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
T. Yoshihara et al., 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, PCCP PHYS C, 2(5), 2000, pp. 993-1000
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.