almost unaffected. This resembles the strategies evolved in biocat-
alytic systems, where activation, regioselectivity, and branching
between alternative substrate transformation pathways are
carefully controlled by the formation of enzyme–substrate
complexes.
The convenient triggering of selective reactions by light and
the regulatory effects of light intensity variations are crucial
benefits of the photochemical approach toward biomimetic model
compounds. These two important aspects of biological systems,
which are otherwise hardly achieved in synthetic molecular
devices, will be briefly discussed in the following sections.
C.1. Controlling reaction pathways
In photochemical reactions, the population of excited states of
different orbital origins can result in quite different reactivity
patterns. Therefore, reaction products may occur, which are not
accessible at all in thermochemical pathways. Especially in
organometallic and coordination compounds, the primary pho-
toproducts obtained are not always resulting from the lowest-
lying excited state levels. Wavelength-selective excitation may
then be exploited to channel the product formation process and
to control a possible branching between different reactivity
patterns.
As already mentioned in the previous section, also the funda-
mental laws of spin conservation may completely close or at least
slow down certain reaction channels. ISC and spin inversion thus
can strongly influence the balance between competing processes
with a different regio- and stereoselectivity ( 5 ). While such
effects are very common in metalloenzyme redox catalysis, their
rational exploitation in bioinorganic photochemistry and photo-
catalysis is still in its infancy (3,6).
Successful fine-tuning of the branching reactivity patterns
observed in metal complexes and organometallics requires a pro-
found set of experimental and spectroscopic data. Especially in
complicated systems with various close-lying electronic excited
states of different orbital parentage, an in-depth interpretation
of the experimental results can only be supported and further
refined by highly sophisticated quantum chemical calculations
(128,129). A well-documented example of such a situation with
a competitive spin-multiplicity- and wavelength-dependent pho-
toreactivity is the selective bond cleavage of [HMn(CO) 5 ] and
related organometallic manganese hydride complexes (Fig. 14).
The carbonyl-hydrido complex [Mn(H)(CO) 3 (DAB)] displays
nine low-lying singlet and triplet excited states, which all are
PHOTOSENSITIZATION AND PHOTOCATALYSIS 257