Steady-State and Laser Flash Photolysis Studies of Bridgehead-Substituted Dibenzobarrelenes.

Article abstract of DOI:10.1021/jo00214a026

The photochemistry of bridgehead-substituted dibenzobarrelenes 1a-h (Scheme I) has been investigated by steady-state photolysis, product analysis, and time-resolved laser flash photolysis. 4b-Substituted dibenzosemibullvalenes 8b,c,f,g are formed as major photoproducts from dibenzobarrelenes 1b,c,f,g,bearing acetoxy, formyl, ethyl, and methoxy 9 -bridgehead substituents, whereas 8b-substituted dibenzosemibullvalenes (or products derived from them) (7 and 6e) are formed from 9-hydroxy- and 9-cyano-substituted dibenzobarrelenes 1a,e.Laser flash photolysis gives rise totransient phenomena attributable to the triplets of dibenzobarrelenes (Φ_T = 0.2-0.7 in benzene).These are characterized by broad and diffuse absorption spectra, short lifetimes (0.07-11 μs), and varying degrees of quenchability by azulene, ferrocene, β-carotene, and p-methoxyphenol.The rate constants for triplet quenching by the latter quenchers as well as oxygen and di-tert-butylnitroxy radicals are presented.