Triplet of cyclooctatetraene: reactivity and properties
Using 7 MeV electron pulses from a linear electron accelerator, the triplet excited state of cyclooctatetraene (COT), a commonly used triplet scavenger in dye lasers, has been characterised in hydrocarbon solvents. In deoxygenated cyclohexane and benzene, 3COT shows an absorption due to a triplet–triplet (T–T) transition between 300 and 400 nm with a peak at 350 nm in cyclohexane and at 360 nm in benzene, and with a lifetime of 100 µs in cyclohexane. Unlike many other solute triplets, 3COT exhibits comparatively low reactivity towards molecular oxygen and the measured rate constant was 3 × 107 dm3 mol–1 s–1. The triplet nature of the transient was confirmed by T–T energy-transfer processes from triplet donors like biphenyl, p-terphenyl and pyrene in a deoxygenated cyclohexane matrix and the molar absorptivity of 3COT at 350 nm was estimated to be 7000 ± 300 dm3 mol–1 cm–1. In benzene, the T–T energy transfer rate constants using various 7-aminocoumarin laser dyes as donors were estimated. These values (of the order of 109 dm3 mol–1 s–1) are 6.5 for Coumarin 47 (C47), 5 for C102, 6 for C120, 2.8 for C152 and 1.5 for C153. Similarly, rate constants were also measured using aromatic triplet donors. These values in the same order are 3.8 for p-terphenyl, 13.3 for biphenyl, 3.3 for both naphthalene and benzil, 0.74 for pyrene, 0.3 for anthracene and 0.7 for acridine. Based on the results of equilibrium studies in cyclohexane with 3anthracene as the donor, the energy of the first excited 3COT (ET) was estimated to be 41 ± 1 kcal mol–1.