Type II Photochemistry of Ketones in Liquid Crystalline Solvents. The Influence of Ordered Media on Biradical Dynamics

Article abstract of DOI:10.1021/ja00335a028

The Norrish type II photochemistry of five alkylphenones, PhCO(CH2)nH (1a, n=4; 1b, n=10; 1c, n=17; 1d, n=19; 1e, n=21), 10-nonadecanone (2), and 2-undecanone (3) was studied in the isotopic, smetic, and solid phases of n-butyl stearate.The ratio of elimination-to-cyclization products for ketones 1c-e and 2 exhibits a strong phase dependence with a 7-8-fold increase in the smectic phase relative to the isotropic phase.The ratio of isomeric cyclobutanols from 2 shows a similar change.Further increases in the elimination-to-cyclization ratio are observed for 1d in the solid phase.The product ratios for ketones 1a, 1b, and 3 are the same in all the phase studied.Transient absorption studies on the intermediate 1,4-biradical produced from laser flash photolysis of 1d yield lifetimes of 64 +/- 5 and 70 +/- 5 ns in the isotopic and smectic phases, respectively.These results are explaned in terms of the structures of the various phases of n-butyl stearate and the accepted behavior of Norrish type II biradicals.