regarded as toxic or carcinogenic, and reactions involving ruthe-
nium complexes should therefore be considered at least as
environmentally problematical. Moreover, [Ru(bpy) 3 ]^2 þ
derivatives absorb only a limited share of the photochemically
relevant region of the solar spectrum, and the photoinduced pri-
mary processes of such compounds are typically limited to one
electron chemistry, which requires further coupling to other
redox-active components acting as mediators for multielectron
transformations ( 173 ).
In contrast, bioinspired compounds such as the copper diimine
complex shown inFig. 25 have been suggested as a versatile and
readily tunable alternative to conventional sensitizers used
today for various photocatalytic applications, artificial photosyn-
thetic devices, and dye-sensitized solar cells ( 138 ). It combines
some of the highly desirable features such as long-wavelength
absorption well adapted to the solar spectrum and intramolecu-
lar coupling of the low-lying excited states to an acceptor ligand
providing a preponderance for multielectron reactivity. Further,
the compound exclusively consists of abundant and environmen-
tally benign building blocks including a biocompatible redox-
active transition metal.
FIG. 25. Comparison of the sunlight harvesting features of tris(2,2^0 -
bipyridyl)ruthenium ( 9 ) and a copper-based multielectron transfer pho-
tosensitizer ( 138 ) carrying ap-acceptor ligand of the BIAN-type ( 13 )
already described in Fig. 16.
PHOTOSENSITIZATION AND PHOTOCATALYSIS 281