The photoreduction of chloranil (Q) to the hydroquinone
(QH2) in benzene by benzhydrols and by related
arylmethanols has been investigated. The products of
photooxidation of the benzhydrols are benzophenones, in
lieu
of formation of benzpinacols. Three distinct mechanisms of
oxidation−reduction have been identified from quantum
yield determinations and laser flash photolysis experiments, including
quinone triplet quenching via H-atom and
electron transfer paths. Direct excitation of ground state quinone
complexes has also been investigated. The quenching
of the triplet state of the quinone by benzhydrol proceeds normally
(k
q = 1.3 × 106
M-1 s-1) and gives
semiquinone
radical (QH•, λmax = 435 nm) and the
benzhydryl radical (λmax = 535 nm). The latter
intermediate decays by
pseudo-first-order kinetics through hydrogen atom transfer with ground
state quinone (Q). Triplet quenching by
bis(4-methoxyphenyl)methanol proceeds at a more rapid rate
(k
q = 5.5 × 109
M-1 s-1) leading to
an intermediate
that is identified as the chloranil radical anion (λmax
= 450 nm). A similar intermediate is observed on Q
quenching
by 1-naphthylmethanol and acenapthenol with the appearance of an
accompanying naphthalene radical cation absorption
(ca. 670 nm). The radical ion transients, which are assigned to
contact ion-pairs (triplet excited complexes) of the
quinone and the various electron donors, decay to semiquinone radicals
(QH•) by first-order processes occurring in
the 100 ns time regime. The transient behavior is interpreted in
terms of a hydrogen atom transfer mechanism for
photoreduction with benzhydrol and, for the more robust electron
donors, a mechanism involving electron transfer
followed by proton transfer between geminate radical ions. For the
electron transfer donors, ground state charge-transfer (CT) complexes can be observed (λmax ca. 500 nm).
Selective CT excitation leads to quinone photoreduction
with reduced quantum yield. The results are discussed in terms of
the time resolution of sequential electron/proton
transfer steps for photogenerated ion-pairs, the occurrence of one
photon−two electron transfer photoredox mechanisms,
and the kinetically distinct pathways for decay of singlet and triplet
intimate radical ion-pairs.
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