Photochemical cis⇔trans isomerization of 1,2-bispytazylethylene

Abstract

The direct and sensitized cis$\rightleftharpoons$trans photoisomerization of 1,2-bispyrazylethylene is investigated in various conditions. Quantum yields of the direct trans→cis photoisomerization increase as the polarity of solvents increase because of the proximity of the lowest $^1(n,π^*)$ and $^1(π,π^*)$ states. Intersystem crossing is also enhanced because these two states are extensively mixed as the same reasons. Quantum yields of direct cis→trans isomerization are lower than those of trans→cis isomerization. The observed quantum yields of trans→cis isomerization by benzophenone sensitization decrease as the concentration of benzophenone increases, whilst 1,2-bispyrazylethylene undergoes fairly efficient geometrical isomerization when photosensitized by biacetyl. 1,2-Bispyrazylethylene shows $^Φfl$ and $^Φnr$ values between those of ortho and meta aza-analogues of stilbene in an inert solvent at room temperature. The fluorescence quantum yields of trans-1,2-bispyrazylethylene at low temperature, however, do not increase to near unity at 77k but far less than unity in contrast to stilbene and their azaanalogues. The pH and salt effects on the energy of $^1(n,π^*)$ and $^1(π,π^*)$ states change the photoisomerization and fluorescence to the different directions. 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 measured triplet lifetime of trans-1,2-bispyrazylethylene by laser spectroscopy is same as the calculated lifetime by azulene quenching studies. Azulene quenching on the photoisomerization and nanosecond laser spectroscopy results also indicate an efficient intersystem crossing on direct excitation. From these results, it is concluded that the direct photoisomerization of 1,2-bispyrazylethylene proceeds through the triplet manifold in contrast to stilbene which undergoes direct cis$\rightleftharpoons$trans photoisomerization t...