Photochemical Trans Cis Isomerization of 1,2-Bis(hetroaryl)ethylenes: 1,2-Bis(pyrazinyl)ethylene

Article abstract of DOI:10.1021/ja00265a027

The direct and sensitized trans (*) cis photoisomerization of 1,2-bis(pyrazinyl)ethylene is investigated in various conditions.Quantum yields of the direct trans -> cis photoisomerization increase with increasing solvent polarity because of the proximity of the lowest ¹(n,?^*) and ¹(?,?^*) states.Azulene quenches the direct and sensitized photoisomerization, giving the same Stern - Volmer constant indicating that the triplet state is the reactive state in both cases.The directly measured triplet lifetime of trans-1,2-bis(pyrazinyl)ethylene by laser spectroscopy is the same as the lifetime calculated from the azulene quenching studies.Laser spectroscopy also indicates efficient intersystem crossing following direct irradiation.From these results, it is concluded that the direct photoisomerization of 1,2-bis(pyrazinyl)ethylene proceeds through the triplet manifold in contrast to stilbene which undergoes direct trans (*) cis photoisomerization through the singlet manifold.