and followed by oxidation of both products by O 2 :

cit^2 
þO 2! 3 
oxoglutarateþCO 2 þO 2 
 ð 52 Þ

CrIIaqþO 2 !CrIIIaqþO 2 
 ð 53 Þ

2O 2 
þ2Hþ!2HO 2 !H 2 O 2 þO 2 ð 54 Þ

H 2 O 2
!

hv<300 nm

2OH ð 55 Þ

CrIIaqþ4OHþ4OH
!CrO 42 
þ4H 2 O ð 56 Þ

Consistent with the proposed reaction scheme, the quantum
yield of chromate(VI) production increased significantly with
pH, that is, for [CrIII(cit)OH]
it was higher than for [CrIII(cit)].
In the case of trisoxalatochromate(III), [Cr(C 2 O 4 ) 3 ]^3
, a similar
pathway of LMCT photochemistry was suggested, although dis-
tinct from edta^3
, the C 2 O 4 
radical ligand is more susceptible
to release(95,245). The LMCT excitation:

CrIIIðÞC 2 O (^43)
hi 3

!

hvðÞLMCT

* CrIIðÞC 2 O 42 ðÞC 2 O 4 

hi 3 


ð 57 Þ

is followed by back electron transfer

 CrIIC

ðÞ 2 O 42 C 2 O 4

ðÞ

hi 3 

! CrIIIðÞC 2 O (^43)
hi 3

ð 58 Þ
and/or by PET accompanied by dissociation of the oxidized
ligand:
CrIIC
ðÞ 2 O 42 C 2 O 4
ðÞ

hi 3

! CrIIðÞC 2 O (^42)
hi 2

þC 2 O 4 
ð 59 Þ
Quenching of the Cr(II) species by molecular oxygen yields
[CrIII(C 2 O 4 ) 2 (H 2 O) 2 ]
and/or CrO 42
, whereas the C 2 O 4 

radicals readily generate OHradicals and finally are oxidized
to CO 2 (222,225).
Recent flash photolysis results showed, however, that beside
the photoredox mechanism illustrated byEqs. (51)–(53), also sol-
vated electron generation could be recorded within nanoseconds
(t1/2¼0.16ms) ( 251 ).
CrIIðÞC 2 O 42 ðÞC 2 O 4 

hi 3

! CrIIIðÞC 2 O 42 ðÞC 2 O 4 

hi 2

þe
aq ð 60 Þ
The e
aq lifetime increased significantly when one oxalate
ligand in [CrIII(C 2 O 4 ) 3 ]^3
was substituted by a more electrophilic
METAL COMPLEXES AS SOLAR PHOTOCATALYSTS 327