Stilbenes photodimerization

The mechanism of stilbene photodimerization was first investigated by Stegemeyer (39) who observed that dimerization occurs upon irradiation of t-1, but not c-1. Thus, 11 and 12 are the syn and anti dimers, respectively, of t-1. Increasing the t-1 concentration was observed to decrease the fluorescence intensity of It, leading to the proposal of a singlet excimer mechanism for photodimerization as outlined in eqs. 2-6 ... 

Complete inhibition of trans-stilbene photodimerization in water was induced by P-CD. Deaggregation of stilbene-associated structures that are... 

Ultraviolet irradiation of the methiodide or hydrochloride of 2-styryl-pyridine (XCIII) in the solid state results in transformation of the trans-isomers to the corresponding dimers (XCVa, b) on the other hand irradiation in benzene solution gives both isomerization and dimerization.309 Dimer (XCVa) was produced in low yield on irradiation of XCIII in the powdered form in the presence of air.308 This is in contrast to the reported stability of XCIII toward photodimerization.109 Similar dimerizations have been reported in the case of 2,4-dichloro-3-cyano-6-styrylpyridine,164 2-styrylquinaldine (XCV) and frans-4,4 -diguanyl-stilbene bis(2-hydroxyethane sulfonate) (stilbamidine) (XCIV).82,83... 

A large number of t-1 derivatives are known to undergo photodimerization (56) however, quantitative details and cycloadduct stereochemistry are available in only a limited number of cases. Stilbenes substituted with electron donating ring substituents are reported to dimerize more efficiently than t-1 (57) styrylpyridines (58) and styrylthiophenes (59) less efficiently. 

The stereochemistry of photodimerization in the solid state and solution has been reported for several halogenated derivatives of t-1 (Table 2) (59-62). Solid state photodimerization of stilbenes, like other alkenes, is subject to topochem-ical control viz, the two reactive double bonds must be parallel and separated by < 4.2 A (63). The photostability of t-1 in the solid state (39b,59) is consistent with its reported crystal packing (64). The halogenated stilbenes 15-20 serve to illustrate the variety of stereochemical outcomes observed for solution and solid state dimerization (eq. 11). 

Photodimerization of the stilbene chromophore in small or medium ring systems is highly dependent upon the ring size. Indeno[2,l-a]indene (5) yields a 1 2 mixture of head-to-head head-to-tail dimers 28 and 29 (eq. 12) upon irradiation in... 

Keywords trans-stilbene, cinnamic acid, [2+2]photodimerization, photopolymerization, cyclobutane... 

Rao et al. observed that photoirradiation of tran.v-stilbene in crystalline y-cyclodextrin inclusion complexes yields a single isomer of. svn-tctraphenylcy-clobutane (119) [109] stereoselectively in high yield (70%). In contrast, the photodimerization of stilbene in solution is very inefficient, and no photodimer was observed even after prolonged irradiation of pure stilbene crystals. 

The photostationary Z/E ratio of stilbene, (Z/E)pss, is known to be significantly dependent on the excitation wavelength. Thus, the Z/E ratio is almost unity (48 52) upon irradiation at 254 nm, but is remarkably enhanced up to 93 7 upon irradiation at 313 nm. This apparently surprising change is readily interpreted in terms of the following equation, which relates the (Z/E)pss ratio with the relative extinction coefficient of the two isomers at the excitation wavelength and the relative efficiency (quantum yield) of the forward and reverse reactions (Z/E)pss = ee/ezx z E/E z [201]. Preparative-scale direct irradiation should be done at low stilbene concentrations, since photodimerization of (fi)-stilbene may compete with the photoisomerization as the concentration increases [202-206]. 

CDx and in particular 7-CDx are known to accommodate two aromatic moieties under certain circumstances. Hence if two appropriate prochiral guest molecules are included in the same CDx cavity, regio- and enantioselective bimolecular photoreactions are expected to occur [108]. Indeed, it is known that the presence of CDx not only accelerates the rate but also modifies the product distribution of photocyclodimerizations of anthracene derivatives [109-111], coumarin derivatives [113-115], stilbene derivatives [116,117], stilbazole [118], and tranilast [119]. For instance, Tamaki and coworkers reported significantly enhanced quantum yields of photodimerization of anthracenesulfonates and anthracenecarboxyl-ates in the presence of 3- and 7-CDx [109-111]. These anthracene derivatives form 2 2 and 2 1 guest-host complexes with (3- and 7-CDx, respectively, and... 

