Photosensitizing properties of 2,4-dichlorobenzoic acid and chlorinated biphenyl carboxylic acids, potentially key components of chromophoric dissolved organic matter

Article abstract of DOI:10.1039/b506971h

2,4-Dichlorobenzoic acid and a suite of models of chlorinated biphenyl carboxylic acids were found to be efficient sensitizers of the reactive oxygen species singlet oxygen (¹O₂). The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b506971h

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Photosensitizing properties of 2,4-dichlorobenzoic acid and chlorinated
biphenyl carboxylic acids, potentially key components of chromophoric
dissolved organic matter{
Anne L. Boreen and Kristopher McNeill*
Received (in Bloomington, IN, USA) 24th May 2005, Accepted 23rd June 2005
First published as an Advance Article on the web 14th July 2005
DOI: 10.1039/b506971h
The data was fit to an exponential growth and decay curve
(eqn. (1)), where Ag and A‘ are the amplitude of the exponential
growth of 1O2 and the amplitude at t 5 ‘, respectively; tg and td
are the lifetime of the growth and decay of 1O2, and t is the time.15
2,4-Dichlorobenzoic acid and a suite of models of chlorinated
biphenyl carboxylic acids were found to be efficient sensitizers
of the reactive oxygen species singlet oxygen (1O2).
1
A recent report by Repeta et al. identified 2,4-dichlorobenzoic acid
(2,4-DCBA) and isomers of tetrachlorobiphenylcarboxylic acid
(CBPAs) as components of marine chromophoric dissolved
organic matter (CDOM).1 While not the first report of these
chlorinated compounds in natural waters,2,3 the high concentration
in ocean water (,1–10 mg L21)1 coupled with their high extinction
coefficients, suggests that such compounds may account for a large
percentage of the total absorbance of solar radiation by marine
CDOM (.2%).1 Along with being the dominant light absorber in
most surface waters, CDOM is also the principle photosensitizer,
having been implicated in the formation of numerous reactive
species including singlet oxygen (1O2), hydroxyl radical and
superoxide.4 Because CDOM is a complex mixture, it has been
difficult to ascertain which functional groups are responsible for
the sensitization of reactive species. The identification of individual
components of CDOM by Repeta et al., however, allows us to
study the photosensitizing ability of these specific compounds. In
particular, we have investigated the ability of these compounds to
The amount of O2 formed was calculated from the difference
between Ag and A‘ as the use of the integrated area of the total
1
signal would incorporate any quenching of O2 by the substrate,
1
artificially lowering the measured amount of O2.
ꢀ
ꢁ
ꢂ
ꢃ
À
Á
td
{t=td
g
Signal~ Ag{A?
e{t=t {e
zA? (1)
tg{td
The quantum yield (W) of 1O2 formation was calculated by
1
comparing the amount of O2 produced by each substrate (S) to
that generated by perinaphthenone, a well-defined reference (R), in
Table 1 (A) Structures of the biphenyl carboxylic acids proposed to
be components of marine CDOM by Repeta et al.a (B) Model
compounds of proposed marine CDOM components used in this
studyb
1
sensitize the formation of O2, a reactive oxygen species found in
sunlit surface waters known to cause the degradation of
anthropogenic and naturally occurring organic substrates.5–12
The structural assignment of the chlorinated biphenyl carboxylic
acid components of CDOM is ambiguous1 and the structures
proposed are not commercially available. This prompted the use of
model compounds that incorporate the essential structural
characteristics of the compounds proposed (Table 1).
Generation of 1O2 was accomplished with a laser flash
photolysis apparatus,7,13,14 using the 266 nm{ output of a
Nd:YAG laser as the excitation source. The phosphorescent signal
of 1O2 at 1270 nm was detected 90u to the incident laser beam with
an ultra-sensitive Ge photodiode detector. The signal was collected
using a digital oscilloscope, and four transients were averaged to
enhance the signal to noise ratio. Representative transients can be
seen in Fig. 1, with all solutions optically matched to an
absorbance of 1 AU.
a
Department of Chemistry, University of Minnesota, 207 Pleasant Street
SE, Minneapolis, MN 55455, USA. E-mail: mcneill@chem.umn.edu;
Fax: 612-626-7541; Tel: 612-625-0781
Proposed chlorine substitution pattern is either 29,39,59; 29,49,59 or
29,49,69. BA = benzoic acid; 2,4-DCBA = 2,4-dichlorobenzoic
b
acid; TCB = 1,2,4,5-tetrachlorobenzene; BPA = 4-biphenylcarboxylic
acid; 29-Cl-BPA = 29-chloro-4-biphenylcarboxylic acid; 39,49-di-Cl-
{ Electronic supplementary information (ESI) available: molar absorptiv-
ities of each substrate in the various solvents at 300, 266 and 254 nm and
details of the environmental half-life calculations. See http://dx.doi.org/
10.1039/b506971h
BPA
2,29,4,49-tetrachlorobiphenyl.
