Quantum yield benzvalene isomerization

Foote, Mallon, and Pitts48 report a gas-phase isomerization of benzene at 1849 A with a quantum yield of approximately unity, and tentatively identify the isomer as benzvalene although they suggest the... 

Non-sensitized isomerization of unsubstituted Dewar benzene and benzvalene was found to proceed in high quantum yield (45, 46) 48). 

On the other hand, photochemical activity is in marked contrast with triplet formation. Chemical relaxation in benzene increases with higher energy excitation, but the absence of even approximate primary quantum yields makes quantitative correlation of photochemical activity with other data impossible. No attempt will be made here to review the extensive literature on benzene photochemistry (see, e.g.. Ref. 124). Suggestive evidence to link chemical activity with the fast nonradiative decay beginning with vibrational levels near 2500-3000 cm- above zero-point energy can be found in the paper of Kaplan and Wilzbach describing their 2537 A vapor phase experiments. Vapor yields of benzvalene formation and of the benzene-r/a isomerization increase as excitation moves from 2537 to 2370 A, but the sum of observed photochemical yields remain well below unity. The correlation is far from quantitative. 

Table 6.4 shows the principal photoreactions of aromatic compounds that we discuss in this chapter. Upon irradiation, aromatic compounds, such as benzenes, naphthalenes and some of their heterocyclic analogues, undergo remarkable rearrangements that lead to some non-aromatic highly strained products, such as benzvalene and Dewar benzene (entry 1), which can be isolated under specific conditions. Quantum and chemical reaction yields are usually low however, photochemistry may still represent the most convenient way for their preparation. While bulky ring substituents usually enhance the stability of those products, aromatic hydrocarbons substituted with less sterically demanding substituents exhibit ring isomerization (phototransposition) (entry 2). 

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