Using crystal optics to demonstrate single-layer localization of a solid-state chain reaction

Upon warming to 225 K, single crystals of 11-bromoundecanoyl peroxide (BrUP ), in which radicals have bean created by photolysis at lower temperature, undergo partial decomposition by a radical chain reaction ca. 40 cycles lon g. FTIR allowed monitoring two chain products: CO2 and an alpha-lactone tha t decomposes further at 260 K. When initiation is confined to alternate mol ecular layers by polarized photoselection, the chain reaction reduces the c rystal symmetry from tetragonal to monoclinic. Desymmetrization is easily o bserved by optical microscopy, although it is difficult to detect by X-ray diffraction. Accurate monitoring of birefringence using a Senarmont 1/4-wav e plate, and comparison with FTIR kinetics, proves that the chain reaction occurs within single molecular layers 2 nm thick.