Absolute Rate Constants for the Reaction of Benzil and 2,2′-Furil Triplet with Substituted Phenols in the Ionic Liquid 1-Butyl-3-methylimidazolium Hexafluorophosphate: A Nanosecond Laser Flash Photolysis Study

Abstract

The triplet excited state reactivity towards phenolic hydrogen of the α-diketones benzil and 2,2′-furil in the ionic liquid 1-n-butyl-3-methyl imidazolium hexafluorophosphate [bmim.PF6] was investigated employing the nanosecond laser flash photolysis technique. Irradiation (λmax = 355 nm) of benzil yields its triplet excited state with λmax at 480 nm and τT = 9.6 μs. Under the same conditions, 2,2′-furil shows a triplet-triplet absorption spectrum with bands at 380, 410, 450, and 650 nm and τT = 1.4 μs. Quenching rate constants (kq) of the reaction between benzil triplet and substituted phenols ranged from 1.4 × 107 L mol−1 s−1 (para-chlorophenol) to 1.8 × 108 L mol−1 s−1 (para-methoxyphenol). A new transient was formed in all cases, assigned to the benzil ketyl. Similar results were obtained for the quenching of 2,2′-furil triplet by phenols, for which kq ranged from 1.9 × 108 L mol−1 s−1 (para-chlorophenol) to 2.2 × 108 L mol−1 s−1 (para-methoxyphenol). The 2,2′-furil ketyl radical was also observed in all cases (λmax = 380 nm). The quenching rate constants are almost independent of the substituent and diffusion-controlled (kq ~ 108 L mol−1 s−1). The proposed mechanism for the phenolic hydrogen abstraction by benzil and 2,2′-furil triplet may involve a proton-coupled electron transfer reaction, ultimately leading to the radical pair ketyl/aryloxyl.