Stilbenes are chromophores that can undergo photoisomerization as do azobenzenes, but they also show photodimerization (see Figure 6.2). Besides other parameters, the packing of the chromophores influences the ratio of... 

Wolf, M. O.j and Fox, M. A. Photoisomerizaiion and photodimerization in self-assembled monolayers of cis- and tri2 s-4-cyano-4 -(10-mercaptodecoxy)stilbene on gold. Langmuir 1996. 12, 955-962. 

Photochemical [2+2] cycloaddition of alkenes in the crystalline state is synthetically very useful because it usually produces only one stereoisomer predicted ftom the crystal structure. On the other hand, this stereospeciflcity of the reaction can be a disadvantage because of inaccessibility to other stereoisomers. In order to circumvent such a problem, we explored compelled orientational control of the photodimerization of particular compounds like ranj-cinnamic acids and anthracenecarboxylic acids [74-78]. During our study, photochemistry of fluoro- and chloro-substituted ranj-stilbene-4-carboxylic acids and their methyl esters and alkaline and alkaline earth salts in the crystalline phase was likewise studied in order to synthesize specific stereoisomers selectively (Scheme 41) [79]. Most of these stilbene compounds dimerized to give exclusively or mainly syn head-to-head cyclobutane dimers. Some were photochemicaUy inert. 

The photodimerization of olefinic systems such as cinnamic acid and stilbenes has been studied and reported for many years a2>. 

As mentioned above, some arylalkenes, such as stilbene, form complexes with Ag+. Such complexes are also formed between Ag+, as added silver perchlorate, and simpler alkenes. Typical of this is the interaction with 1-methylenecyclohexane when a crystalline complex is formed. Irradiation of this complex in the polycrystalline state or in solution in methanol affords isomerization to 1-methylcyclohexene by a 1,3-hydrogen migration path. Further irradiation brings about the stereospecific formation of the exo,trans,exo-dimer (24) of 1-methylcyclohexene. Less specific photodimerization is also reported for the irradiation of the /S-pinene (25)/silver perchlorate complex69 The mechanism was thought to involve a silver/cyclohexenyl radical similar in type to that observed in the y-radiolysis of silver/cycloalkene complexes70,71. 

Photodimerization, first reported by Ciamician and Silber in 1902 [458], requires a rather high stilbene concentration. The quantum yield increases from <0.01 for 0.01 M frans-stilbene to 0.33 for 0.55 M trans-stilbene [459], The excited t singlet state is the reactive species in this process. Pure ds-stilbene shows no dimerization, a fact that has been explained by the short lifetime of the first excited singlet state [294], As Saltiel and Charlton have pointed out [27] short-lived transients, observed at high frans-stilbene concentrations [460], may be associated with dimerization rather than... 

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. 

Bimolecular Reactions j8-CD is known to hinder the [2-h2] photodimerization of stil-benes through complexation within its cavity. " In contrast, y-CD can simultaneously include two stilbenes in its larger cavity in both solution and solid phase to significantly facihtate the [2-i-2] photodimerization reaction. 

Stilbene and its derivatives, which had been widely investigated in CDs and CAs, can be included in CB[8] cavity to form 2 1 complexes. Two ( )-diaminostilbene dihydrochloride 68 (Scheme 1.13) can insert into CB[8] cavity to give a 2 1 host-guest complex which has a good solubility in water. Photoirradiation of the complex gave [2-h2] photodimerization product la,2o,3)3,4)3-telrakis(4-aminophenyl)cyclobutane 69 as major product, along with a trace amount of la,2)3,3a,4j8-tetrakis(4-aminophenyl)cyclobutane 70. No formation of the isomerization product (Z)-68 was observed, which is significantly different from the photoreaction of ( )-68 in the absence of CB[8]. The stereoselectivity of photodimerization in the presence of CB[8] (synlanti = 95/5) is much better than that obtained with y-CD (synlanti = 80/20). 

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