material. This may involve either energy transfer or electron
transfer ( 1 ). In a strict mechanistic sense, however, the sensitizer
must not be consumed in the process. Scientific work on this
topic dates back to the pioneering days of photography, when
the sensitivity of silver halides was for the first time extended
to longer wavelengths by using various organic dyes and
pigments ( 85 ). Later, the dye sensitization of solids and
semiconductors was systematically explored ( 86 ), which finally
led to the development of modern solar cell materials ( 87 ).
Biological effects of sunlight and synthetic organic dyes on
enzymes, microbes, and higher organisms have been discovered
in the end of the nineteenth century and soon were exploited
for medical applications ( 88 ). Today, the optimization of
sensitizers for photodynamic therapy is still a very active branch
of research in bioinorganic photochemistry and molecular
photomedicine (6,89).
The majority of organic compounds and biological metabolites
is colorless but may be severely damaged under UV-light expo-
sure. Therefore, spectral sensitization is a crucial prerequisite
for the efficient accumulation of permanent photoproducts in all
kinds of synthetic processes driven by light. The big advantage
of photosensitized processes avoiding undesired side reactions
caused by secondary photolysis now gradually becomes
recognized as extremely useful for photocatalytic organic synthe-
sis driven by visible light( 90 ).
Here, we will mainly focus on two other important aspects of
photosensitization: the fundamental role of deeply colored com-
pounds as light-harvesting antenna chromophores for solar
energy conversion and the possibility of reaching spectroscopi-
cally hidden, but photochemically active excited state levels by
means of spectral sensitization.
Porphyrins and related tetrapyrrole pigments represent the
most important class of sensitizers in both natural and artificial
photosynthesis (8,87,91). These compounds are ideally suited for
collecting light in the far-red and NIR spectral region, which
represents a natural limit for directly driving energetically uphill
bond-formation processes suitable for the photochemical storage
of solar energy. To reproduce the spectroscopic and light-
harvesting features of the chlorophylls is therefore an important
goal of biomimetic and bioinspired chemistry (5,92,93). A com-
parison of natural and synthetic photosensitizers with quite sim-
ilar absorption characteristics is given in Fig. 8.
Besides metallophthalocyanines 7 as photosensitizers(91,93),
some intensely colored perylene diimine derivatives such as
8 have also been proposed as functional chlorophyll analogues
248 GÜNTHER KNÖR AND UWE MONKOWIUS