Isomers, reversible photodimerization

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... 

Reversible photodimerizations of l- -alkylthymine in single crystals depend on the crystal structures. However, the orientation of the thymine bases in the single crystal does not correspond to the isomer of the photodimer formed because thymines undergo disrotatory motion during the photodimerization process in the single crystals. The most important factor of the crystal structure for the photodimerization is the distance between the thymine base and the terminal methyl group of alkyl chain because this can influence the rotation of the thymine bases. 

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]. 

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... 

Further, the cyclobutane rings in the photodimerized product of 32 can be cleaved by heating to 220°C for 12 h under vacuum in two different ways to yield a mixture of trans and cis isomers of pvba quantitatively as monitored by the H-NMR spectroscopy. This transformation as depicted in Fig. 20 appears to be the first attempt to reverse the cyclized product in CPs. From the integration of H-NMR spectmm, the ratio between the cis and trans isomers has been observed to be 66.5% and 33.5%, respectively. Formation of other isomer of pvba has also been supported by a new peak observed in photo luminescence at 370 nm, which is different from trans pvba and the dimer. Under similar conditions, the photoproducts of 33 and 34 showed scission of cyclobutanes up to 56% and 67%, respectively. Similar to 32, the new peaks also have been observed during the scission of cyclobutane, which are related to cis isomer of pvba. However, the intensities of these peaks are very small for integration from H-NMR spectral data. 

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