Electron Spin Resonance Study of Short-Lived Free Radicals in Photoreduction of Benzophenone

Abstract

By observing electron spin resonance spectra during photolysis at room temperature, kinetic behaviors of short-lived free radicals are studied in photoreduction of benzophenone in alcohols, especially in ethanol, with and without sodium methoxide. In neutral solution, diphenylhydroxymethyl radicals and hydroxyethyl radicals are observed simultaneously. This is a firm evidence, obtained from ESR, of hydrogen abstraction of excited benzophenone from ethanol. Results indicate that (1) free radicals are the most efficiently generated by the light of ∼350 nm, (2) hydroxyethyl radicals transform into diphenylhydroxymethyl radicals if the concentration of benzophenone is high, and (3) the latter radicals disappear following the second order reaction. In the presence of sodium methoxide, diphenylhydroxymethyl radicals transform into benzophenone ketyl anions. Rate constants are determined, being 3×107 and ∼104 mole−1·1·sec−1 for the combination reaction between diphenylhydroxymethyl radicals and the proton transfer reaction from diphenylhydroxymethyl radical to methoxide ion, respectively.

Study of Benzophenone Photolysis in SDS Micelles in the Presence of 2,4,6-Tri-tert-butylphenol: Distinctive Features of SNP in Radical Pairs with a Natural Abundance of 13C Nuclei

“…It should be noted that variation of the relative contributions of the two relaxation mechanisms while maintaining their sum invariant ( k rel = const) has almost no effect on the calculated SNP spectra and kinetics. Taking into account the parameters of anisotropic interaction for 13 C constant in RPI A ∥ = 5.0 mT, A ⊥ < 1.5 mT A : A ≥ 2.5 × 10 17 s -2 18 we evaluate the correlation time of modulation of anisotropic interaction τ c = 46 ps which is in a good agreements with literature values. , …”

“…Taking into account the parameters of anisotropic interaction for 13 C constant in RPI A | ) 5.0 mT, A ⊥ < 1.5 mT A:A g 2.5 × 10 17 s -2 18 we evaluate the correlation time of modulation of anisotropic interaction τ c ) 46 ps which is in a good agreements with literature values. 4,6 Let us now take a closer look at the SNP spectra using calculations. We note that there remain only two adjustable parameters: the magnitude of exchange interaction at the contact radius J 0 and RP recombination ability k s τ while HFI constants a for 13 C nuclei in positions I (a ≈ 2.5 mT), III (a ) 8.1 mT), IV (a ) -0.95 mT), V (a ) 13.9 mT) can be found in the literature.…”

“…There have been quite a few studies of benzophenone photolysis in SDS micelles, which is employed as a model reaction. ,− Magnetic field effects (MFE) and recombination kinetics of radical pairs formed upon photolysis of benzophenone in SDS micelles in the presence of 4-phenylphenol have been considered in detail in ref . The authors have evaluated the rate constants of radical escape from SDS micelles as well as the relaxation rates.…”

Light-Activated Replication of Block Copolymer Fingerprint Patterns

“…For instance, the half-life of a diphenylhydroxymethylcentered radical generated by benzophenone hydrogen abstraction is only 0.3 s in 0.44 M ethanol solution. 69 While other chemistries can photochemically graft to preformed polymer, benzophenone is often favored due to its stability toward heat and ambient light, and tolerance of water in the grafting reaction. 61 In addition to benzophenone, the conformal layer also contains acrylate groups capable of covalently bonding to the photosensitized block copolymer by propagating the radicals generated by hydrogen abstraction.…”

“…Note that this route requires two radicals to recombine during their relatively short lifetimes. For instance, the half-life of a diphenylhydroxymethyl-centered radical generated by benzophenone hydrogen abstraction is only 0.3 s in 0.44 M ethanol solution . While other chemistries can photochemically graft to preformed polymer, benzophenone is often favored due to its stability toward heat and ambient light, and tolerance of water in the grafting reaction …”