Photodimer

In contrast, the photochemistry of uracil, thymine and related bases has a large and detailed literature because most of the adverse effects produced by UV irradiation of tissues seem to result from dimer formation involving adjacent thymine residues in DNA. Three types of reaction are recognizable (i) photohydration of uracil but not thymine (see Section 2.13.2.1.2), (ii) the oxidation of both bases during irradiation and (iii) photodimer formation. 

The formation of photodimers of the cyclobutane type from thymine (111 R = Me) occurs most effectively when a frozen aqueous solution of the substrate is irradiated. After the independent recognition of such dimers in two laboratories about 1960, it was some six years before the main constituent was identified beyond doubt (66JCS(C)2239) as the cis-syn entity (113), a U-shaped molecule in which the planes of the six-membered rings... 

Photodimers of IV-methylisoindole and isobenzofuran have been isolated [8+8] dimers are the major products. The [8 + 8] dimer (30) obtained by UV irradiation of an acetone solution of isobenzofuran at -60 °C has anti stereochemistry (78HCA444, 82CC1195). 

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). 

Cyanoanthracene photodimer [33998-38-8] M 406.4, dec to monomer above 147°. Purified by dissolving in the minimum amount of CHCI3 followed by addition of EtOH at 5° [Ebied et al. J Chem Soc, Faraday Trans 7 75 1111 7979 76 2170 1980]. 

On photolysis in benzene solution, the bisamides 22 (n > 14) form internal [2 + 2] photodimers 23, the maximum quantum yield being noted with n = 26. The shorter alkyl chain analogs 22 (n = 2-12), under the same conditions, form intcrmolccular dimers.236... 

Photoirradiation of both neat and benzene solutions of 2-cyclohexenone (66b) gives a complex mixture of photodimers [40]. However, photoirradiation of a 1 1 complex of 66b with the chiral host (S,S)-(-)-l,4-bis[3-(o-chlorophenyl)-3-hydroxy-3-phenylprop-l-ynyl]benzene (167) in the solid state (Scheme 24) gave (-)-anf/-head-to-head dimer 168 of 46% ee in 75% yield [40]. This reaction was found to proceed in a single crystal-to-single crystal manner. The mechanism of the reaction was studied by X-ray crystal structural analysis [41]. 

All of the mejo-substituted photodimers have been found to have the head-to-tail structure. 

The decarbonylation of dibenzyl ketone has been shown to result from the carbonyl triplet state by its ability to be quenched by 1,3-cyclohexadiene or l,3-pentadiene.<66) Using 1,3-cyclohexadiene as quencher, photodimers of the cyclohexadiene were obtained. Since these are formed only by triplet sensitization,<66) the quenching of ketone triplet states, rather than their excited singlets, was assured. Further evidence for a triplet reaction follows from the fact that decarbonylation could be sensitized by acetone under conditions where the sensitizer absorbed 93% of the light. 

Dauben et al. have investigated the scope of the photochemical type A rearrangement/60 They conclude that the rearrangement occurs only if the fourth carbon atom of the 2-cyclohexenone ring is fully alkyl-substituted. If this requirement is not met, photodimers are the major products. This substituent requirement is necessary but not sufficient to ensure rearrangement since the presence of other groups can inhibit the reaction. 

Formation of the photodimer is reversible, the dimer being split with high quantum yield with light of wavelength about 235 nm.(82> Similar dimerizations have been observed with several other pyrimidine derivatives, as... 

Irradiation of cycloocta-2,4-dien-l-one (55) in pentane gives a racemic photodimer, anti-tricyclofSAO.O Jhexadeca- , 11 -diene-3,16-dione (60) in 10% yield along with polymeric materials 34). Efficient and enantioselective photodimerization of 58 was achieved by irradiation of the 2 1 inclusion complex 59 formed between 2 a and 5813). When a solution of 2a and an equimolar amount of 58 in ether-hexane (1 1) was kept at room temperature for 12 h, 59 was obtained as colorless needles of mp 105 to 108 °C. Irradiation of 59 in the solid state for 48 h gave (—)-60 of 78 % ee in 55 % yield. 

