Cis and Trans Olefin Radicals: Equilibrium and Photoisomerization

Electrochemical reduction of 1,2-bis(l-methyl-4-pyridino)ethylene dication gives the same reduction potentials (-0.53 and -0.72 V vs SCE in CH₃CN) for both cis and trans isomers. Only the absorbance spectrum of the trans radical (∊(max) 57 000 M^-1 cm^-1 at 517 nm) is observed. However, by using pulse radiolysis the radicals of both cis and trans isomers are observed in aqueous solution. Although their absorbance spectra are similar, the extinction coefficients are very different. The cis radical is intrinsically stable but undergoes rapid electron exchange with the trans dication isomer. Cis and trans radicals equilibrate through electron exchange, the limiting rate constants in both directions of the equilibrium being 2 X 10⁹M^-1 s^-1(cis to trans) and 3 X 10⁸M^-1 s^-1(trans to cis). The photoisomerization of the olefin radicals is essentially one-way: the quantum yield for the cis radical is 0.2, while that of the trans is less than 0.01.