Mechanism for photoisomerization of an olefin in the presence of electron acceptors as studied by magnetic field effects

An external magnetic field is found to retard the photoisomerization of trans-3,3-dimethyl-1-(2-naphthyl)-1-butene ( t -DMNB) in degassed acetonitrile in the presence of electron acceptors. From this and other observations, the precursor of the isomerization is indicated to be the olefin triplet arising from intersystem crossing of a singlet radical pair consisting of a t -DMNB cation and an acceptor anion, identified by nanosecond flash photolysis.