Photoinduced Electron Transfer in Rhenium(I)–Oligotriarylamine Molecules

Two molecular triads with an oligotriarylamine
multielectron donor were synthesized and investigated with a view
to obtaining charge-separated states in which the oligotriarylamine
is oxidized 2-fold. Such photoinduced accumulation of multiple redox
equivalents is of interest for artificial photosynthesis. The first
triad was comprised of the oligotriarylamine and two rhenium­(I) tricarbonyl
diimine photosensitizers each of which can potentially accept one
electron. In the second triad the oligotriarylamine was connected
to anthraquinone, in principle an acceptor of two electrons, via a
rhenium­(I) tricarbonyl diimine unit. With nanosecond transient absorption
spectroscopy (using an ordinary pump–probe technique) no evidence
for the generation of 2-fold oxidized oligotriarylamine or 2-fold
reduced anthraquinone was found. The key factors limiting the photochemistry
of the new triads to simple charge separation of one electron and
one hole are discussed, and the insights gained from this study are
useful for further research in the area of charge accumulation in
purely molecular (nanoparticle-free) systems. An important problem
of the rhenium-based systems considered here is the short wavelength
required for photoexcitation. In the second triad, photogenerated
anthraquinone monoanion is protonated by organic acids, and the resulting
semiquinone species leads to an increase in lifetime of the charge-separated
state by about an order of magnitude. This shows that the proton-coupled
electron transfer chemistry of quinones could be beneficial for photoinduced
charge accumulation.