Photodimer isomers

Chemical ionization enables one to distinguish between photodimer isomers such as (I) and (II). The mass spectra consist mainly of two ions, namely the (M + 1) ion and the (M/2 -I- fragment ion. The loss of the elements... 

Figure 4. Isomers of polyamides containing thymine photodimer.

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 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. 

Studies on the photochemical reactions of dihydropyridines have proven to be interesting. There are a number of 1,4-dihydropyridines that are known to disproportionate when irradiated (equation 19) (B-76PH240). Analogous intramolecular reductions have also been observed by other workers (55JA447). In contrast to these results, the 1,4-dihydropyridine (59) rearranged to its 1,2-dihydro isomer (60). Further irradiation resulted in dimerization. Interestingly, the photodimer (61) cyclized to the cage compound (62). 

Phane compounds, i.e. two chromophores held face-to-face by at least two hydrocarbon links (4) Sandwich dimers, which are chromophore pairs produced by photolysis of photodimers in rigid matrices and (5) Bichromophoric compounds having a single saturated hydrocarbon linkage, which form intramolecular excimers as allowed by the rotational isomers of the linkage. In each case, we will utilize the intermolecular excimer formed in solution as the standard against which the properties of constrained excimers will be measured. 

For instance, head-head photodimers are predicted from the crystal structures of 9-cyanoanthracene and 9-anthraldehyde, but the head-tail isomer is produced. Craig and Sarti-Fantoni and later others found that photoreactions of 9-cyanoanthracene and 9-anthraldehyde take place at defect sites [96,215], Systematic photochemical and crystallographic studies by Schmidt and co-workers uncovered many cases of substituted anthracenes which behave in an unexpected fashion (Scheme 40) [216,217]. Examples shown in Scheme 40 clearly illustrate that, unlike cinnamic acid derivatives, the stereochemistry of the product dimer from anthracenes cannot be predicted on the basis of crystal packing. An example from the laboratories of Venkatesan is noteworthy in this context [218], Irradiation of crystals of 7-... 

The relative linearity of a Stern-Volmer plot of 1/< > versus 1/[A] for the photoreaction of cyclohexenone supports this mechanism and suggests that only one excited state is involved (30). The [2+2] addition of the triplet to a ground state molecule is not concerted, and there is a large solvent polarity effect on the regioselectivity for the head-to-head (HH) versus head-to-tail (HT) photodimers, with the more polar head-to-head isomer being favored in polar media. The polarity effect is attributed to the large difference in the dipole moments of the transition states leading to the products. 

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. 

Since the thymine photodimer was first isolated, its structure and the mechanism of the photodimerization reaction were studied extensively. Four different isomers of cyclobutane thymine dimer were proposed, but NMR spectroscopy was demonstrated to be insufficient to distinguish between them [46]. The structures of thymine photodimers are presented in Figure 13.3. A few years later the structure of thymine photodimers... 

It should be noted here that thymine photodimerization may occur by a non-concerted mechanism, involving free radical intermediates. Indeed, photoproducts other than cis-syn dimer, such as the next most abundant thymine dimer, so-called 6 4 adduct, were observed in irradiated DNA. However, the quantum yield of cis-syn photodimer formation (r/j 0.02) is more than an order of magnitude higher than that of the 6 4 adduct ( 0.0013) which in turn is an order of magnitude higher than the quantum yields for other thymine isomers [68]. This specificity can lead to the conclusion that the thymine photodimerization occurs predominantly via concerted 2 + 2 cycloaddition mechanism. A time-resolved study of thymine dimer formation demonstrated that thymine cyclobutane dimers are formed on a timescale of less than 200 nsec, while the 6 4 adduct is formed on a timescale of few milliseconds [69]. The delay in the formation of the latter was attributed to the mechanism of its formation through a reactive intermediate. 

Phenylbenzoxazole (121) forms an amorphous photodimer, either the 1,3-diazetidine (122) and/or the isomer (123). The dimer reverts to the monomer in solution the reaction is very fast under acid catalysis and 116 kJ mol-1 is released. 2-Phenylbenzoxazole is thus an efficient system for the storage and conversion of light energy (82CC380). 

