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

Photochemical hydrogen atom transfer (HT) and electron transfer (ET) reacti ons of 1,4-anthraquinone (1,4-AQ) have been studied in acetonitrile at 295 K by means of CIDEP (chemically induced dynamic electron polarization) tech niques and laser flash photolysis. It was shown on the basis of CIDEP measu rements that both HT and ET reactions in the excited state of 1,4-AQ origin ated from the triplet state. Quantitative investigations on the photochemic al reactions of 1,4-AQ were carried out by laser photolysis at 355 nm. The HT reaction from 4-phenylphenol (PhPhOH) to the (3)(pi,pi*) state of 1,4-AQ ((3)1,4-AQ*) proceeded rapidly with a rate constant (k(HT)) of 5.3 x 10(9) d mol(-1) s(-1), where the efficiencies for HT (phi(HT)) and induced quenc hing (IQ, phi(IQ)) were obtained to be 0.57 and 0.43, respectively. Similar HT reactions were also observed for (3)1,4-AQ* with phenol (k(HT) = 2.3 x 10(9) d mol(-1) s(-1), phi(HT) = 0.49, phi(IQ) = 0.51) and 2,6-di-tert-buty lphenol (k(HT) = 1.9 x 10(9) d mol(-1) s(-1), phi(HT) = 0.47, phi(IQ) = 0.5 3). The observed rapid HT reactions were shown to be due not to hydrogen at om abstractions but to protic hydrogen atom transfer reactions. The ET reac tion from 1,2,4,5-tetramethoxybenzene (TMB) to (3)1,4-AQ* took place with a rate constant (k(ET)) as high as 8.5 x 10(9) d mol(-1) s(-1), which was cl ose to the diffusion-controlled rate constant with efficiencies of ET (phi( ET) = 0.77) and IQ (phi(IQ) = 0.23). These fast reactions may proceed via t riplet exciplexes between (3)1,4-AQ* and phenols (or TMB) with charge trans fer character.