Hydrogen atom transfer and electron transfer reactions in the triplet π,π* state of 1,4-anthraquinone studied by CIDEP techniques and laser flash photolysis

Abstract

Photochemical hydrogen atom transfer (HT) and electron transfer (ET) reactions of 1,4-anthraquinone (1,4-AQ) have been studied in acetonitrile at 295 K by means of CIDEP (chemically induced dynamic electron polarization) techniques and laser flash photolysis. It was shown on the basis of CIDEP measurements that both HT and ET reactions in the excited state of 1,4-AQ originated from the triplet state. Quantitative investigations on the photochemical reactions of 1,4-AQ were carried out by laser photolysis at 355 nm. The HT reaction from 4-phenylphenol (PhPhOH) to the ³(π,π*) state of 1,4-AQ (³1,4-AQ*) proceeded rapidly with a rate constant (k_HT) of 5.3×10⁹ dm³ mol⁻¹ s⁻¹, where the efficiencies for HT (ϕ_HT) and induced quenching (IQ, ϕ_IQ) were obtained to be 0.57 and 0.43, respectively. Similar HT reactions were also observed for ³1,4-AQ* with phenol (k_HT=2.3×10⁹ dm³ mol⁻¹ s⁻¹, ϕ_HT=0.49, ϕ_IQ=0.51) and 2,6-di-tert-butylphenol (k_HT=1.9×10⁹ dm³ mol⁻¹ s⁻¹, ϕ_HT=0.47, ϕ_IQ=0.53). The observed rapid HT reactions were shown to be due not to hydrogen atom abstractions but to protic hydrogen atom transfer reactions. The ET reaction from 1,2,4,5-tetramethoxybenzene (TMB) to ³1,4-AQ* took place with a rate constant (k_ET) as high as 8.5×10⁹ dm³ mol⁻¹ s⁻¹, which was close to the diffusion-controlled rate constant with efficiencies of ET (ϕ_ET=0.77) and IQ (ϕ_IQ=0.23). These fast reactions may proceed via triplet exciplexes between ³1,4-AQ* and phenols (or TMB) with charge transfer character.