Reprinted from the Journal of Photochemistry and Photobiology; A: Chemistry, 57, 97-109 (1991).

©Elsevier Sequoia, 1991.

End-to-end Intramolecular Quenching of Aromatic Ketone Triplets in Aqueous Solutions of 2,6-di(O-methyl)cyclodextrins

William J. Leigh,* Mark S. Workentin and David Andrew

Department of Chemistry, McMaster University, Hamilton, Ontario, Canada L8S 4M1

Abstract: The photochemistry of para-(4-hydroxyphenyl)ethoxyacetophenone (1) in aqueous solutions of alpha-, beta- and gamma-2,6-di-(O-methyl)-cyclodextrins (a-, b-, and g-MCD, respectively) was studied using laser flash photolysis techniques. In acetonitrile solution, the triplet state of 1 (lifetime ~ 15 ns) decays by intramolecular phenolic hydrogen abstraction to yield the corresponding 1,13-phenoxyketyl biradical. Laser flash photolysis of aqueous solutions of 1 and equimolar a- and b-MCD yields transient absorptions centred at 395 nm, which decay with multiexponential kinetics. Two-component decay analyses show that the main portions (greater than 70%) of the decay in the two solutions have lifetimes of about 900 and 440 ns respectively, whereas the minor components are 4-6 times shorter lived. The transients have been shown to be a result of the triplet state of 1 on the basis of quenching studies with sorbic acid. In 1:1 solutions of 1 and g-MCD under similar conditions, no transients of lifetime greater than approximately 12 ns are observed. The data are consistent with inclusion complex structures in which only one of the aryl rings of the solute is included in the a- and b-MCD cavities, whereas both are included in the g-MCD cavity. It is suggested that in the a- and b-MCD complexes, the conformational motions required for formation of the intramolecular abstraction geometry are accompanied by partial disengagement of the solute from the cavity. The effects of increasing the relative concnetration of MCD on the triplet lifetime of 1 are consistent with the formation of termolecular 1:2 1:MCD complexes at high MCD concentration for the b- and g-MCD systems.