Intramolecular Light-Driven Accumulation of Reduction Equivalents by Proton-Coupled Electron Transfer

The photochemistry of a molecular
pentad composed of a central
anthraquinone (AQ) acceptor flanked by two Ru­(bpy)32+ photosensitizers and two peripheral triarylamine (TAA) donors
was investigated by transient IR and UV–vis spectroscopies
in the presence of 0.2 M p-toluenesulfonic acid (TsOH)
in deaerated acetonitrile. In ∼15% of all excited pentad molecules,
AQ is converted to its hydroquinone form (AQH2) via reversible
intramolecular electron transfer from the two TAA units (τ =
65 ps), followed by intermolecular proton transfer from TsOH (τ
≈ 3 ns for the first step). Although the light-driven accumulation
of reduction equivalents occurs through a sequence of electron and
proton transfer steps, the resulting photoproduct decays via concerted
PCET (τ = 4.7 μs) with an H/D kinetic isotope effect of
1.4 ± 0.2. Moreover, the reoxidation of AQH2 seems
to take place via a double electron transfer step involving both TAA+ units rather than sequential single electron transfer events.
Thus, the overall charge-recombination reaction seems to involve a
concerted proton-coupled two-electron oxidation of AQH2. The comparison of experimental data obtained in neat acetonitrile
with data from acidic solutions suggests that the inverted driving-force
effect can play a crucial role for obtaining long-lived photoproducts
resulting from multiphoton, multielectron processes. Our pentad provides
the first example of light-driven accumulation of reduction equivalents
stabilized by PCET in artificial molecular systems without sacrificial
reagents. Our study provides fundamental insight into how light-driven
multielectron redox chemistry, for example the reduction of CO2 or the oxidation of H2O, can potentially be performed
without sacrificial reagents.