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’

Abstract

The fates of phenylacyl/1-naphthoxy singlet radical pairs generated upon irradiation of 1-naphthyl phenylacetate (1a) and 1-naphthyl 2-phenylpropanoate (1b) in three unstretched and stretched polyethylene films and isotactic and syndiotactic polypropylene films have been investigated. From dynamic fluorescence measurements, the primary locus of reactions by 1 is within amorphous regions of the films. The reaction cages afforded by these media inhibit escape of the radical pairs and mediate their reorientational motions leading to photo-Fries and related products. In essence, the cages act as stiff-walled templates. In addition, a method is described to measure the rate constants for the singlet radical pairs. Thus, the rate constants (leading to the keto precursors) of the 2-phenylacyl-1-naphthols from the radical pairs of 1 (>108s−1) are >6 times the rate constants for the 4-isomers. Film stretching increases this selectivity but there is no obvious correlation between the rate of the in-cage radical pair recombinations and macroscopic polymer, properties such as degree of crystallinity and frequency of branched chains. By contrast, formation of (the keto precursors of) 2-benzylic-1-naphthols (from in-cage recombinations after phenylacyl decarbonylation) is slower than for the 4-benzyl-1-naphthols.