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Abstract
The photochemistry of ene-dione 1 has been studied in each of three different media: ethyl acetate solution, the pure crystalline phase and poly(methyl methacrylate) films. Topochemical [2+2] photodimerization is followed in the solid state owing to a favorable orientation of adjacent molecules in the crystal lattice. In the other media, an ill-defined photooligomerization process predominates accompanied by lesser amounts of hydrogen abstraction-initiated photorearrangement. The biradicals produced by hydrogen abstraction can collapse to various stable products depending on their conformation. The rate of interconversion of biradical conformers is viscosity-dependent, thus giving rise to different photoproducts in solution and in the polymer films.