Pulsed laser photolysis experiments are carried out in aromatic exciplex systems, investigating the generation of triplet states under varying conditions of dissolved oxygen and solvent polarity. Four different intersystem-crossing (ISC) paths, leading to the triplet states of the donor (D) or the acceptor (A), are identified: (I) intersystem crossing within the thermalized exciplex, 1(A"D+)*, formed in nonpolar solvents. This is a "slow" ISC mechanism, competing with fluorescence emission from 1(A"D+)*. (II) Triplet states are generated in polar solvents via the homogeneous recombination of solvated radical ions: As" + Ds+ -* 3A* + D (br 3D* + A). (Ill) Intersystem crossing may occur in both polar and nonpolar solutions from nonrelaxed donor-acceptor pairs. This "fast" process circumvents the thermalized exciplex in hydrocarbon solvents and the separated ions in methanol or acetonitrile. (IV) Triplet states may also be formed as a result of quenching of the exciplex fluorescence by molecular oxygen: 1(A"D+)* + 02 -*
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