For certain reasons described elsewhere in more detail(8,93),
our first choice fell on the combination of tetrapyrrole
photosensitizers including porphyrins or phthalocyanines with
redox-active group 14 and group 15 elements such as tin, lead,
antimony, or bismuth. Close relationships between the electronic
structures of low-valent group 15 metalloporphyrins and the
reduced heme groups of cytochrome P450 and chloroperoxidase
enzymes had already been recognized before by a comparison of
their absorption and magnetic circular dichroism spectra ( 186 ).
Functional similarities with the hemoproteins were also
reflected by the facts that the tin complex of protoporphyrin IX
is able to act as an efficient competitive inhibitor of heme
oxygenase ( 187 ), and that lead compounds are blocking the natu-
ral metallation process of protoporphyrin IX catalyzed by heme
synthetase ( 188 ).
By studying the catalytic properties of a series of antimony(III)
porphyrin complexes in the presence of dioxygen, it could be
demonstrated that the reactivity of these compounds toward O 2
activation can be triggered and controlled by light activation
( 155 ). A metal centered sp-excited triplet state of the low-valent
main group metal center has been identified to be responsible
for this photoreactivity. The population of the dioxygen
activating state can be spectrally sensitized by the coordinated
porphyrin antenna chromophores, which allows the controlled
generation of oxy- and hydroperoxo-metalloporphyrin species
with long-wavelength visible light. It could also be shown that
catalytic two-electron photoredox processes involving reactive
metalloporphyrin species with a direct functional analogy to the
hemoprotein compound 0 and compound I intermediates shown
in Fig. 21 are involved in the substrate transformations
catalyzed by these visible light-driven metalloenzyme models.
These compounds, for example, display cytochrome P450 and
chloroperoxidase-type reactivity under visible-light and solar
irradiation (3,155,189,190).
More recently, we have continued to study this interesting
kind of bioinspired photocatalysis and expanded our studies
toward metallocorrole compounds (Fig. 22), as it is well
established that these ring-contracted macrocycles tend to stabi-
lize the formation of high-valent metal complexes ( 192 ). This
strategy for the first time in tetrapyrrole chemistry opened the
possibility to create high-valent lead(IV)- and bismuth(V)-oxo
species as potential redox catalysts, which are currently
investigated in our group (191,193). The molecular structures of
the heme compound 0 and compound 1 analogous species 24
and 25 obtained in the course of biomimetic oxygen activation
PHOTOSENSITIZATION AND PHOTOCATALYSIS 273