Trans-stilbene irradiation

When either the cis or the trans isomer of an alkene is irradiated, a mixture of both isomers will be formed in a particular ratio, which is dependent on the wavelength of light used. For example if either trans-stilbene or cis-stilbene is irradiated at 313 nm, the final composition of the reaction mixture will consist of a mixture of 93% cis and 7% trans (Figure 8.2). 

Figure 8.2 Product of irradiation of either trans-stilbene or cis-stilbene at 313nm...

The stilbenes have played a crucial role in the development of modern photochemistry. Direct or triplet sensitized irradiation of trans-stilbene (t-1) in dilute solution results in isomerization to cis-stilbene (c-1) as the exclusive uni-molecular photochemical reaction (1-3). Direct irradiation of c-1 results in isomerization to both t-1 and trans-4a,4b-di-hydrophenanthrene (2), which revert to c-1 both thermally and photochemically and can be trapped by oxidants such as iodine or oxygen to yield phenanthrene (3) (4-6). Triplet sensitized irradiation of c-1 yields only t-1. These unimolecular isomerization pathways are summarized in eq. 1. 

Meier et al. described hydrocarbon dendrimers with trans-stilbene chromo-phores in the core and in the periphery. Owing to the flexible nature of the arms intra- and intermolecular C-C bonds could be formed by irradiation [28] (see Section 4.1.5.3 for further information). 

Photosensitized electron transfer reactions conducted in the presence of molecular oxygen occasionally yield oxygenated products. The mechanism proposed to account for many of these reactions [145-147] is initiated by electron transfer to the photo-excited acceptor. Subsequently, a secondary electron transfer from the acceptor anion to oxygen forms a superoxide anion, which couples with the donor radical cation. The key step, Eq. (18), is supported by spectroscopic evidence. The absorption [148] and ESR spectra [146] of trans-stilbene radical cation and 9-cyanophenanthrene radical anion have been observed upon optical irradiation and the anion spectrum was found to decay rapidly in the presence of oxygen. 

Temperature in the photoreactor was controlled to +o.l°C by immersion in a constant temperature Dewar flask. For experiments using cyclohexane as the solvent, the reactor was filled with a solution of trans-stilbene, loosely stoppered, and, after reaching the desired temperature, irradiated. Periodically, aliquots were withdrawn and analyzed by gas chromatography. 

In nonpolar solvents, the cyclobutane 44 is the only product upon irradiation of trans-stilbene/dimethyl fumarate mixtures. Exciplex emission, the detection of a weak ground state complex, as well as the stereospedfity of the cyclobutane formation support the proposed mechanism outlined in Eq. (26). 

Irradiation of trans-stilbene in the presence of 2-chlorobenzothiazole leads to a variety of products among which are heteroarylated stilbenes and 9-heteroarylated phenan-threne520. 

Although Ceo is easily reduced, it is very difficult to oxidize [46, 53, 54, 72], The only definitive electrochemical oxidation of Cgo occurs at a potential of -F 1.76 V vs SCE in benzonitrile, and is irreversible [54]. The radical cation was reported to be produced by y-irradiation at 77 K in a glass, and to absorb near 980 nm [65, 66]. Attempts to generate the radical cation (Ceo) by electron transfer to singlet excited dicyanoanthracene, which has a reduction potential near + 2.0 V [73] were unsuccessful. This method has been used to generate, for example, trans-stilbene radical cation [73, 74]. The ion pair probably does not... 

The photo-induced electron transfer of l,4-bis(methylene)cyclohexane in acetonitrile-methanol solution with 1,4-dicyanobenzene (DCB) affords two products, both consistent with nucleophihc attack on the radical cation followed by reduction and protonation or by combination with DCB ). In the absence of a nucleophile, the product mixture is highly complex, as is the case imder electro-oxidative conditions. Under UV irradiation, trans-stilbene imdergoes dimerization and oxygenation (to benzaldehyde) by a single-electron mechanism in the presence of a sensitizer such as 2,4,6-triphenylpyrilium tetrafluoroborate (TPT). This reaction was found to yield a similar product mixture with the sulfur analogue of TPT and their relative merits as well as electrochemical and photophysical properties are discussed. 

