inorganic chemistry

includes all kinds of radical reactions and electron transfer
processes, which can be catalyzed by metal ions ( 123 ). Some
spin-forbidden proton transfer reactions have also been
characterized ( 124 ).
Paramagnetic metal complexes or radical species present in
the coordination sphere of active sites can open up initially
closed reaction channels when their own spin is combined with
those of the reactants. In photosynthetic reaction centers, the
fate of the radical-ion pairs generated by photoinduced charge
separation is, for example, defined by the exchange interactions
with a close-by iron site. Also, the course of bond cleavage pro-
cesses, hydrogenations, and isomerizations can be influenced by
the presence of paramagnetic compounds. All the basic aspects
of this ubiquitous phenomenon termedspin-catalysis( 125 – 127 )
can also be exploited for bioinspired photocatalytic systems,
where spin crossover and intersystem crossing (ISC) processes
can be triggered by light, and the degree of spin–orbit coupling
can be employed to partially control such effects ( 5 ).

C. SELECTIVITY ANDREGULATION

The interaction between light and matter, unlike thermal acti-
vation of compounds in the form of heat, is always a very selec-
tive process. Photochemical activation may induce the twisting,
stretching, or destabilization of certain bonds located in a specific
region of an individual molecule, while other subunits remain

(a)

Energy Energy

Reaction coordinate Reaction coordinate

S S

S±1 S±1

(b)

FIG. 13. Energy curves along the reaction coordinate of a spin-forbid-
den two-state process (a). In such a situation, spin crossover can accel-
erate product formation by opening up a novel low-energy reaction
pathway (b). Adapted from Ref. ( 121 ).

256 GÜNTHER KNÖR AND UWE MONKOWIUS