In a similar way, photochemical remote control of chemical and
biological processes can serve to mimic or influence important
aspects of these natural regulatory systems. The most simple
version of deactivation and activation of any photochemical pro-
cess can be achieved by switching between dark- and light-
adapted conditions, which is comparable to the presence or
absence of an inhibitor. Photosubstitution reactions of metal
complexes and light-induced fragmentation processes can be
employed to trigger the signal transduction function of small gas-
eous molecules and other neurotransmitter substances. Another
common strategy is the photochemical cleavage of a light-sensi-
tive protection group to trigger the release of otherwise hidden
(so-called caged) bioactive or biomimetic compounds(130,131).
The big disadvantage of most of these simple strategies is that
the desired function is only available for one time in an irrevers-
ible light-responsive process.
In many types of photocatalytic reactions, however, the varia-
tion of incident light intensity is directly related to the actual
photostationary concentration of the active species involved( 5 ).
This feature can then be easily applied for a certain degree of
continuous up- and down-regulation of an already running biomi-
metic process under ambient conditions. An even higher level of
control can be reached, whenever a wavelength-selective
response of the system is built in. Some photochromic com-
pounds, for instance, ( 132 ) enable a reversible switching of
catalytic activity and other types of light-dependent processes.
This is a very powerful tool for the construction of biomimetic
and bioinspired enzyme models.
In the field of artificial photosynthetic devices, regulatory
strategies could also be advantageous. An interesting recent
example for such an approach is given inFig. 15. The multi-
chromophoric system consists of a covalently linked porphy-
rin–fullerene donor–acceptor core designed for photoinduced
charge separation ( 133 ). In the periphery, additional aromatic
antenna subunits and a photochromic switch are situated. Under
intense white-light conditions, the spiro-dihydroindolizine-based
regulator subunit 10 opens up to form a larger photostationary
concentration of its deeply colored betaine form, which in compe-
tition to the donor–acceptor moiety also absorbs in the visible
spectral region and efficiently quenches the donor excited state.
In a certain sense, this self-regulating molecule mimics the way
green plant photosynthesis responds to potentially damaging
light levels by controlling the fraction of excitation energy that
can drive PET processes.
PHOTOSENSITIZATION AND PHOTOCATALYSIS 259