lead to low-energy CT interactions. This includes the observation
of mixed-valent compounds with ligand-to-ligand charge transfer
(LLCT) transitions between reducing and oxidizing ligands
simultaneously present at a metal-binding site, or the possibility
of metal-to-metal charge transfer (MMCT) or intervalence charge
transfer transitions in polynuclear metal-binding sites and clus-
ter structures with ligand bridged reducing and oxidizing metal
centers ( 56 ).
In all these cases, the degree of coupling between the donor
and acceptor sites crucially determines the electronic structures
and optical properties. Whenever strong covalent bonding occurs,
as is the case for bioorganometallic systems, the degree of molec-
ular orbital delocalization may become dominant and the
description of electronic transitions with localized orbitals and
discrete oxidation states is then no longer appropriate. Neverthe-
less, this simplified classification has proven to be very useful to
discuss different types of excited states in both inorganic photo-
chemistry and bioinorganic spectroscopy(56,57). A representa-
tive overview on the diversity of bioinorganic chromophores
following this terminology is given in Table III.
As can be clearly seen from the data provided inTable III, the
excitation energies of different bioinorganic chromophores are
completely covering the photochemically relevant spectral region
from ultraviolet to NIR light. Moreover, the observed intensities
of the absorption features directly involving metal-binding sites
strongly vary and span many orders of magnitude. To populate
a desired excited state manifold more efficiently, it may therefore
300 400 500 600
Wavelength (nm)Absorbance700 800FIG. 7. Electronic absorption spectrum of the oxidized rubredoxin
protein from the dinitrogen fixing bacterium Clostridium pas-
teurianum. Adapted from Ref. ( 58 ).
246 GÜNTHER KNÖR AND UWE MONKOWIUS