The quenching of triplet fluoranil (FA) by molecular oxygen was studied by IR laser fluorimetry and nanosecond laser photolysis in various solvents. In acetonitrile, CCl4, chloroform, and cyclohexane, the charge-transfer (CT) interaction between triplet fluoranil and solvent (Sol.) is insignificant and the quantum yield of triplet formation and the probability of O2(1-DELTA(g)) generation from triplet fluoranil are close to unity. In aromatic solvents, a substantial decrease in the quantum yield (PHI(DELTA)infinity) of O2(1-DELTA(g)) formation (limiting value at [O2] --> infinity) was observed; for example, in p-xylene the estimated value is less-than-or-equal-to 10(-2). In the weakly electron-donating solvents, benzene and chlorobenzene, triplet exciplexes with partial charge transfer [3(FA(delta-)...Sol.(delta+))] and lifetime of approximately 150 ns were observed. The absorption spectra of the triplet exciplexes are slightly broadened compared with the triplet-triplet spectrum of free fluoranil and have broad structureless CT bands in the near IR region, The values of PHI(DELTA)infinity are approximately 0.4-0.5, and the rate constants for quenching by oxygen [(6-8) x 10(8) M-1 s-1] of the triplet exciplexes are lower than for triplet fluoranil. Molecular oxygen, a strong electron acceptor, probably interacts mainly with the FA(delta-) fragment of the triplet exciplex where excitation is localized, leading to efficient O2(1-DELTA(g)) production. Fluoranil forms weak CT complexes with aromatic solvents in the ground state, and their direct excitation produces contact ion-radical pairs (CIRP). In benzene and chlorobenzene, the main deactivation channel for the CIRP is intersystem crossing, leading to triplet exciplex formation.
