Cis-Trans Isomerization of Stilbene

The direct photolysis of cis- or tronj-stilbene in solution gives rise to photoisomerization. In addition, dihydrophenanthrene is formed from the cis isomer  

The equilibrium concentration of dihydrophenanthrene under oxygen-free conditions is very small. However, this reaction, in the presence of O3 and I2, has been developed to the point where it is a good method for ring formation. 

The mechanism for the direct photodiemical cis-trans isomerization of stilbene has been a highly controversial subject. However, a recent review by Saltiel and co-workers greatly helps to clarify this area of research by painting a detailed and beautifully consistent picture. We will make extensive reference to this review.  

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The cis-trans isomerization of stilbenes is technically another photochromic reaction (18). Although the absorption bands of the stilbene isomers, occur at nearly identical wavelengths, the extinction coefficient of the lowest energy band of cis-stilbene [645-49-8] is generally less than that of stilbene [103-30-0],... 

Most of the olefins shown so far, for which reversibility of the bromonium ion formation had been demonstrated, are particular olefins, in which either steric bulk impedes the product forming step, or ring strain in the dibromide product retards this step. In order to check the general occurrence of the reversibility during the bromination reaction, a further approach, based on the cis-trans isomerization of stilbene derivatives during the bromination of the cis isomers, was devised. 

The previous sections have shown that cis-trans isomerization of stilbene can take place via the lowest triplet state of stilbene. The question to be considered now is whether the isomerization upon direct photolysis takes place via the singlet state, the triplet state, or a vibrationally excited ground state.a 7 81 50)... 

Evidence that eliminates the triplet mechanism as the mode for the cis-trans isomerization of stilbene upon direct photolysis has been provided by azulene quenching studies.(48) Using the experimentally determined decay ratio a/(l — a) and the triplet mechanism, it is possible to calculate what the effect of azulene is upon the pss. The predicted and observed azulene effects on the direct photoisomerization are shown in Figure 9.6. The failure of the triplet mechanism in predicting the very small changes observed in the pss provides a crucial test that is the basis for rejecting the triplet mechanism. 

The effects of nitro substituents on the cis-trans isomerization of stilbenes has been reviewed70 (equation 63). The trans-to-cis isomerization occurs from a triplet excited state, whereas the reverse cis-to-trans isomerization occurs through a main route which bypasses the triplet state. A nitro substituent usually causes a significant enhancement of the quantum yield of the intersystem crossing. Nitro substituent effects on the photoisomerization of trans-styrylnaphthalene71 (equation 64), trans-azobenzenes72 and 4-nitrodiphenylazomethines73 (equation 65) have been studied for their mechanisms. 

It was suggested by Gegiou et al. that the photoisomerization of azobenzene probably proceeds via a mechanism different than the cis-trans isomerization of stilbene. Such an isomerization process in azobenzene could involve a pyramidal inversion of a nitrogen atom, in contrast to stilbene and its derivatives, where rotation about the central double bond is required. Based on the data, the AT->AC isomerization mechanism may be similar to that of the triphenylmethane dyes, whose ground state structure is known to resemble a three-dimensional, propeller-shaped D3 structure with the phenyl rings rotated 32 degrees from the central plane. 

The cis-trans isomerization of stilbene (4) has been thoroughly studied. The potential-energy diagram for a rotation about the CC double bond that preserves a twofold symmetry axis is shown in Figure 7.3. 

Figure 7.3. a) Schematic state correlation diagram for the cis-trans isomerization of stilbene along the symmetric path (by permission from Orlandi and Siebrand, 1975) b) modified Orlandi-Siebrand diagram (by permission from Hohineicher and Dick, 1984). 

On direct irradiation the formation of dihydrophenanthrene (5, DHP) accompanies the cis-trans isomerization of stilbene. Thus, one has... 

Using azulene as a triplet quencher it has been shown that triplet states are not involved in the cis-trans isomerization of stilbene on direct excitation. Triplet-sensitized cis-trans isomerization, however, is observed and proceeds in both directions through a minimum in the triplet potential energy surface at a twisted geometry, often referred to as the triplet phantom state P. ... 

