Quantum chain process

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. 

K.K., Veerman, M., Mortko, C.J., and Garcia-Garibay, M.A. (2008) Solid state photodecarbonylation of diphenyl-cyclopropenone a quantum chain process made possible by ultrafast energy transfer. Journal of the American Chemical Society, 130, 1140-1141. 

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. 

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... 

Figure 5 Effect of additive on a quantum chain process. (Adapted from Ref. 97.)...

The conditions necessary to observe a quantum chain process (a quantum gain) in solid matrices using the domino mechanism were reported by Ebbesen et al. to find new application of the amplification of the effect of photon in molecular optics [99]. The quantum yield of reaction from A to B (Oq) in solution is described as Eq. (4), where O is the quantum yield of production of the reactive excited state and k, and ki are the rate constants of energy transfer and unimolecu-lar deactivation, respectively. Since the molecular diffusion is slow in the solid matrices, the equation is modified by using Perrin s equation as shown in Eq. 

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. 

The text of a lecture on the benzophenone-sensitized c/j-Zraws-photochemistry of penta-1,3-diene and hexa-2,4-diene has been published.80 A quantum-chain process is described. Wrighton and Schroeder61 have reported the photochemical hydrosilylation of dienes by irradiation in the presence of Cr(CO) and a silane. The following example is typical ... 

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]. 

The quantum chain process in the photoisomerization of the C=C double bond was first reported by Hammond et al. [80a], followed by Hurley and Testa [80b] and Caldwell [81]. Subsequently, the former work [80a] was shown by Saltiel [82] to be erroneous. Thus 1,3-pentadiene and... 

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