absorbed by a photosensitizer or photocatalyst and transferred to
the substrate molecule.
Due to sunlight energy that is limited at the ground level to
e415 kJ/mol (l290 nm), only unique organic substrates (S)
are able under these conditions to undergo photoconversion into
products (P):

S
!

hvðÞsunlight 

S!P ð 1 Þ

The Cr(VI) example quoted above illustrates, however, that
besides the monomolecular also bimolecular photoprocesses are
effective:

S
!

hvðÞsunlight 

S!

Q

P ð 2 Þ

where Q represents a molecular entity that deactivates
(quenches) an excited state of the substrate molecule, either by
energy transfer or electron transfer or by a chemical mechanism.
Moreover, photosensitization processes are observed in which a
reaction of a nonabsorbing substrate is induced by energy trans-
fer or electron transfer from an excited light-absorbing photosen-
sitizer (Sens):

Sens
!

hvðÞsunlight 

Sens!

S

P ð 3 Þ

The electron transfer sensitization is called reductive or oxida-
tive, depending on the action of the excited sensitizer as electron
donor or acceptor, respectively. Photosensitization may be
treated as one branch of the more general family of catalytic
reactions called photocatalysis, which involves light absorption
by a catalyst precursor (pC) or by a substrate:

pC
!

hvðÞsunlight

C!

S

P ð 4 Þ

S
!

hvðÞsunlight

*S!

C

P ð 5 Þ

Photocatalysis includes various reactivity types which may be
divided into two categories depending on the role of light on: (i)
photogenerated catalysis consisting of reactions catalytic in pho-
tons and (ii) catalyzed photolysis, in which processes are non-
catalytic in photons (Fig. 1).
The photoinduced catalytic reactions in which both light and
catalyst are not consumed, are of importance in organic syntheses,
but are of minor significance in environmental chemistry. Much
more important are those in which the solar energy is consumed

294 ZOFIA STASICKA