Films reversible photodimerization

Also, water-soluble poly(vinyl alcohol) was modified by thymine derivatives. This yielded a material that reversibly photodimerizes upon UV irradiation. Photocrosslinking experiments of this polymer were carried out in the film state on quartz plates by irradiation with light at 280 nm. The poly(vinyl alcohol) containing thymine units were found to be more light sensitive than uracil units in a similar polymer. In addition, photodimerization reaction of an ester derivative of the thymene substituted poly(vinyl alcohol) was found to be faster than that of an acetal derivatives, but the sensitivity was lower, because... 

Reversible Photodimerization Annealing of the Thin Film Powder X-ray Diffraction Thermal Analysis of the Thin Films Isomers of the Photodimers Reverse Reaction in Spin-Coated Film Crystal Structure and Photodimerisation... 

FIGURE 104.5 Typical repeated reversible photodimerization (1-n-hexadecylthymine in spin-coated film). 

Mochizuki, E., Yasui, N., Kai, Y, Inaki, Y, Wang, Y, Saito, T, Tohnai, N., and Miyata, M., Reversible photodimerization of ester derivatives of thymine having long alkyl chain in solid film. Polymer /., 32, 492, 2000. 

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. 

Another interesting photochemical reaction that occurs with the monolayers is dimerization. This is exempUfled by the photochemical behaviour of the SAM of 7-(10-thiodecoxy)coumarin (52) on polycrystaUine gold. Irradiation at 350 nm results in the (2 -f 2)-cycloaddition of the coumarin moieties. The photodimerization is a reversible process by irradiating at 254 nm. Better regioselectivity in the cycloaddition is obtained when the solid monolayer is irradiated rather than when it is in contact with benzene. The dimer formed is the yn-head-to-head dimer identified as 53 . Self-assembled monolayers of cis- and frani-4-cyano-4 -(10-thiodecoxy)stilbene (54) are also photochemically reactive. Irradiation of a thin film in benzene solution using A, > 350 nm results in the formation of a photostationary state with 80% of the cis-isomer present. Irradiation in the solid shows that cis.trans isomerism occurs but that trans.cis-isomerism fails. Prolonged irradiation brings about (2 - - 2)-cycloaddition of the stilbene units to afford cyclobutane adducts. Such dimerization is a well established process . The influence of irradiation at 254 nm or 350 nm of self-assembled monolayers of 10-thiodecyl 2-anthryl ether on polycrystaUine... 

---