chlorine atom. In the case of the rhodium surface complex,
the excitation is of rhodium-to-titanium charge-transfer type
resulting in a Ti(III) center and a Rh(IV) species. These
heterosupramolecular metal complexes in the presence of air
photocatalyze the exhaustive visible light oxidation of pollutants
like halocarbons and atrazine. They are active even in diffuse
indoor daylight.
Keywords:Chloroplatinate(IV); Flatband potential; Haloge-
norhodate(III); Photocatalysis; Solar detoxification; Titanium
dioxide; Visible light.
I. IntroductionIn connection with the chemical utilization of solar energy the
photochemistry of inorganic compounds has received increasing
attention. It started in the 1970s, declined in the 1990s, and
came up again in the present decade. For the desired conversion
of light to chemical energy, three key steps can be considered as
schematically summarized byEqs. (1)–(7) for the sensitization of
the redox reaction AþD¼Ared and Dox by a transition metal
complex. They consist of photo-induced charge separation
(Eq. 1), electron exchange with substrates affording primary
redox products (Eqs. 3 and 4), and conversion of the latter to sta-
ble final products (Eqs. 5 and 6). In the case ofsolar energy stor-
age, the over-all reaction has to be endergonic, whereas forsolar
energy utilization it can also be exergonic. It is noted that
although many systems undergo the first reaction step, only a
few enable also the crucial electron exchange steps due to the
highly favored charge recombination (Eq. 2). Even if these two
steps proceed (Eqs. 3 and 4), efficient back electron transfer
(BET) between the primary redox products (Eq. 7) in many cases
prevents efficient formation of the final redox products (Eqs. 5
and 6). Thus, the basic problem of the conversion of light to
chemical energy is how to inhibit the primary and secondary
charge recombination processes according to Eqs. (2) and (7),
respectively. In homogeneous systems, the problem is partially
solved by making one of the redox steps, for example, Eq. (4),
so fast that it successfully competes with recombination. A typi-
cal example is the evolution of hydrogen upon irradiating an
aqueous solution of a tris(bipyridyl)ruthenium(II) complex in
the presence of methylviologen (corresponds to A) and a reducing
agent like triethylamine (corresponds to D). In this system, the
372 HORST KISCH