=
39,49-dichloro-4-biphenylcarboxylic acid; PCB-47
=
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biphenyl, 4-chlorobiphenyl and 4,49-dichlorobiphenyl (0.48, 0.56
and 0.58 in CH3CN, respectively),19 with the exception of PCB-47,
which has a much lower yield of 1O2. This is believed to be due to
the presence of two ortho-chlorine substituents. The excited state of
biphenyl compounds is thought to be planar; a substitution pattern
involving ortho-substituents induces strain in this excited state,
decreasing its lifetime, and thus decreasing the amount of 1O2
generated.20–23
The efficiency of 1O2 formation does not differ substantially
between the phenyl and the biphenyl derivatives. Also, when
comparing BA to 2,4-DCBA, addition of chlorine substituents
increases the quantum yield of 1O2 production. The biphenyl
compounds, however, do not follow this trend, indicating that
chlorination of the carboxylic acid-substituted ring may be more
influential on the efficiency of 1O2 formation than chlorination of
the non-acid substituted ring.
Fig. 1 Representative 1O2 phosphorescence transients for models of the
proposed components of CDOM in acetonitrile, with perinaphthenone
used as a reference sensitizer. Transients were obtained using an excitation
wavelength of 355 nm, with all solutions optically matched to an
absorbance of 1 AU.
We were interested in the photostability of 2,4-DCBA and the
chlorinated biphenyl carboxylic acids, as this will directly influence
their persistence in surface waters. Thus, direct photolysis quantum
yields were determined. Solutions of each substrate (5–50 mM
depending on the molar absorptivity at 254 nm) were photolyzed
in unstoppered quartz test tubes using the 254 nm output of low
pressure mercury vapor bulbs in a photochemical reactor
(Rayonet). Samples were held vertically, and the sample holder
was spun on a turntable to ensure even irradiation. Aliquots were
removed at various timepoints and analyzed for substrate
degradation via HPLC with UV-detection. The pseudo-first-order
degradation of substrate was compared to that of a reference
compound with a known W of direct photolysis (uridine,
Wdirect 5 0.0185)24 according to eqn. (3), yielding the W of direct
photolysis.
the corresponding solvent (eqn. (2)).
À
Á
Á
Ag{A?
WS~
S WR
R
À
(2)
Ag{A?
The calculated W of 1O2 formation for each of the model
CDOM components in acetonitrile (CH3CN), ethanol (EtOH), or
deuterium oxide (D2O) are listed in Table 2.
1
The W of O2 production for these substrates are quite high,
indicating that these components of CDOM are efficient
1
generators of O2, and will contribute significantly to the steady-
state 1O2 concentration in sunlit surface waters. The values
measured here are of similar magnitude to literature values for
kSe2R54nm½Rꢀ
(3)
Wdirect, S
~
Wdirect, R
kRe2S54nm½Sꢀ
Table 2 Quantum yield of 1O2 formation and direct photolysis for
the model CDOM compounds. Errors represent the standard deviation
of multiple experiments
Direct photodegradation was found to be orders of magnitude
less efficient than formation of 1O2 for all compounds except TCB
and PCB-47, for which the two processes are competitive. The
direct photolysis yields for 29-Cl-BPA and PCB-47 are of similar
magnitude to the direct photolysis W reported for 2-chlorobiphenyl
in H2O (0.021).20 The elevated yields for these two compounds are
again believed to be attributable to the presence of ortho-chlorine
substituents and the strain these bulky substituents place on the
planar excited state causing rapid degradation.20–22 The calculated
environmental half-lives for direct photolysis at 30u N range from
4 h for 29-Cl-BPA to 3 years for BPA, again emphasizing the
enhanced photolability of compounds containing ortho-chlorine
substituents (see ESI{ for calculation details.)
Loss of ortho-chlorine substituents upon photolysis of chlori-
nated biphenyl compounds is well-known,20–22,25 and during
photolysis of 29-Cl-BPA in EtOH, the production of BPA was
observed. The identity of this photoproduct was confirmed by
HPLC chromatographic overlap with an authentic standard of
BPA and ESI-TOF MS analysis of the photolysate. The yield of
BPA from 29-Cl-BPA degradation was 31% (Scheme 1).
