The selective eletron-transfer quenching of the radical anions of
dicyanoanthracene, phenazine, and
anthraquinones in the excited state by a quencher such as fumaronitrile
or dicyanobenzene is investigated in
N,N-dimethylformamide solution at room
temperature using the pulse radiolysis−laser flash
photolysis
combined method. The radical anions generated by pulse radiolysis
do not change upon irradiation with a
laser flash at 532 nm. The radical anions in the excited state
decay into the ground state within the laser
flash (5 ns). Lifetimes of approximately 4 ns are estimated for
three radical anions in the excited state assuming
a diffusion-controlled rate constant for the electron-transfer
quenching. The shorter lifetimes of 1.0−1.4 ns
for methyl and chloro substituents on anthraquinone are discussed in
terms of internal conversion from the
excited to the ground state of the radical anions accelerated by
rotation of the substituents. The energy gap
between the excited and ground states of the radical anions is a
significant factor for the rate of the internal
conversion. The quencher radical anion−neutral molecule pair is
suggested as an intermediate in the electron-transfer quenching of the radical anions during the excited state by the
quencher and is discussed with respect
to separation and back electron transfer in the pair.
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