Sensitizer 9,10-dicyanoanthracene isomerization

TABLE 6 Quantum Yields of Dicyanoanthracene-Sensitized Isomerization of Stilbenes iu Acetonitrile ... 

The 9,10-dicyanoanthracene sensitized irradiation of c/i-stilbene results in nearly quantitative isomerization (>98%) to the trans isomer with quantum yields greater than unity. Therefore, the isomerization was formulated as a free radical cation chain mechanism with two key features (1) rearrangement of the c/i-stilbene radical cation and (2) electron transfer from the unreacted cis-olefin to the rearranged (trans-) radical cation. 

The general features of the isomerization are compatible with a free radical cation chain mechanism, featuring electron transfer from unreacted olefin to rearranged radical cation. This chain mechanism was firmly established in several other isomerizations by the observation of quantum yields greater than unity. Thus, the dicyanoanthracene sensitized irradiation of m-stilbene results in nearly quantitative isomerization (> 98%) to the trans-isomer. In this system, the quantum yield increases with increased ds-stilbene concentration, solvent polarity, salt concentration, as well as decreasing light intensity [159]. 

Behavior of olefin radical cations has not been vjell investigated until recently. Lewis reported that 9,10-dicyanoanthracene (DCA) sensitized irradiation of stilbene led to its cis to trans isomerization (33). The resulting c - from electron transfer from cis-isomer to DCA isomerizes to t , v hich subsequently accepts an electron from cis to give trans and regenerate leading to a chain process. The quantum yield for cis to trans isomerization increases with increasing cis-isomer concentration. 

In MeCN the products of photo -oxidation of Ph2C=CH2, cis- and trans-PhCH=CHPh, and Ph2C=CPh2 using 9,10-dicyanoanthracene and 9-cyano-anthracene as sensitizers include benzophenone, benzaldehyde, epoxides, and products of cu-trfln5-isomerization. " A correlation is established between the rate constants for electron-transfer processes and those determined from the acceptor concentration-dependence of product formation. These observations appear to implicate a sensitizer radical anion that subsequently reduces O2 to Oj-... 

As described above, upon photoexcitation, stilbene undergoes a two-way isomerization. However, upon 9,10-dicyanoanthracene (DCA) sensitization in acetonitrile, cis-stilbenes undergo essentially one-way isomerization via a radical cation chain mechanism. Either a unimolecular cis - - trans - or a bimolecular process involving addition of cis - to the neutral stilbene may lead to trans isomers [139-156]. 

The 9,10-dicyanoanthracene (DCA)-sensitized c/s,tram-photoisomerization of highly electron-rich 1,2-diarylcyclopropanes and diarylmethylenecyclopropanes efficiently proceeds via a radical cation chain transfer mechanism.The photoisomerization of electron-rich l-diarylvinylidene-c/s-2,3-dimethylcy-clopropane (c-2d) in acetonitrile was also sensitized by DCA via photoinduced electron transfer. The photoisomerization efficiently takes place to give a 3 7 PSS mixture of c-2d and t-2d. However, the less electron-rich 1-diarylvinylidenecydopropanes c-2a-c under the same reaction conditions slowly isomerized... 

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