Valence isomerization photochemical

These various photoproducts are all valence isomers of the normal benzenoid structure. These alternative bonding patterns are reached from the excited state, but it is difficult to specify a precise mechanism. The presence of the t-butyl groups introduces a steric factor that works in favor of the photochemical valence isomerism. Whereas the t-butyl groups are coplanar with the ring in the aromatic system, the geometry of the bicyclic products results in reduced steric interactions between adjacent t-butyl groups. 

The photochemical valence isomerization of the compounds 11, 12, and 14 to the 7-azaquadricyclane derivatives 76-78 is accomplished by... 

Many thermal and photochemical valence isomerizations leading to cyclobutanes have been investigated with regard to mechanistic aspects. However, preparative applications also exist, especially when eight-membered unsaturated compounds are used as substrates. 

Zhu et al.,26 characterized the radical cation of 1,4-perinaphthadiyl as an intermediate in the photochemical valence isomerization of perinaphthocyc-lobutane radical cation to 1,8-divinylnaphthalene radical cation. 

Valence Isomerism.—Photochemical energy storage via the photosensitized valence isomerization of norbornadiene to quadricyclene has continued to attract attention. The effects of excitation wavelength and suppression of side reactions have been studied and the efficiency of various photosensitizers has been compared. It has been concluded that such systems are technically feasible but economically non-competitive at present. Similar valence... 

A third example, the photochemical valence isomerization of the tricycle 34 to form 1-cyanoheptalene (35), is worthy of mention. It proceeds at 254 nm with a quantum yield nearly 100 times that at 365 nm. In this case, the rate of reaction depends on the initial concentration, but not on the instantaneous concentration. This is an indication that the isomerization proceeds via an exciplex... 

On irradiation, 1,2-dihydronaphthalenes undergo photochemical valence isomerization to give 1,1 a,2,6b-tetrahydrocyclopropa[()]indenes (1). ... 

The photochemical valence isomerization of 1,2-dihydronaphthalenes to l,la,2,6b-tetrahydrocycloprop[Z>]indenes (e.g., 30 31) is well estab-... 

Norbornadiene undergoes a photochemical valence isomerization to quadricyclane with unit quantum yield when photosensitized with acetophenone. The proposed reaction scheme (14) includes two triplet state intermediates norbornadiene triplet (14b) and the 3,5-nortricyclanediyl biradical (14c). Based on the full Breit—Pauli with a rather modest qual-... 

Pyridine undergoes thermal as well as photochemical valence isomerizations analogous to benzene. 

Photochemical valence isomerization was also successfully applied to generation of [4]paracyclophane (3a). Tsuji and Nishida reported that irradiation of [4.2.2]propelladiene (25a) in ethanol at 77 K produced a reactive species possessing an electronic absorption at 340 nm which was assigned to 3a (Scheme... 

Even such weak nucleophile as alcohols underwent nucleophilic addition to highly strained [4]paracyclophane (3a), which was generated by photochemical valence isomerization of the Dewar isomer 25a (Scheme 8) [24]. 

The goal to utilize photochemical valence isomerization between norbomadiene and quadricyclane for solar energy collection and storage was reported by others [185]. Nagai et al. synthesized five different polymers with trifluoromethyl-substituted norbomadiene moieties in the side chains and in the main chain [185] ... 

Perfluoropentaethylmethylbenzene isomerizes photochemically to a mixture of Dewar-benzene isomers. Further irradiation converts these into the prismanes. These valence isomers of benzene are remarkably stable, and appear to owe their stability, in part, to destabilization of the parent benzene by non-bonded interactions. 

This paper describes the synthesis of some new functional monomers bearing both polymerizable vinyl groups and photorespon sive NBD moieties, and the thesis of polymers containing pen dant 2,S-Nl3D-carbo late moieties by cationic polymerization and radical polymerization of these monomers. Also investigated were photochemical valence isomerization of pendant NBD moieties in the polymer films, and photochemical reversion of the produced QC groups in the polymer films. 

Photochemical Valence Isomerization between Pendant NBD moiety and the QC Group in Polymer Film... 

From the results of photochemical valence isomerization be tween pendant NBD moiety and the QC group in the polymer film, the following condusions can be drawn. 

The photochemical valence isomerization between pendant NBD moiety and the QC group in polymer film is strongly affected by both the wavelength of the photo-irradiation and Ity the com position of the polymer backbone. 

Many polyfluorobenzenes undergo photochemical valence-bond isomerization to give, in most cases, one or more para-bonded isomers. Hexafluoro-benzene gives up to 60% of hexafluorobicyclo[2,2,0]hexa-2,5-diene (111)... 

Several types of photochemical valence isomerizations of dienes are known. The reactions shown in Eqs. (82)-(87) are typical. They represent... 

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