W Formation
W Direct
photolysisb
of 1O2
a
Compound
2,4-DCBA
Solvent
CH3CN
EtOH
0.63 ¡ 0.03
0.64 ¡ 0.02
ndc
nd
D2O–H2Od 0.45 ¡ 0.02
0.0055 ¡ 0.0005
nd
nd
0.0027 ¡ 0.0006
0.26 ¡ 0.02
nd
BA
CH3CN
EtOH
D2O–H2O 0.37 ¡ 0.03
CH3CN
CH3CN
EtOH
H2O
CH3CN
EtOH
H2O
0.42 ¡ 0.03
0.39 ¡ 0.02
TCB
BPA
0.47 ¡ 0.02
0.49 ¡ 0.04
0.68 ¡ 0.06
nd
0.58 ¡ 0.02
0.76 ¡ 0.07
nd
0.49 ¡ 0.03
0.66 ¡ 0.08
nd
nd
0.00020 ¡ 0.00005
0.008 ¡ 0.001
0.028 ¡ 0.002
0.08 ¡ 0.01
nd
29-Cl-BPA
39,49-di-Cl-BPA CH3CN
EtOH
H2O
CH3CN
nd
0.0011 ¡ 0.0001
0.089 ¡ 0.005
PCB-47
a
0.084 ¡ 0.007
W of 1O2 production by perinapthenone in CH3CN 5 0.99,16,17
EtOH 5 0.92,16 and H2O 5 0.98.17 Values are corrected for
b
screening by substrate.18 The actinometer was photolyzed in H2O,
and the direct photolysis quantum yields of TCB, 29-Cl-BPA and
Thus, the direct photolysis of 29-Cl-BPA in EtOH yields BPA,
and although this product was not observed in photolyses
conducted in H2O, dechlorination products such as hydroxyl-
substituted derivatives are still anticipated,20,25 and these products
PCB-47 were not corrected for differences in the index of refraction
between CH3CN or EtOH and H2O. nd 5 not determined.
c
d
Singlet oxygen quantum yields were measured in D2O due to the
extended lifetime of 1O2 relative to H2O.
1
may still function as O2 sensitizers.
4114 | Chem. Commun., 2005, 4113–4115
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1O2 formation and direct photolysis due to increased absorption by the
substrates (See Table S1. ESI{). Quantum yields of single chromophores
are generally wavelength independent, and are thus applicable to
environmental systems.26
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Scheme 1 Conversion of 29-Cl-BPA to BPA in photolyses conducted in
EtOH at 254 nm. The quantum yield of dechlorination was calculated as
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aquatic environment depends not only on their quantum yields of
1O2 formation and direct photolysis, but also on their ability to
absorb solar radiation. The chlorine substituents on 2,4-DCBA
increase the efficiency of 1O2 formation, but also cause the
absorption to undergo a bathochromic shift relative to BA,
allowing for greater solar radiation absorption. Similarly, the
biphenyl compounds generally absorb longer wavelengths more
efficiently than the phenyl compounds. The absorbance of both
29-Cl-BPA and PCB-47 above 300 nm, however, is substantially
less than the other biphenyl compounds due to the non-planarity
of the molecules, and thus the amount of solar radiation absorbed
by biphenyl compounds containing ortho-substituents is expected
to be diminished relative to biphenyl compounds with other
substitution patterns.
This work suggests that chlorinated benzoic and biphenyl
carboxylic acids, if present in the marine environment at the levels
found by Repeta et al., will be significant sources for photo-
1
1
chemically generated O2. Based on the W of O2 formation by
samples of marine CDOM (y0.0611), the W of 1O2 production by
2,4-DCBA, and assuming 2,4-DCBA is responsible for 2% of the
total absorbance by CDOM, this single chromophore would
1
account for approximately 15% of the O2 generated by marine
CDOM. This work also indicates that these compounds are
weakly photolabile, which should lead to vertical concentration
gradients as they are depleted from marine surface waters.
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the substitution pattern, as any CBPA containing ortho-chlorine
substituents absorb less solar radiation, and are expected to
undergo chlorine loss in conjunction with sensitization of 1O2.
Their degradation products as well as the other isomers of CBPAs
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1
and BPA itself, however, are also efficient O2 sensitizers.
Notes and references
{ The use of wavelengths shorter than 300 nm (266 and 254 nm), although
not environmentally relevant, were used to calculate the quantum yields of
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Chem. Commun., 2005, 4113–4115 | 4115