Effect of conformation on ring opening of cis and trans dimers of acenaphthylene in the triplet state. Direct detection of hydrocarbon biradical, 1,1'-biacenaphthene-2,2'-diyl.

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Triplet sensitized photodecomposition of cis and trans dimers of acenaphthylene (c-D and t-D) has been studied in benzene by means of a nanosecond laser flash photolysis. An aromatic hydrocarbon biradical, 1,1′-biacenaphthene-2,2′-diyl (1) was found as an intermediate for the decomposition of triplet t-D to acenaphthylene monomers. The rate of bond opening was directly measured to be 8.3×106 s−1 at 20 °C. The temperature dependence of the rate yields the frequency factor 2.8×107 s−1 and activation energy 3.1 kJ mol−1. The lifetime of 1 is 330 ns; this is insensitive to temperature changes (6–64 °C). Oxygen quenches 1 with the rate constant of 9×109 M−1 s−1 (1 M=1 mol dm−3). On the other hand, the triplet state of c-D did not show any evidence of the presence of 1 as an intermediate. The difference of ring opening mechanisms between the two isomers is interpreted in terms of different steric hindrances between two acenaphthene rings. The triplet-triplet (T-T) absorption spectra of both dimers resemble each other. No characteristic absorption due to intramolecular triplet excimer of naphthalene was observed even for c-D.
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