Stilbene, photochemical isomerization photoisomerization

Photoisomerization of Stilbene Especially detailed study of the mechanism of photochemical configurational isomerism has been done on Z- and E-stilbene. The isomerization involves a twisted singlet state that can be attained from either the Z- or the E-isomer. Spectroscopic data have established the energies of the singlet and triplet states of both Z- and E-stilbene and of the twisted excited states that are formed from both isomers. Some features of the potential energy diagram are due to structural... 

The ground-state synthesis of imines, oximes, and azo compounds invariably produces the more stable -isomers and photochemical isomerization is practically the only means for preparing their less-stable Z-isomers, as in the case of olefins such as stilbene. For example, the pharmacologically active Z-isomer of isonicotinaldehyde was prepared by photoisomerization (Scheme 50)." A less stable E-isomer of methyl p-nitrophenylhydroximate O-methyl ether can be isolated in 38% yield by UV irradiation of the Z-isomer in benzene, followed by preparative TLC (Scheme 51)." ... 

Category 4. Photoisomerization. The most common reaction in this category is photochemical cis-trans isomerization." For example, cw-stilbene can be... 

It was only recently that these phenomena were detected photochemically using photochromic molecules whose chromophores undergo a reversible photoisomerization reaction. The photochemical reactions involved in these processes are mainly the trans-ds isomerization of aromatic azo compounds and stilbene derivatives as well as the ring opening/dosure reaction of spirobenzopyran derivatives . ... 

The photoisomerization of all types of azobenzenes is a very fast reaction on either the singlet or triplet excited-state surfaces according to the preparation of the excited state, with nearly no intersystem crossing. Bottleneck states have lifetimes on the order of 10 ps. The molecules either isomerize or return to their respective ground states with high efficiency. So photoisomerization is the predominant reactive channel, and the azobenKnes are photochemically stable. Only aminoazobenzene-type molecules and pseudo-stilbenes have small quantum yields of photodegradation. 

Silver ions cause perturbation of the (E)-(Z) photoisomerization pathway for both stilbene and azobenzene . The efficiency of silver ions in this respect is compared with the effect of Nal which can only induce a heavy atom effect. Ag+ clearly forms complexes with both compounds. Observation of cis-trans conversion in olefin radical cations shows that electron transfer can bring about isomerization of stilbene derivatives. The efficiency of such processes obviously depends on the presence and nature of any substituents. Another study deals with photochemical generation, isomerization, and effects of oxygenation on stilbene radicals. The intermediates examined were generated by electron transfer reactions. Related behaviour probably occurs through the effect of exciplex formation on photoisomerization of styrene derivatives of 5,6-benz-2,2 -diquinoyE. ... 

The discussion of cis-trans photoisomerization of alkenes, styrene, stilbene, and dienes has served to introduce some important ideas about the interpretation of photochemical reactions. We see that thermal barriers are usually low, so that reactions are very fast. Because excited states are open-shell species, they present new kinds of structures, such as the twisted and pyramidalized CIs that are associated with both isomerization and rearrangement of alkenes. However, we will also see familiar structural units as we continue our discussion of photochemical reactions. Thus the triplet diradical involved in photosensitized isomerization of dienes is not an unanticipated species, given what we have learned about the stabilization of allylic radicals. 

Most of the papers in this section are concerned with photo-oxidation reactions, but a few papers refer to photochemical reactions in polymer matrices. Thus the kinetics of the photo-oxidation of anthracene and naphthacene in solid polystyrene (PS),170 the photoionization of aromatic hydrocarbons dissolved in PMMA and PS,171 the photoreactions of naphthalene in cellulose triacetate,172 the cis-trans isomerization of stilbene residues in the side-chains of polymers,173 intrachain photodimerization in polymers,174 photoisomerization of 1,2-diphenyl-cyclopropane by peptides containing naphthalene in a side-chain,176 and photochemical transformations of poly(vinyl p-azidobenzoate)176 have been reported. 

The effed of thienyl groups on the photoisomerization and rotamerism of symmetric and asymmetric stUbenes has been studied [73]. Stationary and pulsed fluorimetric techniques, laser flash photolysis, and conventional photochemical methods and theoretical calculations were used for investigating photochemistry of flve symmetric (bis-substituted) and asymmetric (mono-substituted) analogues of -stilbene, where one or both side aryls are 2 -thienyl or 3 -thienyl groups. It was shown that the presence of one or two thienyl groups and their positional isomerism afled the spectral behavior, the relaxation properties (radiative/reactive competition), the photoisomerization mechanism (singlet/triplet), and the ground-state rotamerism. 

Photochemical and photophysical properties of a poly(propylene amine) dendri-mer (2) functionalized with -stilbene units have been studied [102]. Z-photoisome-rization and photocyclization of the Z-isomer of the stilbene units were investigated in air-equilibrated acetonitrile solutions. The quantum yields of the E Z photoisomerization reaction and the fluorescence quantum yield of the E were found to be equal to 0.30 and 0.014, respectively. Stilbene dendrimers prepared by coupling 4,4 -dihydroxystilbene with first-, second-, third-, or fourth-generation benzyl ether-type dendrons underwent photoisomerization with the same efficiency as that of 4,4 -dimethoxystilbene [103], The lifetime of the core structure was found to be shorter then 1 ns. According to [104], polyphenylene-based stilbene dendrimers, Gl, G2, and G3, underwent mutual cis-trans isomerization upon direct irradiation with 310 nm light at room temperature. In a solvent glass at 77 K, one-way cis-trans isomerization was observed for G2. 

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