Laser flash photolysis study of the phenolic hydrogen abstraction by 1,2-aceanthrylenedione triplet

the nanosecond laser flash photolysis technique has been used to study the reactivity of 1,2-aceanthrylenedione (1) triplet towards phenols. using acetonitrile as solvent, the quenching rate constant kq ranged from 3.3 x 105 l mol-1s-1 (phenol) to 1.3 x 107 l mol-1s-1 (4hydroxyphenol). phenols with electron donating substituents were more reactive. the kinetic isotope effect (kh/kd) observed for the quenching of 1 with 4-methoxyphenol was 1.5. the hammett plot for the reaction of the triplet of 1 with phenols gave a reaction constant r = 1.04 in acetonitrile solution. the low value for the quenching rate constant measured for this triplet with 1,3-cyclohexadiene (kq=2.0 x 104 l mol-1s-1) and the diffusion controlled quenching rate constant obtained for b-carotene suggest that the energy of the lowest triplet state for 1,2-aceanthrylenedione is located between 88 and 219 kj mol-1.