Photoreactions of substituted o-cresyl acylates in cyclohexane and in polyethylene films. The influences of intra- and inter-molecular 'crowding' effects.

Yu-Zhe Chen, Richard G Weiss

Index: Photochem. Photobiol. Sci. 8(7) , 916-25, (2009)

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Abstract

Photo-Fries rearrangements and associated photoreactions of four o-cresyl acylates were investigated in cyclohexane and in unstretched and stretched polyethylene (PE) films with different degrees of crystallinity. The esters differ in the number of phenyl substituents attached to the methyl group of the cresyl part and the length of the acyl chain. The influences of intramolecular structural factors and intermolecular environmental effects on the fates of the excited singlet states and the singlet state aryloxy/acyl radical pairs generated subsequently from them are explored. The results indicate that there are definable limits to the selectivity of the photoreactions of aryl esters that are dependent on their shapes and the ability of their environments to interact with them. Quantum yields for the reactions of the o-cresyl acylates were measured in cyclohexane and their values can be understood on the bases of a combination of conformational and electronic factors. The PE cavities holding the less globularly shaped o-cresyl acylates act as templates for the formation of the photoproducts: the ratios of the 2- and 4-position photo-Fries rearrangement products from o-cresyl acetate and 2-benzylphenyl acetate are higher when irradiated in films that are stretched or have higher film crystallinity; control by the PE cavities over the fates of the radical pairs is diminished for the o-cresyl acylate with two phenyl groups appended to the methyl of the cresyl part. The cavities of the PE films exert an interesting influence on the ability of the excited singlet states to undergo concerted decarboxylation reactions as well.