reactivity patterns of certain excited state species. These
analogies can be readily exploited for triggering the corresponding
substrate transformations in biomimetic photocatalysis. A selec-
tion of different building blocks, which, however, can display a
very similar chemical reactivity, is given in Fig. 19.
The functional equivalence between the CT excited states of
certain metalloporphyrin photosensitizers carrying a high-valent
metal-oxo moiety and the porphyrin radical intermediate sup-
posed to be critically involved in cytochrome P450 oxidations
has already been recognized several years ago ( 155 ). Indeed,
the biological system and the biomimetic photocatalysts display
very similar reactivity patterns. In their electronic ground state,
the light-driven enzyme models could be readily characterized
and studied (3,155). In contrast, despite decades of continuous
efforts, a detailed spectroscopic characterization of the Fe(IV)-
oxo-porphyrin cation radical thought to be the key intermediate
in P450 catalysis has been absent for a long time (153,156).
(^1920)
21
N
O•
C
O•
*MO•*MO•R22COMOnπ*hnLMCThnFIG. 19. Different ways to introduce oxyl radical reactivity: nature
employs metal bound tyrosyl radicals ( 19 ) or high-valent metal oxo
fragments in many active sites (65,153). Nitroxyl radicals such as
2,2,6,6,-tetramethylpiperidin-1-oxyl (TEMPO, 20 ) are reactive species
used in organocatalysis ( 154 ). The excited states of carbonyl functional
groups ( 21 ) and metal oxo-fragments ( 22 ) display a radical pair charac-
ter, which may become very attractive for biomimetic photoredox
processes upon spectral sensitization (3,5).
266 GÜNTHER KNÖR AND UWE MONKOWIUS