field theory or on the interpretation of valence-bond state corre-
lation diagrams turn out to be extremely useful for the discus-
sion and prediction of reactivity patterns such as radical
coupling mechanisms or bond-formation processes between
nucleophilic and electrophilic species (160,161). Such types of
easily generalized models are very important for the rational
design of biomimetic photocatalytic systems, as they are able to
directly connect the elementary reactions occurring between
reactants, products, and CT excited states in an intuitive and
pictorial way ( 161 ), which can help to adjust the critical reaction
barriers as described already before (Fig. 11).
With the continuous development of theoretical and computa-
tional photochemistry(162,163), also the available quantitative
tools for predicting light-induced reactivity are improving rapidly.
It is impossible here to cover the immense progress made in this
field comprehensively, and therefore, the reader is encouraged to
consult and follow the relevant literature. As a starting point, we
can only mention a few topics considered to be of prime interest
for bioinorganic photosensitization and photocatalysis.
A lot of valuable information can now be derived from a
detailed computational analysis of excited state potential energy
hypersurfaces. This includes the location of areas where either
an extended touching of two surfaces occurs or surface crossing
regions such as funnels or conical intersections exist(164,165).
Studying not only the minima of these crossing points but also
a broader energetic region along the so-called extended conical
intersection seam ( 166 ) allows to shape product distributions
resulting from branching photochemical pathways, and it is quite
clear now that nature makes extensive use of this possibility by
exploiting control and selectivity effects induced by the protein
environment.
The construction of valence-bond state correlation dia-
grams ( 167 ) is also a powerful tool for predicting and shaping
critical barriers or describing reactivity patterns in homogeneous
catalysis. This conceptual approach can be readily adopted for
the field of bioinorganic photocatalysis, as low-lying CT excited
states are frequently found to strongly mix with the critical
transition states of bioinorganic reactions (161,167).
Another important trend already mentioned in previous
sections is the increasing attention to angular momentum con-
servation and spin catalysis (Fig. 13). Attempts are now also
made to find analogies between established semiempirical scales
such as the spectrochemical and nephelauxetic series and local
spin-philicity parameters included in calculations using
spin-polarized conceptual DFT descriptors ( 168 ).
268 GÜNTHER KNÖR AND UWE MONKOWIUS