergoes ZZ- ZE and ZZ->EE one-way photoisomerization in the triplet state in a quantum chain process [94]. On the other hand, ZE and EE isomers undergo mutual isomerization. However, ZE - EE isomerization proceeds in a quantum chain process, since its quantum yield increases linearly with the total concentration of 19 on 9-fluorenone sensitization. On 9-fluorenone sensitization, all the isomers exhibited the same T-T absorption = 390 nm, Tt 1.6/is) attributable to EE at 1 /is after the laser pulse [94,95]. Furthermore, the photostationary mixture becomes richer in EE with increased initial ZE concentration. Therefore, the isomerization between ZE and EE takes place similarly to that of 5b with a dual mechanism. Thus the triplet state of 19 is composed of EP and EE in equilibrium, where EP means the perpendicular geometry at one double bond and E geometry at the other double bond of the diene (Scheme 5) unimolecular deactivation from EP gives EE and ZE, and bimolecular deactivation from EE with ZE isomer gives solely EE. 

The types of compounds used as antioxidants can be used in such studies, but there are other substances that act as inhibitors. Molecular oxygen inhibits many free-radical chain processes. The triplet oxygen molecule is extremely reactive toward many organic radicals ... 

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