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 Ñash 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(p,p*) state of 1,4-AQ (31,4-AQ*) proceeded rapidly with a rate constant of 5.3 ] 109 dm3 mol~1 s~1, where the efficiencies for HT and induced quenching (k HT ) ( / HT ) (IQ, were obtained to be 0.57 and 0.43, respectively. Similar HT reactions were also observed for 31,4-AQ* / IQ ) with phenol dm3 mol~1 s~1, and 2,6-di-tert-butylphenol (k HT \ 2.3 ] 109 / HT \ 0.49, / IQ \ 0.51) (k HT \ 1.9 ] 109 dm3 mol~1 s~1, The observed rapid HT reactions were shown to be due not to / HT \ 0.47, / IQ \ 0.53). hydrogen atom abstractions but to protic hydrogen atom transfer reactions. The ET reaction from 1,2,4,5tetramethoxybenzene (TMB) to 31,4-AQ* took place with a rate constant as high as 8.5 ] 109 dm3 mol~1 (k ET ) s~1, which was close to the di †usion-controlled rate constant with efficiencies of ET and IQ (/ ET \ 0.77) These fast reactions may proceed via triplet exciplexes between 31,4-AQ* and phenols (or TMB) (/ IQ \ 0.23). with charge transfer character.
Phosphorescence and TREPR spectra have been measured for
2-methyl-1,4-naphthoquinone (MNQ), 2,3-dimethyl-1,4-naphthoquinone (DMNQ), and 2-methoxynaphthoquinone (MeONQ)
in several matrices at low
temperatures. The |D| value of ZFS parameter
decreases with increasing the electron-donating character
of
the substituent group. For MNQ, the T1 state is
clearly assigned to be 3nπ* in character. The
broad
phosphorescence spectra and minor effects of the matrix polarity on the
ZFS parameters lead to the conclusion
that the character of the T1 states are mainly
3ππ* in DMNQ and MeONQ. The T1 states
of the present
naphthoquinone derivatives have more or less mixed character of the
3nπ* and 3ππ* states.
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