The first example of such a reversible proton translocation
cycle occurring within a protein core( 118 ) was described for the
GFP chromophore 5 already mentioned in the previous sections
(Fig. 3). In the presence of light, excited state proton transfer
should also be relevant for many other biological systems carry-
ing phenols such as tyrosine residues or other deprotonable
moieties in their active sites. In inorganic photochemistry, the
acid–base properties of the coordinated ligands can also be dras-
tically modified upon excitation. Although some attempts to gen-
eralize the Förster concept for coordination compounds have
been made ( 119 ), the possible benefit of utilizing such effects is
still underestimated. Nevertheless, it is already quite clear that
metal complexes with potential proton translocation sites on
the ligand periphery also exhibit immense changes in their
nucleophilic character and their acid–base properties upon light
absorption ( 120 ). The chemical bonds most frequently involved
in the context of excited state proton transfer reactions include
OH, NH, and CH. Typical photoinduced changes in acidity or
basicity are characterized by pK-value variations of 4–6 units,
which is in the range of the transition-state effects observed in
hydrolytic enzyme catalysis ( 5 ). These effects could therefore
become a very useful functional tool for many applications in bio-
mimetic and bioinspired photocatalysis.
B.3. Spin
When chemical bonds are formed or broken, the valence
electrons of the participating species are redistributed. In some
cases, the necessary changes in electron angular momentum
(spin) in the course of a chemical reaction may represent the
decisive rate-limiting factor (Fig. 13). For example, the majority
of stable organic substances are diamagnetic with a singlet
ground state. Spontaneous reaction with dioxygen, which has a
triplet ground state, is therefore constrained due to spin-barrier
effects. These limitations are immediately circumvented, when
O 2 is converted into singlet oxygen by photosensitization or by
other means.
Rate acceleration is the most fundamental aspect of catalysis.
To elucidate the electronic mechanisms of spin–acceleration phe-
nomena therefore represents an important topic in both bio-
inorganic and biomimetic dioxygen activation (5,122). Many
other types of substrate transformations catalyzed by metal
complexes or redox enzymes also involve key steps with a change
in spin along their reaction coordinates. The abundance of such
phenomena seems to be much wider than initially thought. This
PHOTOSENSITIZATION AND PHOTOCATALYSIS 255