Knowledge Bank

One possible answer to the as yet unexplained channel three relaxation of $1B2u$ benzene is a reversible photochemical isomerization to one of several $C6H6$ isomers. We have examined in detail the spectra of three isomers, fulvene, benzvalene and Dewar benzene. Excitation of fulvene into its second excited singlet state causes an efficient $(Bz254nm=0.65$ at $0.5$ Torr) photo-rearrangement to benzene. Benzvalene photochemistry is more complex due to thermal instability, but it appears also to photo-rearrange with high quantum efficiency to benzene.