Triplet of cyclooctatetraene: reactivity and properties

Abstract

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 (E_T) was estimated to be 41 ± 1 kcal mol^–1.