Redox Reaction of Benzyl Radicals with Aromatic Radical Ions Photogenerated. The Marcus Inverted Region and the Selective Formation of Carbocations or Carbanions

Efficient redox reactions of benzyl-type radicals were achieved by
irradiating an aromatic donor/acceptor
pair with substituted dibenzyl ketones as a radical precursor in
MeOH−MeCN. In this system, the aromatic radical
ion pair was generated by photoinduced electron transfer followed by
one-electron oxidation and reduction of
photogenerated benzyl radicals (R•) by the radical ions
to afford benzyl cations (R+) and anions
(R-). The cations
and anions were trapped by MeOH to yield ROMe and RH, respectively.
The relative product ratios were determined
for a variety of donor−acceptor pairs, reflecting the relative
efficiencies of the redox reaction of benzyl radicals
with a steady-state concentration of radical ions. The selective
formation of carbocations or carbanions was achieved
in some sets of donor/acceptor pairs. Assuming that the radical
ions exist in a 1:1 ratio in the steady state, the
product ratios are equal to the relative electron transfer rates, which
are analyzed in terms of the free energy changes
of the processes. The present results indicated that the rates
became maximal at the energy gap of ca. −0.5 eV,
representing a novel example of the Marcus inverted region. This
interpretation is discussed in comparison with
other related cases and in relation to recent theories on electron
transfer rates.