EPR Study of the Biphotonic Nature of the Solute-Sensitized Photodecomposition of Polydimethylsiloxane

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Abstract
The photolysis of solutions of naphthalene in polymeric dimethylsiloxane (PDMS) has been examined by EPR (electron paramagnetic resonance) spectroscopy at 77°K. It was found that the naphthalene solute sensitizes the decomposition of the solvent by a stepwise two-photon absorption process involving the first triplet state of naphthalene as an intermediate. The absorption of the second photon within the naphthalene triplet manifold of states is followed by an energy transfer process to the polymer which then decomposes. Decomposition occurs via Si–C and C–H bond rupture. The stabilities of the resultant radicals, ≡Si·, CH3·, —CH2·, and H·, which are identified by means of their EPR spectra (except for H· which is not observed at 77°K) are also discussed. The latter species presumably reacts immediately, possibly by a hotatom reaction, with a polymer molecule to form H2 and —CH2·. It is shown that the CH3· radical is also unstable at 77°K and decays by a hydrogen abstraction reaction to form CH4 and the —CH2· radical with a half-life of approximately 80 min.