Photoisomerization behavior of 1-(9-anthryl)-2-(2-pyrazinyl)ethene, a diaza analogue of 1-(9-anthryl)-2-phenylethene

The fluorescence properties and photoisomerization behavior for trans and cis isomers of 1-(9-anthryl)-2-(2-pyrazinyl)ethene (APzE). a diaza analogue of 1-(9-anthryl)-2-phenylethene (9-APE), were studied. The fluorescence maximum (λ 1 ). quantum yield ( Φ t ) and lifetime ( τ t ) of t -APZE were solvent-dependent reflecting the intramolecular charge transfer character in the lowest excited singlet state. However, in polar protic solvent, some discrepancy was observed probably due to protonation on the nitrogen atoms. The photoisomerization of t -APzE was observed even in very nonpolar solvent, in contrast that the parent t -9-APE did not photoisomerize in any solvent and photoisomerization of monoaza analogues t 1-(9-anthryl)-2-( n -pyridyl)ethenes ( n -APyE) was very inefficient in nonpolar solvent. The quantum yield of the trans → cis photoisomerization ( Φ t → c ) of t -APzE was also solvent-dependent. Not only the inverse dependence for Φ f -and Φ t → c , of t -APzE on the temperature and the solvent polarity but also hiacetyl-sensitized photoisomerization experiment indicate that the photoisomerization of t -APzE occurs, at least in part, via the excited singlet state. In air-saturated solution, another side photoproduct was observed in all solvent examined and identified as anthraquinone, a photo-oxidation product. For c -APzE, the τ t , is too short to measure and Φ t , is relatively low. Quantum yields of fluorescence and cis→trans photoisomerization of c -APzE in various solvent and temperature indicate singlet mechanism for the cis → trans photoisomerization of c -APzE in polar solvent but mixed singlet/triplet mechanism in nonpolar solvent. On irradiation of c -APzE, the photo-oxidation product was not generated at all in any solvent. In n -hexane, photocyclization competing with photoisomerization was observed both for t - and c -APzE.