inorganic chemistry

and its concentrations are too low for the existence of the dimeric
Cr 2 O 72
form. Both monomeric anions absorb the near-UV part
of the solar spectrum (lmax¼374 nm, e¼4880 for CrO 42
and
lmax¼352 nm,e¼1350 for HCrO 4
( 253 )) and the absorbed radi-
ation generates the LMCT excited state, which undergoes a pho-
toreaction only when a proper electron donor is accessible to
activate the PET between two sites localized in the same molec-
ular entity (innersphere PET, Eq. 66) or from one molecular
entity to another (outersphere PET, Eq. 67).
Oxidation of alcohols at the expense of photoreduction of Cr
(VI) was initially interpreted in terms of the photochemical reac-
tivity of chromate(VI) esters, but time-resolved spectroscopic
studies ( 253 ) proved that the photoreaction mode depends on
the bond strength between chromate and secondary alcohol. In
the case of the strong bond, innersphere transfer of two electrons
was recorded (Fig. 3):

R

1 R 2 CHOCrO 3


þ5Hþþ3H

2 O!R 1 R 2 COþ Cr HðÞ 2 O 6

 4 þ

ð 66 Þ

whereas in the case of weak interaction between chromate(VI)
and an electron donor, outersphere quenching yields one electron
transfer:

HCrO

4


þR

1 R 2 CHOH!H 2 Cr

VO

4


þR

1 R 2 CHO

 ð 67 Þ

In the environment, there are many external electron donors,
which are susceptible to contribute to the Cr(VI) photoreduction,
among others alcohols, oxalate, cysteine (HSCH 2 CH(NH 2 )
COOH), phenol, and its derivatives (2,95,244,245,253– 255 ).

FIG. 3. Scheme for photoinduced charge transfer between excited
chromate(VI) anions and electron donors via inner- or outer-sphere
pathway.

METAL COMPLEXES AS SOLAR PHOTOCATALYSTS 329