Isomerization of stilbene

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],... 

Fig. 13.9. Schematic reaction profile for the EZ isomerization of stilbene. The reaction coordinate 6 is the torsion angle about the double bond. [From FI. Meier, Angew. Chem. Int. Ed. Engl. 31 1399 (1992)]. Reproduced by permission of Wiley-VCH.

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 mechanism for the direct photochemical 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.U)... 

If a common intermediate were not involved in the triplet state isomerization of stilbene, the sum of d>c T and T-.0 could have any value from zero to two. 

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)... 

Figure 9.3. Variation of the stationary states in photosensitized isomerization of stilbenes with various sensitizers. [From G. S. Hammond, Kayaku to Kdgyd (Tokyo) 18, 1464 (1965).]...

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. 

Michael Faraday reported in 1821 that chlorine addition to alkenes is Stimulated by sunlightand today this is taken to indicate the involvement of a free radical process (equation 26). Free radical chain mechanisms were proposed in 1927 by Berthoud and Beraneck for the isomerization of stilbene catalyzed by Br2 (equation 27), and by Wachholtz for bromine addition to ethyl maleate (equation 28).Later studies showed inhibition of halogen addition by reaction of the intermediate radicals with oxygen, and a free radical chain mechanism for solution and gas phase halogenations as in equation (26) was shown (equation 29). Kinetic and mechanistic... 

In the initial work135,136 on the sensitized isomerization of stilbene, it was found that quinone sensitizers gave photostationary states which were trans-rich relative to those predicted by energy transfer (Fig. 3). The original plot of sensitizer triplet energy versus isomer ratio of the photostationary state therefore showed several maxima and minima. Later work on the rates of energy transfer from various sensitizers to stilbene proved that the plot was in fact a smooth curve and that the photostationary states observed with quinones were not the true ones.137 Irradiation of benzene solutions of... 

Fig. 3. Photostationary states obtained in the photosensitized isomerization of stilbene as a function of triplet energy of the sensitizer. (From Wagner and Hammond.1)...

A particularly interesting case of transfer from T2 has been noted238 in the 9,10-dichloroanthracene (DCA) sensitized isomerization of stilbene. As shown below, the DCA ground state which results from quenching of T2 by stilbene (St) can act as a quencher of stilbene triplets and thereby alter the stilbene cis. trans photostationary state. 

In an attempt to probe further into the complex photochemical system involved, the isomerization of stilbenes adsorbed on silica gel was examined.9 It was observed that the time required for establishment of the photostationary state was significantly increased (by a factor of 3) and that the composition of the photostationary state changed from 93y cis isomer in cyclohexane solution to 60% cis isomer in the silica gel matrix. Though not definitive, this evidence supports Fischer s triplet mechanism, 53 as we have previously reported.9... 

Microscopic Friction and Solvation in Barrier Crossing Isomerization of Stilbene in Size-Selected Hexane Clusters, A. A. Heikal, S. H. Chong, J. S. Baskin, and A. H. Zewail, Chem. Phys. Lett. 242, 380 (1995). 

SCHEME 1. The effect of the triplet excitation energy of the sensitizer on the Z/E isomerization of stilbene at the photostationary state. Reprinted with permission from Ref. 7. Copyright (1964) American Chemical Society... 

Extensive investigations on the Z E isomerization of stilbenes have revealed that stilbene undergoes mutual isomerization in the singlet or triplet manifold on direct or... 

Figure 2.20 Potential energy diagram illustrating the room-temperature photoinduced isomerization of stilbene in solution wavy lines represent non-radiative transitions, curved arrows isomerizations and straight lines radiative transitions...

Polo AS, Itokazu MK, Frin KM, Patrocinio AOT, Iha NYM. Light driven trans-to-cis isomerization of stilbene-like ligands in fac-[Re(CO)3(NN)(trans-L)]+ and luminescence of their photoproducts. Coord Chem Rev 2006 250 1669-80. 

Cyclic conjugated earbanions have also been studied and rarely found to give an electron transfer photochemistry in the absence of an electron acceptor. One example was however found in the case of cyclooctadienyl carbanion irradiated in THF and in the presence of chlorobenzene or rrans-stilbene. Dimeric radical products and phenylated cyclooctadiene were formed in the first case while trans-cis isomerization of stilbene was observed in the second one... 

Concerning the 77 dependence of the rate constant of solution reactions, most often investigated experimentally is photoinduced EJZ (trans/cis) isomerization of stilbenes. The isomerization takes place by surmounting over a transition-state barrier on the excited-state potential surface. The reaction is very fast with a rate constant on the order of 10 s- due to a small height (of about 15 kj/mol) of the barrier. The rate constant observed in solvents decreases as 77 increases. To be more exact, however, k often decreases more slowly than 77 , describable by a fractional-power dependence on r/ as kgf, 77- with 0 < a < 1. Therefore, the 77 dependence of k ), deviates from that expected from the Kramers theory. 

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. 

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