Photodimerization of

The smallest subimit of these fullerene polymers is the C j2o dimer [368], Synthesis and examination of the properties of this molecule are, therefore, of considerable interest. Beside the solid-state mechanochemical reaction of with KCN -described in Section 3.2.3 -the dimer Cj2o can be synthesized at 5 GPa and 200 °C in good yields from a bis(ethylene-difhio)TTF-C5Q complex [369]. Cj2o is a dark brown solid that is almost insoluble in most organic solvents. 1 ts solubility decreases with increasing purity of the sample this behavior is also known for pristine C q. 

The dimer dissociates quantitatively to Cjq upon heating to 175 °C or upon reduction and slowly even upon exposure to light [368]. 

Dimers with two methylene bridges and a cyclobutane fusion were synthesized by thermolysis of ethoxycarbonylmethano-l,2-dihydro[60]fullerene. They have a similar structure as 328 [374], 

Photodimerization of dimers, where the two Cgg-spheres are already linked via bisfunctional groups, can be carried out in very good yields [375, 376]. The dimer 

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The role of rose bengal and other sensitizer dyes in the photodimerization of 2-acet5i-l,4-benzoquinone [1125-55-9] involves electron transfer but not singlet oxygen (42) (see Dyes, SENSITIZING). 

Fig. 7. Photodimerization of light-sensitive functional groups attached to polymer backbone stmctuies (a) polymer containing photosensitive cinnamic...

Unusual heterocyclic systems can be obtained by photodimerizations and for five-membered heterocycles with two or more heteroatoms such dimerizations need be effected on their ring-fused derivatives. Cyclobutanes are usually obtained as in the photodimerization of the s-triazolo[4,3-a]pyridine (540) to the head-to-head dimer (541). These thermally labile photodimers were formed by dimerization of the 5,6-double bond in one molecule with the 7,8-double bond in another (77T1247). Irradiation of the bis( 1,2,4-triazolo[4,3-a]pyridyl)ethane (542) at 300 nm gave the CK0ifused cyclobutane dimer (543). At 254 nm the cage-like structure (544) was formed (77T1253). 

The photodimerization of thiochromone 1,1-dioxide (67) has also been studied. In... 

It is well known that the [2+2] photodimerization of diolefinic compounds is allowed to occur photochemically but not thermally, whereas the cyclobutane cleavage reaction occurs both photochemically and thermally. The cleavage reaction occurs with irradiating light of shorter wavelength... 

Single crystal-to-single crystal enantioselective photodimerizations of cou-marin (169a) and thiocoumarin (169b) were found to proceed efficiently in... 

Figure 10.1 Photodimerization of anthracene (a) and trans-cinnamic acid (b).

Reversible Phase Separation Driven by Photodimerization of Anthracene A Novel Method for Processing and Recycling Polymer Blends... 

Figure 10.9 Reversible photodimerization of anthracene induced by light of two wavelengths 365 and 295 nm in a PSAF/PVME (20/80) mixture observed at 25 °C.

The [2 + 2] photodimerization of a, j8-unsaturated sulfones is correctly viewed as a photoreaction of alkenes, rather than the sulfone group, and this aspect has been reviewed recently by Reid, as part of a wider survey of the photoreaction of O- and S-heterocycles. The topic continues to attract considerable interest and a few recent examples, as well as some synthetic applications, will be discussed here. Much of the photodimerization work has been carried out on the benzo[fc]thiophene (thianaphthene) 1,1-dioxide system. For example. Porter and coworkers have shown that both 3-carboxybenzo[i]thiophene 1,1-dioxide (65) and its methyl ester give only the head-to-head (hth), anti dimer (66) on irradiation in ethanol. In a rather unusual finding for such systems, the same dimer was obtained on thermal dimerization of 65. Similar findings for a much wider variety of 3-substituted benzo[fi]thiophene 1,1-dioxides have been reported more recently by Geneste and coworkers . In the 2-substituted analogs, the hth dimer is accompanied by some of the head-to-tail (htt), anti dimer. The formation of the major dimer appears to proceed by way of an excited triplet and the regiochemistry observed is in accord with frontier MO theory. 

