Solvent effects on the quenching of the equilibrating n,π* and π,π* triplet states of 9,10-phenanthrenequinone by 2-propanol

Quenching of the triplet state of 9,10-phenanthrenequinone (PQ) by 2-propanol was studied using laser flash photolysis. The room temperature rate constant is solvent dependent and is 3.3(±1.1) × 108 M−1 s−1 in carbon tetrachloride (CCl4), 1.2(±0.3) × 108 M−1 s−1 in chlorobenzene (CB), and 2.5(±0.8) × 107 M−1 s−1 in acetonitrile (MeCN). The rate constant in CCl4 is the largest known for abstraction of hydrogen from 2-propanol by an excited ketone. An activation energy of 0.017(±0.006) kcal/mol and a pre-exponential A factor of the Arrhenius equation of 4.45 × 108 M−1 s−1 was found in CCl4; an activation energy of 2.14(±0.006) kcal/mol and an A factor of 4.5 × 108 was found in MeCN. These results are consistent with a mechanism involving a reactive n,π* triplet in thermal equilibrium with a non-reactive π,π* triplet, and are in agreement with a previous study of the room temperature phosphorescence of PQ showing that the lowest energy triplet is n,π* in CCl4 but π,π* in MeCN.