Photochemistry of dienones. Part 7. On the photosensitized isomerization of (E)-β-ionone and its isomeric α-pyran. Evidence for exciplex formation between the α-pyran and fluoren-9-one

Abstract

The photosensitized (E)-β-ionone (1)–α-pyran (2) isomerization in benzene as a solvent has been studied in some detail. For xanthone, benzophenone, and 1- and 2-acetonaphthone the (1) : (2) photostationary state (p.s.s.) ratio is 3.5 ± 0.2, whereas for the series of aromatic hydrocarbons studied and the ketones fluoren-9-one and benzanthrone it is 0.37 ± 0.07, both for a total substrate concentration of ca. 0.1M. The low {[(1)] : [(2)]} p.s.s. values observed with the second group of sensitizers are not the result of additional singlet sensitization. They are ascribed to a specific additional route of isomerization for the α-pyran via singlet exciplex formation which route is almost negligible for (E)-β-ionone. This explanation is in line with both the Stern–Volmer plots for the quenching of the fluorescence of fluoren-9-one by the α-pyran and by (E)-β-ionone [the quenching being far less with (E)-β-ionone than the α-pyran], and by the dependence of {[(1)]/[(2)]} p.s.s. on the total substrate concentration in the case of fluoren-9-one as a sensitizer. From the dependence of the photostationary state for the triplet photosensitized isomerization on the triplet energy of the sensitizer, it was concluded that the triplet energies of (E)-β-ionone and the isomeric α-pyran (2) are 54–56 and < 54 kcal mol^–1, respectively.