The former case is illustrated inFig. 4a, showing two alternative
pathways for the redox cycles photocatalyzed by Fe(III) or Cu(II)
chelate complexes: (i) innersphere PET with ligand (L) as electron
donorand(ii)outerspherePETwithanexternalelectrondonor(D).
The latter way of degradation by Fe(III) photocatalysts is of special
meaning in the case of anthropogenic pollutants, such as
herbicides and pesticides (164,186,189,258,259).
The iron photocatalytic cycles have relevant activity in nature
not only due to their cleaning and disinfection functions( 203 )
but also for inducing key biological processes, including amino
acid synthesis, oxygen transport, respiration, nitrogen fixation,
methanogenesis, the citric acid cycle, photosynthesis, and DNA
biosynthesis. Moreover, the vast majority of bacteria require for
growth iron in a definite oxidation state ( 260 ).
It is now realized that copper as metal next to iron and
chromium participates in photoredox cycles and its role cannot
be ignored. The most important part of the cycle is photoreduc-
tion of Cu(II) to Cu(I) induced by solar light and oxidation of
ligands to their environmentally benign forms. Then Cu(I) is
easily re-oxidized to Cu(II), which can coordinate the next
ligand molecule, and thereby the Cu photocatalytic cycles con-
tribute to continuous environmental cleaning. Besides oxida-
tion/reduction, other critical processes relevant to the copper
cycles are adsorption/desorption and precipitation/dissolution
FIG. 4. Two alternative pathways for the redox cycles
photocatalyzed by Fe(III) or Cu(II) complexes (a) and by chromium
compounds (b); L, ligand; D, external electron donor; DOM, dissolved
organic matter.
METAL COMPLEXES AS SOLAR PHOTOCATALYSTS 331