Photosensitized Dehydrogenation of Crystalline Durene: Mechanism of Duryl Radical Formation

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Abstract
The photosensitized dehydrogenation of durene doped with 2,4,5-trimethylbenzaldehyde and naphthalene-d8 gives rise to duryl (2,4,5-trimethylbenzyl) radicals. The rate of production of these radicals was studied as a function of the guest, of photolyzing light intensity, and of temperature. The experimental data show that the mechanism of radical formation is dependent upon the nature of the particular guest involved. In naphthalene-d8/durene mixed crystals, duryl radical is probably formed by a stepwise biphotonic absorption process involving the first triplet state of the guest as an intermediate. The results show that radical formation is not responsible for the temperature dependence of the naphthalene-d8 phosphorescence lifetime in durene host at T > 220°K. In 2,4,5-trimethylbenzaldehyde/durene mixtures, the formation of duryl radical is a monophotonic process, and the over-all reaction is the hydrogen abstraction of aromatic carbonyl compounds with a lowest nπ* triplet state, a process which has been extensively studied in liquid solutions. However, the use of the solid state together with low temperature techniques has revealed the intervention of a new transient species as a precursor to matrix dehydrogenation.