| Cation radical isomerization reactions initiated by photoinduced electron transfer with a variety of photooxidants were studied in methacrylate polymers. A chain mechanism was found to be operative, even in the low dielectric polymer media. Quantum yields were found to be low, but greater than unity in several cases and dependent on reactant concentration. Furthermore, evidence suggestive of very long-lived (days) propagating cation radicals was found.; High conversion of reactant to product, even at low photooxidant concentration, was realized under appropriate conditions. Three hypotheses to account for the high conversion in rigid, low dielectric media were considered: (A) phase separation of the reactant from the polymer matrix into (potentially fluid) regions of high local reactant concentration; (B) diffusion of reactant and/or photooxidant through the medium; or (C) separation of the propagating cation radical species from the anion radical of the photooxidant over long distances. Hypothesis (A) was proved untenable on the basis of both electron microscopy and experiments tethering the reactant moiety to polymer backbone, presumably at random. Hypothesis (B) was proven kinetically incompetent to explain the high conversions observed, on the basis of “Forced Rayleigh Scattering” grating experiments. This, somewhat surprisingly, left hypothesis (C) to explain the high extent of conversion observed in these systems. A possible explanation for the energetically costly long distance separation of charge on the basis of the difference in oxidation potential between reactant and product is proposed.; The potential of these materials as photoresponsive plastics for optical applications utilizing the spatial refractive index modulation that can be achieved is also discussed. |
