Rose Bengal-sensitized dimerization

The effect of co-adsorbates such as alcohols and ACE was scrutinized by direct or sensitized excitation of ACN. In the direct excitation, the cisoid/transoid ratio was reduced from 8.78 with additives to 1.6 2.8 without additives. In the Rose Bengal sensitization, the ratio remained relatively constant at 0.34 0.61. Co-adsorption of alcohols increased ACN mobihty on the silica gel surface because of blocking of stronger binding sites and reduced the cisoid/transoid ratio because increased mobility by solvent molecules would be advantageous for dimerization from the T, state, rather than the S, state, over a longer time span. Moreover, the co-adsorbed alcohols would result in diminished formation of preferred pairs of ACN that would lead to cisoid-l from the S, state owing to the occupation of sites for pair formation by the co-adsorbate. When ACN is diluted with ACE as an inert counterpart, a hybrid pair of ACN with ACE that is unable to dimerize may be created, and therefore the cisoid/transoid ratio reflects the diffusional dimerization via the Tj state. 

The evidence from photosensitization derives from the Rose Bengal (RB) sensitized dimerization of acenaphthylene.1 15 It is observed that one RB molecule can yield, by repeated excitation, dimeric molecules with a high conversion. This requires that dimers move away from the RB molecules and that monomers move to occupy these vacant places. Evidence has been presented1 that the RB molecules are firmly fixed to the surface and thus the acenaphthylene monomers and dimers must be undergoing extensive motion on the surface during... 

A mixture of four dimers (8X) is obtained from the benzophenone-sensitized irradiation of the triene (164). Vacuum sublimation of this mixture affords the dimer (165) which was subsequently converted by conventional chemical paths to the hydrocarbon (166).Photosensitized (Rose Bengal) dimerization of aceanthrylene (167) has been studied in methanol solution. This affords both the syn- and anti-head-to-head and head-to-tail stereoisomers. The results indicate that there is a slight preference for the formation of the syn-adduc t s. ... 

Lead dioxide will oxidise 4,4 -(trimethylene)bis(2,6-di-t-butylphenol) leading to formation of a dispiro-compound by intramolecular cyclisation at the 4,4 -positions, and irradiation of this compound in a methylcyclohexane matrix at — 150°C gives 4,4 -(trimethylene)bis(2,6-di-t-butylphenoxy) diradical as a stable triplet species. Rose Bengal photooxidation of 2,3-dihydroxy-naphthalene and 2,7-dihydroxynaphthalene in basic aqueous solution gives the l,r-dimeric products by coupling of the radicals formed by electron transfer either to the excited sensitizer or to 02( Ag). ° The dimer which originates from 2,7-dihydroxynaphthalene is subject to further oxidation to 6,7-dihydroxyperylene-l,12-quinone. Irradiation of the dimethoxyphenol (82) under constant current electrolysis leads via (83) to formation of the substituted tricyclo[5.4.0.0 ]undec-8-ene (84) which can be converted into ( )-isoitalicene (85 R = Me, R = H). ... 

An earlier report by Maruyama et al. described the formation of dimers (439) from the direct irradiation of the acetylquinone (440). In the presence of an electron-donating sensitizer (Rose Bengal) irradiation of the same quinone yields a different dimer assigned structure (441). A further account of the regio- and stereo-specific dimerization of the quinones (442) to yield (443) has been reported. The possibility of the involvement of an enol intermediate was considered. In one example, that of (444), dimerization failed and the cyclobutene derivative (455) was obtained via a Norrish Type II reaction. 

The chemical and electrochemical oxidations of triphenylphosphine sulphide and phosphorothioates in acetonitrile have been investigated.In both reactions, triphenylphosphine sulphide initially gives a product formulated as a dimeric di-cation (39), but this rapidly decomposes on removal of the solvent or exposure to air. Triphenylphosphine oxide and sulphide form radical anions on treatment with potassium in ether at low temperature or by one-electron reduction at a Hg cathode in DMF. The photo-oxidation (sensitized by polymer-supported Rose Bengal) of triphenylphosphine selenide to the oxide has been shown to be highly solvent-dependent. 

The fluorescence spectra of rose bengal monomers and dye dimers (24) (a) belie the extent of dissociation at C-6 (Figure 2b,c) and are (b) sensitive to proximity effects. The latter is... 

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