of most dyes; the second O 2 singlet state (^1 Sg) is more difficult to
reach (157 kJ mol^1 ), but is much more reactive.
The^3 HS may react with triplet O 2 or with an oxidizable reac-
tant by energy, hydrogen or electron transfer reactions (Fig. 2).
Direct reactions between^3 HS and organic substrates seem less
favored since the triplet state reaction with oxygen is very effi-
cient ( 7 ). The quantum yield of singlet oxygen production deter-
mined at 365 nm as 1–3%, depends strongly on the irradiation
wavelength and the HS nature ( 6 ).
Moreover, due to the extremely short^1 O 2 lifetime in water
(4ms), the competing reaction rate constants should be at least
2 orders of magnitude higher than that of solvent quenching
(2.3 105 s^1 )( 8 ). Furthermore, singlet oxygen (^1 D) was found
to be quenched not only by water molecules but also by HS.
These characteristics make the steady-state concentration of sin-
glet oxygen in natural waters of the order of 10^13 – 10 ^14 and
thus can oxidize only the most reactive acceptor molecules, such
as tryptophan (3-indolylalanine, C 6 H 4 NHCHCCH 2 CH(NH 2 )
COOH), histidine (C 6 H 9 O 3 N 3 ), phenolates (C 6 H 5 OM) ( 9 ).
Photosensitized oxidations by molecular oxygen are however
important because of their impact on living organisms. It is
known that, microorganisms and other living cells can be killed
in the presence of light, molecular oxygen, and sensitizing dye.
Moreover, some of the anthropogenic hazardous organic
pollutants can undergo degradation photosensitized by HS. To
this group belong both pesticides and herbicides, such as DDT
(dichloro-diphenyl-dichloroethane) ( 10 ), atrazine (2-chloro-4-(eth-
ylamino)-6-(isopropylamino)-s-triazine) ( 11 ), differentO 2O 2(^1) O 2
O 2 –
R R
OX
ROX
RH
R
R R+
(^3) HS
FIG. 2. Reactions of humic substances in the excited triplet state
with molecular oxygen and with organic reactants.
298 ZOFIA STASICKA