Reversible photodimerization

The C-C linkage in tire polymeric [60]fullerene composite is highly unstable and, in turn, tire reversible [2+2] phototransfonnation leads to an almost quantitative recovery of tire crystalline fullerene. In contrast tire similarly conducted illumination of [70]fullerene films results in an irreversible and randomly occurring photodimerization. The important aspect which underlines tire markedly different reactivity of tire [60]fullerene polymer material relative to, for example, tire analogous [36]fullerene composites, is tire reversible transfomration of tire fomrer back to the initial fee phase. 

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.

More recent investigations of the photodimerizations of cyclopentenone and cyclohexenone<134) have revealed that both reactions result from the lowest triplet states of the enones (Et > 70 kcal/mole), which is probably 77- 77 in character/1331 Both photodimerizations occur via reversibly... 

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). 

It may be suspected that the genuinely topotactic (as secured by the molecular precision of the AFM [18]) photodimerization of 2-benzyl-5-benzyli-denecyclopentanone [118] might be a good candidate for a quantitative preparative photo dimerization to give the head-to-tail anti-[2+2] dimer. Early quantitative solid-state [2-1-2] photodimerizations (most of the published mechanistic interpretations of which can no longer be accepted) are listed in [110]. These deal with the anti dimerization of acenaphthylene-1,2-dicarboxylic anhydride, the head-to-head syn dimerization of acenaphthylene-1-carboxylic acid, the syn dimerization of 5,6-dichloroacenaphthylene, and the thermally reversible head-to-tail anti dimerization of seven ( )-2,6-di-f-butyl-4-(2-aryl-ethenyl)pyrylium-trifluoromethanesulfonates. All of these reactions proceed fully specific. On the other hand, quantitative photoconversions of a 1 1 mixed crystal of ethyl and propyl a-cyano-4-[2-(4-pyridyl)ethenyl]cinnamates gives mixtures of diesters with one (A>410 nm) or two cyclobutane rings (no cutoff filter). 

An unusual photochemical reaction of 2-pyridones, 2-aminopyridinium salts and pyran-2-ones is photodimerization to give the so-called butterfly dimers. These transformations are outlined in equations (13) and (14). Photodimerization by [2+2] cyclization is also a common and important reaction with these compounds. It has been the subject of particular study in pyrimidines, especially thymine, as irradiation of nucleic acids at ca. 260 nm effects photodimerization (e.g. equation 15) this in turn changes the regular hydrogen bonding pattern between bases on two chains and hence part of the double helix structure is disrupted. The dimerization is reversed if the DNA binds to an enzyme and this enzyme-DNA complex is irradiated at 300-500 nm. Many other examples of [2+2] photodimerization are known and it has recently been shown that 1,4-dithiin behaves similarly (equation 16) (82TL2651). 

The natural propensity of 2-pyridone photodimerization to favor the head-to-tail [4+4] adducts can be reversed by the use of a tether (Sch. 42). For head-to-head 189, photocycloaddition leads to a 1 1 mixture of the cis and trans adducts [122]. For tail-to-tail isomer 192, a high proportion of the cis adduct 194 is generated, although product mixtures for both reactions change under extended irradiation times, favoring the cis isomers [123]. 

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]. 

At low conversions the diffraction efficiency of the holographic grating can be monitored to determine the rate of photodimerization. Kohler et al. have measured an upper limit on the activation energy for photodimerization of 3.6 kcal/mol, which is very close to a value of about 3.1 kcal/mol determined by Nakanishi et al. for the dimerization of distyrylpyrazine [96]. Rates of disappearance of the diffraction pattern can be used to measure the rate of reversion to starting monomer which occurs upon thermal annealing above 100°C. Kohler et al. have measured the activation energy for this process to be 23.7 kcal/mol. 

The reversible photodimerization of polycyclic aromatic hydrocarbons offers a number of advantages as a general technique for refractive-index imaging. We mentioned earlier the well-known reversible dimerization of anthracene and its relatives,... 

The acridizinium crystals showed a change in behavior after repeated cycling (several thousand times) which was attributed to strains caused by the difference in molecular volumes of the two species present. This is a potential problem for any such system and it is probably the most difficult inherent problem in using reversible photodimerization for reversible holographic imaging. 

Peroxide Sensitizers. Poly(methylmethacrylate), PMMA, is the first polymer reported to exhibit an increase in refractive index upon irradiation with uv light (41). Irradiation at either 325 nm (Cd laser) or 365 nm (Hg arc) was found to give an index increase of up to 0.3%, and the resolution, determined from gratings, was shown to be at least 5000 lines/mm. Applications to optical waveguides and holographic elements were suggested. This phenomenon was a serendipitous discovery. The PMMA had been intended for use as an inert matrix for reversible photodimerization when its own photosensitivity was uncovered. 

When the DMSO solution was kept at 120 °C after irradiation, UV-visible and fluorescence spectra showed that the anthracene monomer came back again in more than 90% conversion. These spectral changes showed that the reversible photodimerization and dissociation of 9-anthracene end-groups took place. 

When after formation of photoproduct at one of the two sites the crystal was heated and cooled the photoreaction was reversed. We were able to conclude with the help of the packing calculations of sec. 3.2 and other experimental results that those impurity molecules that reacted could not be those that replaced anthracenes in the regular lattice, but must be at dislocations or regions of abnormal packing, where the host and guest were in face-to-face positions. The reaction is thus belongs to the class of the photodimerizations of anthracene, and 9-cyanoanthracene, not of dimers formed topochemically in the normal lattice. 

Other Photofragmentations - [4+4] Cycloadducts of anthracenes and furan are both formed and cleaved photochemically. (o-Anthrylpolystyrenes have been shown to undergo reversible photodimerization, allowing photochemical control of molar rnass. The cycles can be conducted more than ten times without detectable degradation of the polymers. Polyurethanes and poly-ethers containing coumarin components exhibit similar reversible photopolymerization and cleavage. 

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