inorganic chemistry

( 57 – 59 ) is converted to an OH radical. The latter subsequently
induces oxidation of 4-CP.
Recently, it was reported that also the halogenides of Ru(III),
Ir(IV), and Au(III) are able to form visible light active titania
photocatalysts( 60 ).

IV. Summary and Outlook

Modification of titania powders by halogenometal compounds
affords surface transition metal complexes that are efficient
semiconductor photocatalysts for the visible light mineralization
of organic pollutants. In these hybrid materials, titania can be
considered as a semiconducting“inorganic ligand”bound via oxy-
gen to a platinum(IV) or rhodium(III) center. Different to a con-
ventional ligand titania has a peculiar electronic structure and
provides also a solid surface for the adsorption of substrates.
The quasi-Fermi level of electrons in the titania ligand is shifted
anodically by 0.2–0.3 V as compared to the free“ligand.”In the
case of the platinum complex visible light irradiation results
in a platinum centered excitation followed by homolytic PtCl
bond cleavage and electron injection of Pt(III) into the titania
conduction band. Thus, the reducing and oxidizing surface sites
can be described as a trapped electron, that is, a Ti(III) center,
and a loosely bound chlorine atom. In the case of the rhodium
surface complex, the excitation is of the rhodium-to-titanium
charge-transfer type resulting in a Ti(III) center and a Rh(IV)
species.
The few investigations hitherto performed on the photochemi-
cal properties of transition metal complexes having titania as a
ligand clearly proof the general importance of the field. Until
now this applies only for the high photocatalytic activity in
exhaustive photooxidation reactions. Since this is based on an
efficient light-induced charge separation such type of supramo-
lecular coordination complexes are promising systems also for
other relevant processes like nitrogen fixation and artificial
photosynthesis.

ACKNOWLEDGMENTS

This work was supported by the Bavarian State Program
BayFORREST and by Deutsche Forschungsgemeinschaft.

VISIBLE LIGHT PHOTOCATALYSIS 391