Laser flash–chemiluminescence study of O2(1Δ) photoeliminated from 1,4-dimethylnaphthalene endoperoxide

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Abstract
Deactivation of O2(1Δ) photoeliminated from electronically excited 1,4‐dimethylnaphthalene endoperoxide in acetonitrile and perdeuteroacetonitrile has been monitored by time resolved detection of O21Δg→3Σ−g chemiluminescence at 1.27 μ. Study of the chemiluminescence decay as a function of laser photolysis pulse energy and endoperoxide concentration was undertaken in order to elucidate the processes that cause the previously reported drastic lifetime shortening of O2(1Δ) photoeliminated from this singlet oxygen storing molecule. Analysis of the non‐first‐order decay revealed that, aside from solvent collisional deactivation, quenching of O2(1Δ) by O−2 plays a dominant role at higher photolysis laser pulse energy or endoperoxide concentration. Kinetic evidence suggests that superoxide is generated upon 266 nm endoperoxide photolysis mainly along a two photon excitation path. The ionic fragmentation channel has been confirmed by direct observation of dimethylnaphthalene cation radicals in transient absorption in the red spectral range. In order to find out about the possible role of O2(1Δ) self‐annihilation through pooling of the energy in O2(1Σ+), the rate constant of this process has been determined in a separate series of photosensitization experiments as (2.6±0.8)×107 l mol−1 s−1(C6F6). This value implies that marked contribution of O2(1Δ) pooling to nonexponential singlet O2 decay occurs only at high laser photolysis pulse energies or endoperoxide concentrations.

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