The structural changes that accompanied the [2 + 2] photodimerization of the metastable a -polymorph of ort/zo-ethoxy-tranx-cinnamic acid have been studied [93]. In this study, the photochemical reaction was carried out at 293 K, and observed in situ by single-crystal X-ray diffraction. In the structure of the title compound, the three molecules in the asymmetric unit are arranged to form two potential reaction sites, but only one of these was found to be photoreactive. Since only two out of three molecules in the asymmetric unit take place in the photodimerization reaction, the crystal of the final product contains an ordered arrangement of the photodimer and the unreacted monomer. 

The photochemical behavior of methylene cyclopropenes is a subject of current investigation170 Previous results with some 4,4-diacyl and 4,4-dicyano triafulvenes indicate that mainly dimerization, but sometimes additional solvent incorporation and hydrogen abstraction occurs. In the case of the photodimer of 1,2-diphenyl-4,4-diacetyl triafulvene (180) the structure 460 can be assigned from spectral evidence ... 

In a similar vein, the cleavage of the photodimer of anthracene occurs in the dark in the presence of nitrosonium cation to afford the anthracene cation radical as its 7t-dimer via a thermal electron transfer196 (equation 66). 

This apparently happens in the excited singlet dedimerization of thymine photodimer in a rigid matrix as well as subsequent excited singlet dimerization of the pair of thymine molecules formed, since quantum yields of both processes are unity.176)... 

Analogous [4+4]photodimers are obtained in irradiating naphthalene derivatives (4.28)431). 

Menadione (vitamin K-3, 79) in ethanol in the presence of oxygen was placed in the sun for 10 min to give the 2,3-epoxide (80) [64]. This has been photolysed in micellar solutions to give two main products 2-hydroxy-3-methylnaphtho-quinone and 2,3-dihydro-2-hydroxy-2-methylnaphthoquinone [65]. Solid menadione left on a south-facing windowsill (in Texas ) for a month gave two photodimers the syn head-to-head and syn head-to-tail cyclobutanes. With UV... 

It is well known that pyrimidine bases convert to photodimers upon irradiation to UV light near the X max( > 270 nm). This photochemical reaction has a lethal effect in biological systems due to the photochemical transformation of pyrimidine bases of nucleic acids. However the photodimerization is a reversible reaction and the photodimers split to afford the original monomers very efficiently upon irradiation at a shorter wavelengths as shown in Scheme 1(1). 

Polyamides containing thymine photodimer units in the main chain (17a,b) were prepared by polycondensation of thymine photodimer derivatives (15a,b), which were obtained by the photochemical reaction of the monomeric compound, and various diamines by the activated ester method (Figures 4 and 5) (17, 19). 

Figure 4. Isomers of polyamides containing thymine photodimer.

Photopolymerizations of the bis-pyrimidine derivatives were carried out in solid film by irradiation with UV light from a spectroirradiator. The rate of the photopolymerization was found to depend on the wavelength of light. The highest reactivity was observed at 300 nm (Figure 7a), while the highest reactivity for photoreversal of the photodimer was obtained by irradiation at 240 nm (Figure 7b). The polymers... 

Photodissociation of the polyamides (Figures 4 and 5) was carried out in the solid films. Polymer films (1-2 U m) were cast onto quartz substrates and were exposed to monochromatic 250 nm light from a spectroirradiator. The dissociation of the thymine photodimers was followed by monitoring the absorbance at 270 nm. The results obtained for the polyamide prepared from the reaction of propane diamine and the isomers of the thymine photodimer (cis-syn(17a), cis-anti(17b), and... 

Figure 7. (a) Wavelength dependency of photopolymerization of the 6-cyanouracil derivatives, (b) Wavelength dependency of photoreversal of the photodimer. 

Figure 9. Photoreversal of the polymers containing isomers of thymine photodimer.

The photosensitivity tests were carried out only for the polyamides containing different isomers of thymine photodimers to determine the effect of isomer structure on photosensitivity. 

Polyamides containing thymine photodimer units in the main chains showed excellent resolution values (0.3 p m) and behaved as positive photoresists. It is concluded that polymers containing pyrimidine bases displayed high resolution and high sensitivity when used in both negative and positive photoresists formulations. 

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