Excited state properties of some 1-(9-anthryl)-2-naphthylethene and 1-(9-anthryl)-2-quinolylethene derivatives

The photophysical properties of the lowest singlet and triplet excited states of some trans-1-(9-anthryl)-2-(n-naphthyl)ethenes (n = 1 or 2, t-n-ANE) and trans-1-(9-anthryl)-2-(n-quinolyl)ethenes (n = 2–4, t-n-AQE) derivatives have been studied in methylcyclohexane and acetonitrile at room temperature. The hydrocarbon derivative t-1-ANE exhibits fluorescence and intersystem crossing quantum yields (which are independent of solvent polarity) very similar to those of trans-1-(9-anthryl)-2-phenylethene. For the monoaza analog t-4-AQE, the fluorescence and triplet quantum yields in acetonitrile are much lower than in methylcyclohexane. t-4-AQE undergoes trans → cis photoisomerization even in the nonpolar solvent. t-2-ANE and t-3-AQE show remarkably high fluorescence efficiency. Surprisingly, the excited state properties of t-2-AQE are highly solvent-dependent. In polar solvents, the fluorescence quantum yield is reduced, while the photoisomerization yield increases. But, for both t-2- and t-4-AQE, the triplet quantum yield also decreases with solvent polarity. Therefore, photoisomerization probably proceeds via a singlet manifold. The singlet oxygen quantum yields of t-n-AQE and t-n-ANE are strongly dependent on both the solvent polarity and oxygen concentration, indicating that singlet oxygen is very likely being generated from both the excited singlet and triplet states.

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