The crucial advantage of collecting and coupling redox
equivalents is to avoid free-radical side reactions, as these pro-
cesses tend to decrease both the long-term stability and the over-
all efficiency of the system. Especially, the light-induced
substrate transformations in natural and artificial photosynthe-
sis strongly depend on the feasibility of MET catalysis. For exam-
ple, the fixation of CO 2 to form carbohydrates in a photocatalyzed
four-electron process allows a long-wavelength spectral sensitiza-
tion down to a photon energy of 1.3 eV, corresponding to an NIR
threshold absorption wavelength of about 950 nm. In contrast, a
minimum energy of 3.6 eV (340 nm, UV-light) is necessary to drive
the reaction in highly unfavorable one-electron steps, and two-
thirds of the solar energy suitable for carbon dioxide reduction
are wasted ( 8 ). Also many energetically downhill reactions in bio-
inorganic and bioinspired catalysis require optimized MET
reagents to be sufficiently accelerated and to guarantee a large
total number of possible redox cycles.
The qualitative reaction profile given inFig. 11 shows how a
light-induced single-electron transfer process can be coupled to
suitable follow-up steps to facilitate the formation of permanent
two-electron photoredox products. Larger bond and shape
reorganizations of excited state molecules, which typically
involve the population of CT states or the formation of
Jahn–Teller distorted species, are very helpful to achieve such
FIG. 11. Schematic illustration of a net two-electron transfer
photoredox process. Excitation of reactants (R) forms a metastable
one-electron intermediate (I), which finally can yield an energy-rich
permanent product (P) after a second electron transfer step. Adapted
from Ref. ( 8 ).
PHOTOSENSITIZATION AND PHOTOCATALYSIS 253