An interesting example is photodimerization of waphorone where the use of SC CO2 increases the yield of the aaft -dimer relative to the yn-dimer. Special polymers ean be made by using SC fluids. Apart from SC CO2, SC propane can be very useful and act as a convenient medium. Roche has also reported replacement of an expensive Rh catalyst with a cheaper Ru catalyst using a high-pressure application of hydrogen. 

Intermolecular photocycloadditions of alkenes can be carried out by photosensitization with mercury or directly with short-wavelength light.179 Relatively little preparative use has been made of this reaction for simple alkenes. Dienes can be photosensitized using benzophenone, butane-2,3-dione, and acetophenone.180 The photodimerization of derivatives of cinnamic acid was among the earliest photochemical reactions to be studied.181 Good yields of dimers are obtained when irradiation is carried out in the crystalline state. In solution, cis-trans isomerization is the dominant reaction. 

An excellent review on organic photochemistry in organized media, including aqueous solvent, has been reported.178 The quantum efficiency for photodimerization of thymine, uracil, and their derivatives increased considerably in water compared with other organic solvents. The increased quantum efficiency is attributed to the preassociation of the reactants at the ground state. 

Dallos M, Lischka H, Shepard R, Yarkony DR, Szalay PG (2004) Analytic evaluation of nonadia-batic coupling terms at the MR-CI level. II. Minima on the crossing seam formaldehyde and the photodimerization of ethylene. J Chem Phys 120 7330... 

Similar relationships have been established with regard to the 1,3-dipolar addition and the photodimerization of olefins 62>. The HO—LU... 

Fig, 7.27 a and b. Orbital relationships in 1,3-dipolar additions and photodimerization of olefins, a) 1,3-Dipolar addition, b) Photodimerization of olefins... 

The photodimerization of anthracene has been utilized to produce a number of interesting synthetic derivatives which are essentially photo-isomers of the starting materials,[Pg.28]

We have already discussed one of the earliest photoreactions to be studied, that is, the (4w + 4w) photodimerization of anthracene. That the singlet state was involved in this reaction was conclusively shown in the period 1955-1957. The first reaction in which the triplet state of the molecule was shown to be involved was the photoreduction of benzophenone by Hammond and co-workersa) and Backstrom and co-workers<2) 1959-1961. This was the first in a series of many papers from Hammond s laboratory... 

The photodimerization of simple isolated olefinic bonds is rarely observed because of the absorption of these compounds in the high-energy or vacuum-ultraviolet region. One case reported is that of the photo-dimerization of 2-butene.<2) Irradiation of liquid cw-2-butene with light from a cadmium (A = 229, 227, 214 nm) or zinc (A = 214 nm) lamp was reported to lead to dimers (1) and (2) ... 

Photodimerization of isoprene has also been observed using ferrocene as sensitizerproduct distribution in this case was similar to that observed using high-energy photosensitizers. To account for this behavior, the following mechanism was proposed, where 7 indicates a triplet level of isoprene higher than the lowest triplet ( ) ... 

Table 10.3. Solvent and Concentration Effects on the Photodimerization of Acenaphthylene<41)...

Photodimerizations of other olefins conjugated with aromatic rings are shown below<51 53) ... 

The photodimerization of 2-cyclopentenone has been studied by Eaton and Hurt<82) and Ruhlen and Leermakers<83) ... 

The photodimerization of 2-cyclohexenone to yield cyclobutanes (70) and (71) has also been reported 84 ... 

Rubin and co-workers have investigated the photodimerization of the 4,6-diene-3-ketosteroid (75)(86) ... 

The photodimerization of coumarin has been rather extensively investi-gated.<90 92) Irradiation of coumarin in solution yields a mixture of four stereoisomeric dimers (78)—(81) with dimers (78) and (79) strongly predominating ... 

The photodimerization of some other a,/3-unsaturated carbonyls and acid derivatives is shown below<127 130) ... 

Photochemical Techniques and the Photodimerization of Anthracene and Related Compounds... 

The photodimerization of anthracene, having been first studied by Fritzsche in 1867 (two years after Kekule proposed his revolutionary structure for benzene), was one of the first photochemical systems to be extensively investigated. Fritzsche found that upon exposure to sunlight, benzene solutions of anthracene yielded an insoluble substance which he called Para-photen. Observing that the photoproduct yielded anthracene upon melting, he concluded that he had obtained a polymer of anthracene/9 ... 

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