Reversible photoisomerization of beta-methyl-beta-nitrostyrene: The role of triethylamine.

The photolysis of trans-beta-nitro-beta methylstyrene in acetonitrile solut ion gives the cis-isomer with a quantum yield of 0.8. Addition of triethyla mine to the system leads to a small decrease in the photoisomerization quan tum yield and the appearance of a strong EPR signal. This signal remains fo r several hours at the end of the irradiation. It is demonstrated that the EPR signal is associated to an intermediary produced during the triethylami ne catalyzed thermal back formation of the trans isomer. This intermediary bears an unpaired electron mostly associated to the nitro group and suggest that the thermal back reaction takes place by an initial electron transfer from the amine to the nitro group, leading to a long lived zwitterionic bi radical.