EFFECT ON SINGLET OXYGEN YIELD OF CHARGE-TRANSFER INTERACTION BETWEEN FLUORANIL AND SOLVENT

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.