In the context of artificial photosynthesis and solar fuel pro-
duction, the long-wavelength spectral sensitization of photo-
reactions into the far-red and NIR-region is of prime interest.
Without suitable chromophores, a reasonably high efficiency of
an abiotic solar energy storage process, which is always
characterized by a specific optimum threshold wavelength
(5,96,97), will not be reached.
As already mentioned above, spectral sensitization may also
become indispensable when the light absorption properties of a
potentially photoreactive compound do not permit direct excita-
tion in the desired wavelength region. By application of
sensitizers with adjusted excited state properties, it is, for exam-
ple, possible to induce photochemical reactions of otherwise col-
orless compounds with visible light. Another important
application in photochemistry is the sensitized population of
excited state levels, which are not easily reached by direct
absorption of light due to the limitations of quantum chemical
selection rules. This phenomenon has been extensively exploited
in mechanistic and synthetic organic photochemistry, where
enhanced yields of triplet state population could be achieved in
various dye-photosensitized processes( 98 ).
In the pioneering years of inorganic photochemistry, the basic
inter- and intramolecular sensitization processes were
introduced by Vogler and Adamson( 99 ), which soon was followed
by organometallic examples ( 100 ). In this decade, also the blue-
light absorbing tris(2,2^0 -bipyridyl)ruthenium(II) cation [Ru
(bpy) 3 ]^2 þ, 9 was promoted as an interesting new sensitizer for
energy and electron transfer processes (101,102). In a plethora
of slightly modified forms, [Ru(bpy) 3 ]^2 þ became an extremely
popular prototype of an inorganic photosensitizer ( 103 ). This also
opened fascinating new routes in bioinorganic photochemistry
such as probing and modulating the active site properties of
metalloenzymes with blue light ( 104 ). Derivatives of [Ru(bpy) 3 ]^2 þ
continue to be extensively studied in the context of photo-
catalysis, biomimetics, solar energy conversion, and artificial
photosynthesis ( 105 – 109 ). The structure of the parent ruthenium
polypyridine complex 9 together with a representative applica-
tion of this structural motif in bioinorganic photocatalysis is
depicted below (Fig. 10).
While the MLCT excited state properties of sensitizers such as
9 are easily studied and spectroscopically characterized, these
compounds are not yet the best choice for certain applications.
This includes all kinds of photosystems requiring to collect a
much larger share of the solar spectrum such as artificial photo-
synthetic architectures or solar cells. Even more severely, in PET
250 GÜNTHER KNÖR AND UWE MONKOWIUS