Photo induced dimerization

In a recent report [54], the potential energy curves for both thermal and photo-induced dimerization of thymine resulting in the cis-syn dimer were... 

The pendant functions listed in Figure 3 are often useful and of synthetic interest per se. For example, methacrylate and acrylate esters are polymerizable (cross-linking sites) [19-21] the cinnamate is photorespon-sive (for the photo-induced dimerization of its unsaturated groups) [20] oligo(oxyethylene) [25-27] and carbohydrate groups [35] give hydrophilic and water-soluble polymers, whereas perfluoroalkyl moieties [32-34] enhance hydrophobicity. Thus, poly(vinyl ethers) with cinnamate functions... 

Scheme 2). Photo-Induced dimerization of the carbanion (l6) derived from the bromo-analogue of (10) led to dinltro-derlvative (17) in high yield (Scheme 3). ... 

Cyclohexene 27a undergoes photo-induced dimerization in the presence of xylene sensitizer to give a mixture of stereoisomers in non-stereospecific manner [30]. 

The low solubility of fullerene (Ceo) in common organic solvents such as THE, MeCN and DCM interferes with its functionalization, which is a key step for its synthetic applications. Solid state photochemistry is a powerful strategy for overcoming this difficulty. Thus a 1 1 mixture of Cgo and 9-methylanthra-cene (Equation 4.10, R = Me) exposed to a high-pressure mercury lamp gives the adduct 72 (R = Me) with 68% conversion [51]. No 9-methylanthracene dimers were detected. Anthracene does not react with Ceo under these conditions this has been correlated to its ionization potential which is lower than that of the 9-methyl derivative. This suggests that the Diels-Alder reaction proceeds via photo-induced electron transfer from 9-methylanthracene to the triplet excited state of Ceo-... 

There is always interest in the photochemistry of the pyrimidine nucleic acid bases and related simple pyrimidinones, due to its importance in genetic mutation. In addition to damaging DNA, photo-induced reactions may also repair the damage, as in the reduction, by FADH, of the thymine glycol 64 back to thymine <06JACS10934>. Another report related to repair of DNA involved a model study, by means of the linked dimer 65, of the involvement of tryptophan in the electron-transfer leading to reversion of thymine oxetane adducts <06OBC291>. 

Other olefinic substrates known to dimerize through photo-induced electron transfer sensitization include enamines (72), diarylethylenes (73-75), vinyl ethers (76), styrenes (77,78), and phenyl acetylenes (79). Alternate ring closures (besides cyclobutanes) are sometimes observed, probably via 1,4-radical cationic intermediates. For example, a tetrahydronaphthalene is formed from the radical cation of 1,1-diphenylethylene, eq. 

The radical anion (95), generated from photo-induced electron transfer of halogenated benzocycloalkanes, furnishes ring-expanded products (96) in addition to reduced species and dimers.198... 

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 nucleophilic attack on the radical cation followed by reduction and protonation or by combination with DCB ).63 In the absence of a nucleophile, the product mixture is highly complex, as is the case under electro-oxidative conditions. Under UV irradiation, /nmv-stilbene undergoes dimerization and oxygenation (to benzaldehyde) by a single-electron mechanism in the presence of a sensitizer such as 2,4,6-triphenylpyrilium tetrafluoroborate (TPT).64 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. 

In this system with even number N of dimer units a orbitals HOMO and LUMO are nonbonding with zero overlap [9], Therefore, the photo-induced electron transition between these orbitals is forbidden. The first electron transition with lowest energy in optical spectrum of this system proceeds between HOMO and unoccupied molecular orbital next to LUMO [6]. Simple calculations based on formula (9) give the energy AEt of this transition at N 1 as... 

Among the first examples of photo-induced reactions through radical-ion intermediates was the dimerization of VCZ, discovered by Ellinger [66], and later thoroughly investigated by Ledwith [67, 68]. 

Since coiled chains of proteins are known to uncurl because of ionic repulsions when ionization occurs, Reid (1957) suggested that excited state dissociation acts as a trigger in rapid biological processes. The 7-azaindole dimer, which undergoes photo-induced double proton transfer (see Section 4), has similarities to the adenine-thymine and guanine-cytosine base pairs of DNA. Its excited state proton transfers have been proposed as possible mechanisms of mutagenesis (Ingram and El-Bayoumi, 1974). 

Excited state complexes are relevant in connection with photo-induced electron transfer, since their formation frequently competes with or precedes electron transfer. The simplest examples, excited state dimers (excimers), were discussed by Kautsky as early as 1939 [77], The first organic excimer, the dimer of pyrene, was identified by its characteristic, red-shifted, structureless emission spectrum by... 

Irradiation of benzene solutions of 7e-aryl arenetellurocarboxylates with light from a mercury lamp generated radicals via homolytic cleavage of the Te—CO bond. The free radicals subsequently formed aldehydes, diorgano tellurium compounds, and diorgano ditellurium compounds via hydrogen abstraction, dimerization and elimination of tellurium, or dimerization -. The products of the photo-induced reaction of Te-4-methylphenyl 2-methylthiobenzene(tellurocarboxylate) are given as an example . ... 

As a consequence of photodamage to DNA there is still considerable interest in the photochemical dimerization of pyrimidine derivatives. Thus, the synthesis of the pyrimidine dimers (157), (158) and (159) has been carried out by irradiation of the pyrimidine derivative (160). Sugiki et al. have studied the photochemical dimerization of the long-chain substituted thymine derivatives (161). The results of the measurement of the reduction potentials of thymine and cytosine cyclobutane dimers have been reported. In addition the electron-transfer-induced ring cleavage by electron donation from suitable sensitizers of the adducts (162) and (163) has also been examined. Calculations have been carried out on the photo-induced repair mechanism in DNA both by direct irradiation and by the use of SET induced photocleavage. ... 

---