For the photodimerization of thymine, four isomeric products are assumed to be formed (Fig. 16). It is known that ds-syn and trans-syn isomers are formed by photolysis of l,l -trimethylene-bis-thymine, TpT, and denaturated DNA. The chemical shift of the 5-methyl proton at 1.12 to 1.18 ppm in our case suggests the formation of syn-type isomers. It has been found that when acetone is used as photosensitizer the products exhibit photodimer distribution similar to that obtained without using a sensitizer. 

Stilbenes offer both advantages and disadvantages compared with photodimers. The absorption band which is changed by conversion of the more stable trans isomer to the cis form is much weaker than the strong band of the anthracene system that moves into the ultraviolet after dimerization. The stilbene band is somewhat stronger than the near-uv anthracene band. Trans-4-methoxy-4 -nitrostilbene absorbs at ca. 360 nm (e = 6 x 1(P). ... 

The Batelle workers report an index difference of 0.15 for these isomers, based on the extrapolation of index measurements made on dilute solutions. Their data yield nQ = 3.9 x 10- cm3 for system. This is a large difference, comparable to photodimer changes. 

Even so, the distinction between the two is sometimes a more subtle matter. Thus, in a photoisomerization a common excited state intermediate may undergo a transformation to either of the two isomeric cis-trans species of a planar ground-state molecule. These two transformations are virtually identical in nature, yet one leads back to the original species and is therefore a photophysical primary step, e.g., internal conversion or intersystem crossing, while the other leads to the chemically distinct isomer and should be called a photochemical primary step. As another example, the distinction between the formation of an excimer and of a photodimer lies in the instability and stability, respectively, of the dimeric species in the ground state. Excimer formation is usually considered as photophysical and photodimer formation as photochemical. These examples show that the classification of steps as photochemical and photophysical is in some cases arbitrary. 

Photodimers have been reported as the principal products from irradiation of numerous arene substrates. While there is little selectivity to be considered in most of these reactions, it has been reported that almost all photolyses of 9-substituted anthracenes give head-to-head dimers as depicted in U10). ° An intramolecular version of these photodimerizations has also been reported. Irradiation of di(a-2-naphthylmethyl) ether gave a mixture of the endo cyclomer (111) and the exo cyclomer (112). ° The photodimerization of 2-pyridones has been the subject of extensive investigation for a number of years." "" In most instances the head-to-tail or trans-anti isomer was the major or exclusive product in these reactions. Irradiation of A -methyl-2-pyridone for 15 h provided the trans-anti dimer (113) in 51% yield. This material was accompanied by much smaller quantities of other dimeric species. Vari-... 

The 1,2-cycloaddition reaction can take place in an intramolecular manner (equation 63), although in this example the initial excitation involves the aromatic group . A reaction of a different type is thought to be involved in the first stage of the formation of azulene or naphthalene photodimers from diphenylacetylene (equation 64), though here it is claimed that an intermediate benzocyclobutadiene species has been detected . The intermediate isomer of diphenylacetylene is formed via the triplet state and is relatively long-lived at — 10 °C. The major dimers formed are 1,2,3-triphenylazulene and 1,2,3-triphenylnaphthalene hexaphenylbenzene and octaphenylcubane are also produced . 

Photoaddition - Micellar aggregates of alkylated -stilbazolium salts in water form only head-to-head photodimers, with a preference for the jj/n-isomer, suggesting a significant degree of order within the aggregates. Preferential... 

With the possible exception of Sharp and Hanunond, who isolated but did not identify other dimeric products from the photoreaction of 2-pyridones in 1972, it was not until 1978 that the first attempt was made to isolate and identify isomers other than the heretofore discussed traits head-to-tail photodimers like 3. In a detailed study, Nakamura investigated the chromatographically isolable products and their yields as a function of solvent and concentration effects. These results, partially summarized in Figures 2 and 10, were the first to show the significant levels of the cis head-to-tail isomer 28 that was formed in all solvents. This study remains the only instance where regiochemical fidelity has been violated and head-to-head products 29 and 30 have been isolated. This occurs only in aqueous solution. It is noteworthy that the three very different solvents all give a similar mixture of cis and trans head-to-tail 3 and 28. 

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