Stilbene photochemistry

The photochemistry of a-methylstilbene (5) resembles stilbene photochemistry in many ways. However, as pointed out earlier, both the cis and trcms isomers are nonclassical acceptors of triplet excitation. This suggests that both the cis and trcms triplet states correspond to high-energy vibrational levels of the twisted or phantom triplet. Azulene does not alter the photo-... 

The photoisomerization of stilbene is one of the most extensively studied photoreactions (25). Solvent effects have been thoroughly investigated for both the direct and photosensitized isomerizations, and a model has been developed which attributes these effects to solvent viscosity (26). Increased viscosity inhibits direct photoisomerization of the cis isomer, but facilitates that of trans-stilbene. As a result, the cis/trans ratio of the photostationary state increases with increasing solvent viscosity. The wide range of viscosities which are attainable by pressure manipulation of supercritical carbon dioxide provides an excellent opportunity to probe the effect of viscosity on stilbene photochemistry in the same solvent. 

Stilbene photochemistry continues to provide a mine of riches for the dedicated photochemist, in particular Hochstrasser, Fleming, and their groups. Metcalf et al. have described chiral discrimination in electronic energy transfer processes enantioselective excited state quenching occurs. A study of singlet electronic energy transfer from cyclohexane to benzene appears to require revision of the benzene fluorescence efficiency in cyclohexane to 0.26 0.02 (Johnston and Lipsky). 

One of the features of stilbene photochemistry is its essentially strong dependence on medium polarity and temperature the competition between fluorescence and trans-cis isomerization has been shown to be extremely sensitive to medium viscosity. Solvent polarity can affect both the dynamics and the pathway of the reaction. The dipolar character of asymmetrically substituted stilbenes and polarizability of the traws-stilbene transition state can explain the sensitivity of the photoisomerization rate to medium polarity [5, 6, 12, 31, 66-69]. 

Boratastilbene has recently been synthesized and structurally characterized.26 A study of the consequence of the isoelectronic B for C substitution on the pho-tophysics/photochemistry led to the conclusion that, due to intramolecular charge transfer arising from the inequivalent charge density of the two aromatic rings, nonaggregated boratastilbene is highly emissive relative to stilbene. 

Energy is transferred from molecules electronically excited in a chemical reaction to other molecules which emit the accepted excitation energy in the form of light alternatively the accepting molecules can undergo photochemical transformations. First examples of this photochemistry without light were described by E. H. White and coworkers 182>. Thus the trans-stilbene hydrazide 127, on oxidation, yielded small amounts of the cis- 128 beside the trans-stilbene dicarboxylate in a luminol-type reaction. 

The enormous amount of overactivation in photochemistry is not always required for solid-state cis-trans isomerizations. There are also some thermal E/Z isomerizations of crystalline olefins that are catalyzed by iodine. For example, crystalline czs-stilbenes 91 can be isomerized to give frans-stilbenes 92 without intervening liquid phases (Scheme 8). The isomerizations follow first-order kinetics with various rate constants for 4-MeO, two modifications of 2-MeO, 2-EtO, 2-n-PrO, and 2-i-PrO substitution. The activation energies vary from 20 to 32 kcal mol but could not be interpreted [54]. Similarly, cfs-l,2-diben-... 

While much has been learned about S near the geometry of tS in solution, there have been no reports on the spectroscopic detection of the twisted excited singlet state of stUbene. Information about the twisted excited singlet state of tetrapheny-lethene will be described in this chapter in Section 2.3. Interesting photochemistry also occurs following the excitation of cw-stilbene however, because of the much shorter lifetime of c -stilbene, femtosecond-pulsed lasers must be used." ... 

The photochemistry and photophysics of trans-stilbene derivatives (24) have been utilized by Whitten and co-workers to understand the relaxation characteristics of media such as micelles, monolayers, and LB films [144,145]. For example, the d>,rans to Cis for stilbene derivatives show the following trend solvent system methylcyclohexane > SDS micelle multilayer assemblies (with arachidic acid). In fact, no isomerization is observed in multilayer assemblies. This is the trend expected on the basis of how readily the media can respond to stilbene shape changes during isomerization process. 

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. 

All of the photochemical cycloaddition reactions of the stilbenes are presumed to occur via excited state ir-ir type complexes (excimers, exciplexes, or excited charge-transfer complexes). Both the ground state and excited state complexes of t-1 are more stable than expected on the basis of redox potentials and singlet energy. Exciplex formation helps overcome the entropic problems associated with a bimolecular cycloaddition process and predetermines the adduct stereochemistry. Formation of an excited state complex is a necessary, but not a sufficient condition for cycloaddition. In fact, increased exciplex stability can result in decreased quantum yields for cycloaddition, due to an increased barrier for covalent bond formation (Fig. 2). The cycloaddition reactions of t-1 proceed with complete retention of stilbene and alkene photochemistry, indicative of either a concerted or short-lived singlet biradical mechanism. The observation of acyclic adduct formation in the reactions of It with nonconjugated dienes supports the biradical mechanism. 

Tabushi et al. reported the photochemistry of stilbene-capped (3-CD 118 [108], The unique property of 118 is that its trans cap is completely converted into a cis cap and the cis cap is further converted into a phenanthrene cap. 

Photochemistry offers many examples of large-scale movement at the molecular level. The cis-trans isomerization127-33- of an azo group is a very attractive process for modification of the geometry and the properties of compounds, although several other photochemical reactions can be utilized,134-371 including the photoisomerization of stilbene-like compounds. 

Lee, G.A. (1976) Photochemistry of cis-and trons-stilbene oxides. Journal of Organic Chemistry, 41, 2656-2658. 

Saeva mentions, in an early review [10] on LC materials and aspects of their photochemistry and photophysics, the irradiation of an unspecified stilbene containing polymer and changes in its physical properties. Creed et al. [25,61,62] have reported several observations of the photophysics and photochemistry of a rra/w-stilbene 4,4 -dicarboxylate containing MCLC polyester, 29, one of a series of such polymers with different spacers synthesized by Jackson, Morris, and coworkers [63]. This polymer is partly crystalline in the as cast state and has a N mesophase over a narrow temperature range (177-186°C). In solution, the structured UV-Vis absorption [62] and fluorescence [25,61] spectra are almost... 

Papper, V., Likhtenshtein G.I., Pines D. and Pines E. (1998) Tran.s-4-4 disabstituted stilbenes Linear free-energy relationship in photochemistry and photophysics. In Recent Research Development in Photochemistry and Photobiology. V. 1 Transwold Research Network. Trivandrum, pp. 205-250. 

In 1,2,3-triphenylallyl carbanions, electron transfer photochemistry is found in the presence of acceptors like stilbene as demonstrated by the sensitized cis-trans isomerization of this molecule according to the following pathway [145],... 

In the triarylallyl carbanion study and in the absence of accepting stilbene, EjZ photoisomerization of the irradiated carbanion was observed and the effect of changing the substituent at position 2 was examined. Starting from the zero coefficient at carbon-2 in the non bonding MO of the allyllic system, Tolbert considers that the substituent at the 2 position does not affect the energy of the MO. If an electron transfer mechanism governs the reactivity, the substituent on this position will not modify the process in a significant way. The experimental results were very dependent on the C-2 substitution and this reaction was therefore considered as relevant of the intrinsic photochemistry of the anion [145]. This view has been confirmed in other studies on 1,3-diphenylallyl carbanion the kinetic parameters of the photoisomerizations were found to be inconsistent with an electron transfer mechanism [146, 147]. 

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... 

---