The conception of this functional enzyme mimetic is based on a
combination of several individual key steps already discussed in
the previous sections. A closed photocatalytic reaction cycle in
homogeneous solution could be successfully constructed by cou-
pling a series of complementary functions including bioinspired
dioxygen activation with photoexcited main group metals
(Fig. 22), hydrogen atom abstraction, controlled formation of sub-
strate radicals with photoactivated metal-oxo species (Fig. 19),
and an efficient long-wavelength spectral sensitization of the cat-
alytic system with tetrapyrrole macrocycles acting as robust
antenna chromophores (Fig. 8).
The selective transformation of alcohols into carbonyl com-
pounds with this kind of artificial oxidoreductases has been
directly compared to a series of native enzymes performing
under identical reaction conditions( 3 ). In the dark-adapted form
of the photocatalyst, which is a high-valent antimony porphyrin
complex, the alcohol substrates are already preorganized by
hydrogen bonds in the second coordination sphere of the active
site. The system displays a certain extent of pH-controlled sub-
strate selectivity, which makes competitive secondary reactions
involving the reaction products less favorable. Substrate conver-
sion can be completely switched off in the dark and is readily
regulated by variations of light intensity. The system performs
under very mild reaction conditions at room temperature and
ambient pressure in aqueous solution utilizing dioxygen from
air as a two-electron acceptor. Sunlight or even diffuse daylight
with a threshold wavelength of approximately 600 nm is
activating the catalyst for substrate conversion, which occurs at
a rate ofkcat¼0.05 s^1 corresponding to a turnover frequency of
TOF¼180 h^1 under AM¼2.0 solar irradiation conditions ( 3 ).
A product formation quantum yield ofF¼0.02 has been deter-
mined for monochromatic visible light irradiation at 546 nm.
Gradual degradation of the quite robust photocatalyst occurs
with a quantum yield ofF 3 10 ^5 , which is in a typical stabil-
ity range of natural tetrapyrrole pigments such as chlorophylls
and corresponds to an average turnover number of at least
4000 productive photoredox cycles for each catalyst molecule.
The most significant and surprising result of this proof of prin-
ciple study was the fact that even with such a simple MET pho-
tosensitizer system powered by sunlight, a very promising
biomimetic performance with reaction rates up to three times
higher than that of the natural metalloenzymes catalyzing
exactly the same process in the absence of light could be achieved
(3,5). Further systematic activities heading in this direction are
therefore certainly worthwhile.
PHOTOSENSITIZATION AND PHOTOCATALYSIS 279