Steady-state and laser flash photolysis studies of bridgehead-substituted dibenzobarrelenes

Abstract

The photochemistry of bridgehead-substituted dibenzobarrelenes has been investigated by steady-state photolysis, product analysis, and time-resolved laser flash photolysis. 4b-Substituted dibenzosemibullvalenes are formed as major photoproducts from dibenzobarrelenes, bearing acetoxy, formyl, ethyl, and methoxy 9-bridgehead substitutents, whereas 8b-substituted dibenzosemibullvalenes (or products derived from them) are formed from 9-hydroxy- and 9-cyano-substituted dibenzobarrelenes. Laser flash photolysis gives rise to transient phenomena attributable to the triplets of dibenzobarrelenes (phi/sub T/ = 0.2-0.7 in benzene). These are characterized by broad and diffuse absorption spectra, short lifetimes (0.07-11 /sup +/s), and varying degrees of quenchability by azulene, ferrocene, ..beta..-cartene, and p-methoxyphenol. The rate constants for triplet quenching by the latter quenchers as well as oxygen and di-tert-butylnitroxy radicals are presented.