Formation of p-xylylene from p-xylene by a two-photon process and hexamethyl Dewar benzene from hexamethylbenzene by a one-photon process at 193 nm

While studying a series of methyl-substituted benzenes, C6H6−n(CH3)n with n = 2, 3, 4, 6, and perfluorobenzene in the gas phase using 193-nm laser flash photolysis, we observed the formation of p-xylylene (benzoquinodimethane) due to the elimination of two hydrogen atoms as a result of a two-photon process. The results were explained in terms of an intermediate hot molecule formed by internal conversion which finally led to the ground electronic state. Quadratical dependencies on the photoproducts were observed for toluene, xylene, mesitylene, and perfluorobenzene in the presence of a foreign gas, while linear dependencies were observed for durene and hexamethylbenzene. Dewar-type benzene was detected from photolysis of hexamethylbenzene.