De Mayo and Hasegawa have studied the cis -trans -isomerization of stilbenes (23) mediated by a CdS semiconductor. ... 

Pico second spectroscopic investigations of cis -trans -isomerization of stilbene and the derivative (25) have demonstrated the presence of an intermediate which is involved in the isomerization. cis -trans -Isomerization of several pyridinium analogues of stilbene have been studied. The salts (26) also undergo singlet state trans -cis -isomerization on irradiation. A detailed mechanistic examination of these systems has been carried out and a physical study of the influence of solvent and quencher on the decay of the excited singlet state of the stilbene derivatives (27) has been reported. The cis -trans isomerization of bis -heteroaryl ethenes has been reported in detail. ... 

Figure 1 Potential energy surfaces of cis-trans isomerization of stilbene (1).

On benzopheneone sensitization the quantum yields of cis-trans isomerization of stilbene, d>c, t and t- c. were 0.55 and 0.38, respectively, and the value (1-45) ([c]/[t])pss ( 1.48) [77,78]. Detailed experiments revealed that with sensitizers having excitation energies much higher than those of trans-and cw-stilbene there is a variation in the photostationary state isomer composition as well as in the quantum yields of Isomerization ([c]/[t])psj varies from... 

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. 

Bohning and Weiss 3°) have suggested that cis-trans isomerization of stilbenes may proceed via an intermediate addition product (vide infra) and not by energy transfer. The results obtained in the biacetyl-anthracene system are not completely clear Backstrom and Sandros reported isolation of anthracene dimer while Dubois and Behrens 45> suggested that a 1 1 adduct of unspecified structure is formed. 

The thermal cis - trans isomerization of stilbenes has been reviewed by Cundall [65] and Laidler and Loucks [66]. For the energy barrier between cis- and trans-stilbene ground states values of 172-193 kJ mol -1 have been determined in the gas phase (Table 3a). For liquid solutions an activation... 

Other quenchers that have been used in the benzophenone-sensitized cis trans isomerization of stilbene are /9-carotene, oxygen, and di-tert-butyl nitroxide. /9-Carotene shifts the photostationary state to the trans side similar to the azulene effect [237]. In the presence of di-tert-butyl nitroxide, a radical quencher, the photostationary state is slightly shifted to the cis side Caldwell and Schwerzel [226] have suggested the involvement of the twisted triplet state and a quenching mechanism other than energy transfer, probably vibrational relaxation to the ground state caused by spin exchange. 

Using sealed-tube sample holders, Santoro and co-workers (32-35) investigated a wide variety of organic reactions. Examples are the cis - trans isomerization of stilbene and oleic acid, polymerization of styrene, Diels-Alder reactions, and others. Unstable intermediates in an organic reaction have been detected using DTA techniques by Koch (36). If a solution of an unstable compound is heated, temperature changes characteristic of reactions of the intermediate can be detected. Conversely, the absence of thermal effects indicates that no unstable product is present. 

By monitoring the fluorescence generated by two consecutive picosecond light pulses it has been possible to measure rate constants for the cis trans isomerization of stilbene. The main reaction occurs in the singlet manifold via a perpendicular singlet state with lifetime shorter than a few picoseconds. 

The vexed question of the mechanism of the direct and sensitized cis-trans isomerization of stilbene has been the subject of recent papers.546 The studies of the two groups differ in the favoured mechanism for the direct cis-trans isomerization, one proposing the conventional triplet-state mechanism, the other plausibly invoking the intervention of a doubly excited singlet potential surface with a deep potential minimum at a 90° configuration. 

Most of the papers in this section are concerned with photo-oxidation reactions, but a few papers refer to photochemical reactions in polymer matrices. Thus the kinetics of the photo-oxidation of anthracene and naphthacene in solid polystyrene (PS),170 the photoionization of aromatic hydrocarbons dissolved in PMMA and PS,171 the photoreactions of naphthalene in cellulose triacetate,172 the cis-trans isomerization of stilbene residues in the side-chains of polymers,173 intrachain photodimerization in polymers,174 photoisomerization of 1,2-diphenyl-cyclopropane by peptides containing naphthalene in a side-chain,176 and photochemical transformations of poly(vinyl p-azidobenzoate)176 have been reported. 

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