Hydroxyanthraquinones as sensitizers of singlet oxygen reactions: quantum yields of triplet formation and singlet oxygen generation in acetonitrile

Abstract

Abstract The photophysical properties, in particular those of the triplet states, of α- and β-hydroxy-substituted 9,10-anthraquinones (1-AQ, 2-AQ, 1,2-AQ, 1,4-AQ, 1,8-AQ, 2,6-AQ, 3-Me-1,6,8-AQ and 1,2,5,8-AQ) were studied. In acetonitrile at room temperature, fluorescence occurs from the short-lived (τF ⩽ 1 ns), lowest excited 1(π, π*) states (Es(1) between 65 and 55 kcal mol−1) with quantum yields φF ⩽0.025 (the exception is 1,4-AQ with τF≈2 ns and φ≈0.1). In methylcyclohexane as well as in EPA at 77 K, AQ gives rise to a vibrationally resolved phosphorescence spectrum, from which the energy of the lowest triplet state T1(n, π*) is determined (ET= 61.6 kcal mol−1). For the hydroxy-AQs, the 3(π,π*) state, assumed to have almost the same energy as that of AQ, should represent the second triplet state T2, whereas the lowest triplet state T1 should be a 3(π, π*) state located appreciably below the T2 state. Triplet quantum yields φT as well as singlet oxygen quantum yields φΔ are found to depend on whether the 1(π, π*) state of the hydroxy-AQ is located above or close to the corresponding 3(π, π*) state (φT ⩽ 0.7; φΔ 0.65) or appreciably lower than this state (φT ⩽0.5; φΔ ⩽ 0.3). These results are interpreted by assuming that, for hydroxy-AQs with relatively high S1 levels, intersystem crossing from S1(π, π*) to T2(n, π*) (followed by fast internal conversion to T1(π, π*)) contributes efficiently to the formation of the long-lived T1 state, in addition to the S1(π, π*)→T1(π, π*) transition. For hydroxy-AQs with relatively low S1 levels, the latter transition is almost exclusively responsible for the formation of the T1 state, the quenching of which by 3O2 should be the predominant, if not the only source of O2(1Δg) production.