HYDROXYANTHRAQUINONES AS SENSITIZERS OF SINGLET OXYGEN REACTIONS - QUANTUM YIELDS OF TRIPLET FORMATION AND SINGLET OXYGEN GENERATION IN ACETONITRILE

The photophysical properties, in particular those of the triplet states, of alpha- and beta-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 (tau(F) less-than-or-equal-equal-to 1 ns), lowest excited 1(pi, pi*) states (E(S(1)) between 65 and 55 kcal mol-1) with quantum yields (PHI(F) less-than-or-equal-to 0.025 (the exception is 1,4-AQ with tau(F) almost-equal-to 2 ns and PHI(F) almost-equal-to 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, pi*) is determined (E(T) = 61.6 kcal mol-1). For the hydroxy-AQs, the 3(n, pi*) 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(pi, pi*) state located appreciably below the T2 state. Triplet quantum yields PHI(T) as well as singlet oxygen quantum yields PHI(DELTA) are found to depend on whether the 1(pi, pi*) state of the hydroxy-AQ is located above or close to the corresponding 3(n, pi*) state (PHI(T) greater-than-or-equal-to 0.7; PHI(DELTA) greater-than-or-equal-to 0.65) or appreciably lower than this state (PHI(T) less-than-or-equal-to 0.5; PHI(DELTA) less-than-or-equal-to 0.3). These results are interpreted by assuming that, for hydroxy-AQs with relatively high S1 levels, intersystem crossing from S1(pi, pi*) to T2(n, pi*) (followed by fast internal conversion to T1(pi, pi*)) contributes efficiently to the formation of the long-lived T1 state, in addition to the S1(pi, pi*) --> T1(pi, pi*) 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 O-3(2) should be the predominant, if not the only source of O2(1DELTAg) production.