inorganic chemistry

a favorable situation of the potential energy surfaces (5,8).
Ideally, the thermal back reaction of the products formed should
have a sufficiently high activation barrier to enable a continuous
accumulation of the desired product molecules during many
photocatalytic cycles. According to the Hammond postulate
(113,114), which claims that exothermic reactions have an early
barrier, the structural features of the one-electron transfer inter-
mediates formed photochemically should already be as close as
possible to the transition state of the follow-up electron transfer
step. This requirement gives an important guideline for choosing
the type of excited state to be built into such systems.

B.2. Protons

The most common type of biocatalytic reactions is proton trans-
fer ( 115 ). Nearly, every enzymatic reaction involves one or more
proton-coupled steps. Transition-state proton bridging and intra-
molecular proton transfer (general acid–base catalysis) are
important strategies to accelerate substrate conversion pro-
cesses. Moreover, proton transfer also plays a fundamental role
in bioenergetics ( 116 ).
There are also many well-documented cases of excited state
proton transfer reactions. It has been known for a long time that
the acid–base properties of organic molecules such as phenols are
drastically modified upon light absorption. About 60 years ago,
Förster suggested a simple method for estimating the excited
state pK* values of photoactivated species from thermodynamic
and spectroscopic data ( 117 ), which became very popular
(Fig. 12).

ArOH*

ArOH

ArO– + H+

ArO– *

hnArOH

hnArO–

ΔH

ΔH*

pKa

pK*a

FIG. 12. Förster-cycle for the acid–base equilibria between a phenol
derivative (ArOH) and the corresponding phenolate anion (ArO) in
the ground state S 0 and the first excited singlet state S 1.

254 GÜNTHER KNÖR AND UWE MONKOWIUS