The 4,5-corane (84) is obtained in SOX yield on photo-decarbonylation of the pentacyclic ketone (85). Photochemical decomposition of the carbonate (86), by the loss of carbon dioxide, affords a mixture of products containing oxirane. styrene oxide, bibenzyl and phenylacetaldehyde. Triplet sensitized irradiation yields products solely from benzyl radicals. - An earlier study of the irradiation (at 254 nm) of the carbonate (87) reported that benzaldehyde, phenyl carbene, and carbon dioxide were produced. A reinvestigation of the irradiation of this compound (at 254 nm in acetonitrile) has provided evidence that the cis- and trans-stilbene oxides (88) and (89) are formed as well as deoxybenzoin and smaller amounts of diphenylacetaldehyde and bibenzyl. When methanol is used as the solvent the same products are produced accompanied by benzylmethyl ether, 1,2-diphenylethanol, and 2,2-diphenylethanol. These authors suggest that the oxiranes (88) and (89) are formed by way of... 

The combination of oxygen and iodine is the most satisfactory system found for the photocyclization of stilbenes to phenanthrenes. " Thus a mixture of trans-stilbene (0.01 m.) and iodine (0.005 m.) in cyclohexane on irradiation with a mercury... 

Irradiation of the contact charge transfer complex formed between trans-stilbenes and oxygen molecules in a zeolite NaY matrix at 313 nm leads to generation of the corresponding benzaldehydes in an electron-transfer process from which stilbene cation radicals and superoxide anion radicals arise. By contrast, excitation at 254 nm induces isomerisation and phenanthrene production, but without formation of any oxygenation products. 

Stilbene is a molecule which exists in two isomeric states because of the large energy barrier associated with rotation around the central double bond (see fig. 7.13). It has been known for some time [22] that when stilbene is excited to its first singlet state by absorption of UV light, it rapidly fluoresces to the ground state but with some molecules in the other isomeric form. Thus, if c -stilbene is irradiated with ultraviolet light, one obtains approximately 50% tran -stilbene and... 

Addition to six-membered oxygen heterocycles is also common. The photocycloaddition of 5,7-dimethoxycoumarin to tetramethylethylene has been described, and 4-hydroxycoumarin (326) undergoes facile addition to cyclohexene on direct irradiation to give the cyclobutane (327) analogous additions to a variety of other alkenes have been reported, and the cycloaddition of 4-methoxycoumarin to 2-methylpropene has been employed in a synthesis of l,2-dihydrocyclobuta[c]coumarin. Photoaddition of the 1,2-bisenol lactone (328) to trans-stilbene yields propellane (329), and [ 2 -F, 2] cycloaddition is observed along with other competing photoreactions on irradiation of chromone in the presence of alkenes. ... 

Photoemission from excited single states produced by photoionization of anthracene crystals occurs after two step laser excitation Biphotonic excitation of phenanthrene under 208 nm irradiation is a complex process involving both ionization andT-T annihilation. Change transfer exciton band structures have been characterized with samples of crystalline tetracene . Measurement of the photoionization efficiency in trans-stilbene crystals as a function of excitation energy shows that ionization occurs after rapid vibronic relaxation o. 

A lesser degree of loss of configuration was observed in the reactions of cis- 36> and iraMS-stilbene 35> with tetrachloro-o-benzoquinone to give dioxenes. The m-stilbene reaction (in acetone or acetonitrile) afforded about 80% of cis-adduct while 88% of trans-a.d6.net 35> was obtained from the reaction of trans-stilbene (both reactions at 15°, A >400 nm). Curiously, irradiation of the quinone and cfs-stilbene (or diphenyl-acetylene) in benzene solution produced the mono-phenyl ether (33)... 

