Mediation of photochemical reactions of 1-naphthyl phenylacylates by polyolefin films. A 'radical clock' to measure rates of radical-pair cage recombinations in 'viscous space'

The fates of phenylacyl/1-naphthoxy singlet radical pairs generated upon ir radiation of 1-naphthyl phenylacetate (1a) and 1-naphthyl 2-phenylpropanoat e (1b) in three unstretched and stretched polyethylene films and isotactic and syndiotactic polypropylene films have been investigated. From dynamic f luorescence measurements, the primary locus of reactions by 1 is within amo rphous regions of the films. The reaction cages afforded by these media inh ibit escape of the radical pairs and mediate their reorientational motions leading to photo-Fries and related products. In essence, the cages act as s tiff-walled templates. In addition, a method is described to measure the ra te constants for the singlet radical pairs. Thus, the rate constants (leadi ng to the keto precursors) of the 2-phenylacyl-1-naphthols from the radical pairs of 1 (>10(8) s(-1)) are >6 times the rate constants for the 4-isomer s. Film stretching increases this selectivity but there is no obvious corre lation between the rate of the in-cage radical pair recombinations and macr oscopic polymer, properties such as degree of crystallinity and frequency o f branched chains. By contrast, formation of (the keto precursors of) 2-ben zylic-1-naphthols (from in-cage recombinations after phenylacyl decarbonyla tion) is slower than for the 4-benzyl-1-naphthols. (C) 2000 Elsevier Scienc e Ltd. All rights reserved.