A técnica de fotólise por pulso de laser de nanossegundo foi empregada no estudo da reatividade do triplete de 1,2-aceantrilenodiona (1) ), assim como o fato de que a constante de velocidade de supressão por -caroteno é controlada por difusão, sugerem que a energia do triplete de energia mais baixa para 1,2-aceantrilenodiona situa-se entre 88 e 219 kJ mol -1 .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 k q ranged from 3.3 x 10 5 L mol -1 s -1 (phenol) to 1.3 x 10 7 L mol -1 s -1 (4-hydroxyphenol). Phenols with electron donating substituents were more reactive. The kinetic isotope effect (k H /k D ) 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 = -1.04 in acetonitrile solution. The low value for the quenching rate constant measured for this triplet with 1,3-cyclohexadiene (k q =2.0 x 10 4 L mol ) and the diffusion controlled quenching rate constant obtained for -carotene suggest that the energy of the lowest triplet state for 1,2-aceanthrylenedione is located between 88 and 219 kJ mol -1 .