Stilbene and Related compounds. The influence of sol-gel transformations on the emission spectra of cis and trans-stilbene has been assessed. The results of INDO/S calculations on stilbene has estimated the influence of the excited states involved in isomerization. The photostability of solutions of trans-stilbene to high-energy excitation with a XeCl laser has been studied. Geometric isomerization of both cis- and trans-stilbene is the result of irradiation of the compounds in NaY zeolites. A photostationary state composed of 65% cis and 35% trans is... 

The photoequilibration of cis and trans-stilbene in N-butylpyridinium chloride-AICI3, an ionic liquid system, occurs via the radical cations of the stilbene. A different mechanism is operative when irradiations are carried out in l-ethyl-3-methylimadazolium chloride-AlCh. Some aspects of photochemistry in ionic liquids, such as cis-trans-isomerism, have been discussed in a recent publication. ... 

Squires, Venier, and Aida (1983) describe an experimental technique they use to study the effect of solvent viscosity on the cisitrans ratio of stilbene irradiated in supercritical CO2. They use a dynamic flow technique similar to that described in chapter 4. In their system trau5-stilbene is coated onto glass beads, which are then packed into a high-pressure column. Supercritical CO2 flows through the column and solubilizes some of the trans-stilbene. The C02-stilbene phase is continuously irradiated with ultraviolet light as it flows through a quartz photoreactor at a fixed temperature and pressure. As the solvent viscosity increases, the photoisomerization of the cis isomer is inhibited while that of the trans isomer is facilitated. We should expect to see the cisitrans ratio of stilbene vary as the density of CO2 varies. This viscosity effect is clearly shown in figure 11.11. While there is a small effect of pressure on the... 

Bordwell and collaborators73 have demonstrated that UV irradiation of phenylthiirane 1,1-dioxide (64) affords styrene and sulphur dioxide. The 2,3-diphenyl dioxide (65,66) (as a mixture of cis and trans isomers) also decomposes and affords a mixture of cis and trans stilbene. Loss of S02 occurs on irradiation of 2,3-diphenylthiirene-1,1-dioxide (67) in methanol giving diphenylacetylene in 93% yield74. The photodecomposition of three-ring compounds has been reviewed75. 

The two adducts (254) and (255) are formed from the irradiation of trans-stilbene in the presence of rra 5-3-phenylacrylonitrile. The two adducts are themselves photolabile and are converted into cis- and /ram-stilbene and cis- and... 

Tertiary amines have been reported to undergo addition to singlet trans-stilbene product selectivity appears to be determined by the orientation of deprotonation of the aminium radical intermediate by the stilbene radical anion. Intramolecular addition has been observed in 5-2-(r-methylalkyl)-aniline (220) on irradiation in methanol to give the 25,3/ -2,3-dimethylindoline... 

It stands out that the photochemistry is also dependent of the crystalline phases where the topochemical relations between nearest-neighbor molecules are not the same in the pure guest crystals and in the TOT clathrates. When the TOT/cw-stilbene clathrate was irradiated, photoconversion to trans-stilbene occurred smoothly with the additional formation of small amounts of phenanthrene and of an unidentified product. On the contrary the TOT/tru j-stilbene clathrate remained unchanged when irradiated for long periods of time. In the majority of cases, crystalline cw-cinnamic acid derivatives are entirely converted to tran -isomers on irradiation and the trans-to-cis isomerization is not observed However both TOT clathrates with the trans- and cis-isomers of methyl cinnamate photoisomerized under similar conditions. It is noteworthy that these clathrates and those of the stilbenes (Sect. 2.1.4) are isomorphous. 

Many other examples can be found in the literature. Most familiar isomerization reaction is that of trans-stilbene to cis-stilbene, shown above. It was observed that the quantum yield of stilbene cis-trans isomerization decreased with an increase in viscosity of the medium [89]. In addition, it was also fotmd that in a polymeric matrix, the photo-isomerization is not inhibited, provided that it occurs above the glass transition temperature of the polymer. An example of a fragmentation of a molecule is the decomposition of disulfides upon irradiation with ultraviolet light of the appropriate wavelength ... 

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