Quantum chain, isomerization

Anthrylethylene derivatives displayed trans-to-cis isomerization from the singlet excited state as an adiabatic process (Scheme 9) [106,129]. The cis isomer formed adiabatically is shown to mvolve in energy transfer process to the ground state trans molecule leading to quantum chain isomerization process originating from the singlet excited state. Fluorescence, fluorescence lifetime [130], and... 

On the other hand, in the triplet state of 5b, t is the most stable and is in equilibrium with the slightly less stable p [25] therefore, the triplet state deactivates from both p and t, several thousand times faster from p (kjp = 10" s ) than from t (k, - 10 s ). The deactivation from p gives trans and cis isomers as the typical two-way mode, and that from t gives solely the trans isomer by either unimolecular deactivation or transferring energy to the cis isomer to regenerate c, leading to quantum chain isomerization of cis to trans isomers. The isomerization proceeds with the dual mechanism depicted in Scheme 3. 

The behavior of 20 and 21 contrasts sharply with that of 1,3-pentadiene and 2,4-hexadiene. Therefore, substitution of an anthracene nucleus on the diene can control the isomerization mode leading to a highly specific isomerization of very large quantum efficiency by completely suppressing the quantum chain isomerization from a trans to a cis double bond as observed in 2,4-hexadiene [82]. 

Platinum porphyrin complexes can be prepared by reaction with PtCl2(PhCN)2. Purification of the final complex is by medium pressure liquid chromatography on alumina. The strongly phosphorescent platinum(II) porphyrin complexes are efficient sensitizers for stilbene isomerization. The quantum yields for the cis to trans process are greater than unity because of a quantum chain process in which the metalloporphyrin serves both as an energy donor and an acceptor.1110 Picosecond laser spectroscopy has been used to obtain time-resolved excited-state spectra of platinum octaethylporphyrin complexes, and to probe the excited-state energy levels.1111 Tetrabenzoporphyrin complexes have been prepared for platinum in both the divalent and tetravalent oxidation states. The divalent complex shows strong phosphorescence at 745 nm.1112... 

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. 

The nitrone-oxaziridine ring-chain isomerization has attracted theoretical attention.The quantum yield for the reaction has been measured and the reaction has been examined in a rigid polymer matrix. ... 

The anthryl olefins (la- d) undergo solely cis-to-trans isomerization, and no reverse isomerization takes place at all. Also, surprisingly, the quantum yields of the cis to trans isomerization far exceed unity and increase with the cis-isomer concentration attaining 10-20 depending on the olefins and their concentration. Once a photon is absorbed, many cis-molecules isomerize into trans by a quantum chain process. Therefore, this reaction also can serve for amplification of the effect of photons. We named this type of reaction as one-way isomerization and proposed its mechanism. 

This chapter discusses the typical one-way isomerization taking place as a quantum chain process as observed in 2-anthrylethenes. 

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. 

Quantum chain reactions were already reported 20 years ago on the metallopor-phyrin sensitized isomerization of stilbene (1) and l-(l-naphthyl)-2-(4-pyridyl)-ethene (15) [94,95], In these cases the metalloporphyrin acts not only as a sensitizer but as a mediator of excitation energy. 

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... 

XIII. ISOMERIZATION OF POLYENES WITH A QUANTUM CHAIN PROCESS FOR SEVERAL DIRECTIONS... 

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. 

In the cis - trans one-way isomerization of anthrylethenes. Figure 2 illustrates a typical example of the concentration dependence of the quantum yields of cis - trans isomerization of 6a. increases with the increase in cis-isomer concentration [4,18-20]. The isomerization proceeds in the quantum chain mechanism described in Scheme 2, where S is the sensitizer triplet and the k s are the rate constants for the corresponding processes. 

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. 

More recently, Tokumaru and collaborators found that the O-methyl ether of an aromatic ketone oxime, 2-acetylanthracene oxime O-methyl ether, upon irradiation in benzene underwent one-way Z, -isomerization through a quantum chain process in the triplet state (Scheme 7). The kinetics of the triplet-state isomerization and the potential energy surface of the triplet state were studied by transient spectroscopy. A quantum yield as high as 22 at a concentration of 1.35 x 10 M was obtained. According to their calculation, the Tj potential-energy curves of the substrate are in agreement with the rotation mechanism for one-way isomerization. ... 

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