An investigation of the benzophenone-sensitized photoisomerization of 2,4-hexadiene has been carried out in an attempt to infer if the preferred configuration of the lowest lying triplet state of the diene is in the form of a “1,4-biradical,” which would permit isomerization about both double bonds, or an “allyl-methylene” system, which would permit isomerization about only one double bond per excitation. During the course of the investigation it was found that a quantum-chain mechanism, heretofore unreported for the type of system, is operative during the early stages of photoisomerization. The chain-carrying species is believed to be an electronically excited state of the diene. Although the chain process appears to occur with isomerization of only one end of the diene, under conditions designed to minimize the chain process, the data are consistent with a “1,4-biradical” or rapidly equilibrating “allyl-methylene” system. © 1969, American Chemical Society. All rights reserved.
