ES\u0026T2011 DCNP SI

SUPPORTING INFORMATION

MODELLING PHOTOTRANSFORMATION REACTIONS IN SURFACE WATER

BODIES: 2,4-DICHLORO-6-NITROPHENOL AS A CASE STUDY

Pratap Reddy Maddigapu, Marco Minella, Davide Vione,* Valter Maurino, Claudio Minero

Dipartimento di Chimica Analitica, Università di Torino, Via P. Giuria 5, 10125 Torino, Italy.

http://www.chimicadellambiente.unito.it

* Corresponding author. E-mail: [email protected]

http://naturali.campusnet.unito.it/cgi.bin/docenti.pl/Show?_id=vione

ii

Reagents and materials

2,4-Dichloro-6-nitrophenol (DCNP, purity grade >98%), anthraquinone-2-sulphonic acid, sodium

salt (AQ2S, 97%), NaN3 (99%), NaNO3 (>99%), NaHCO3 (98%), HClO4 (70%) and H3PO4 were

purchased from Aldrich, NaOH (99%), methanol and 2-propanol (both LiChrosolv gradient grade)

from VWR Int., Rose Bengal (RB) from Alfa Aesar.

HPLC determinations

The adopted Merck-Hitachi instrument was equipped with AS2000A autosampler (100 µL sample

volume), L-6200 and L-6000 pumps for high-pressure gradients, Merck LiChrocart RP-C18 column

packed with LiChrospher 100 RP-18 (125 mm × 4.6 mm × 5 µm), and L-4200 UV-Vis detector

(detection wavelength 279 nm). It was adopted an isocratic elution with a 60:40 mixture of

CH3OH:aqueous H3PO4 (pH 2.8), at a flow rate of 1.0 mL min−1

. The retention time of DCNP was

11.1 minutes, the column dead time 0.90 min.

iii

Modelling the formation and the reactivity of ••••OH in surface waters (Vione et al., 2010a).

In natural surface waters under sunlight illumination, the main •OH sources are (in order of average

importance) Coloured Dissolved Organic Matter (CDOM), nitrite, and nitrate. At the present state

of knowledge it is reasonable to hypothesise that these three sources generate •OH independently,

with no mutual interactions. Therefore, the total formation rate of •OH (R•OH

tot) is the sum of the

contributions of the three species:

−

•

−

••• ++= 32 NO

OH

NO

OH

CDOM

OH

tot

OH RRRR (i)

Various studies have yielded useful correlation between the formation rate of •OH by the

photoactive species and the respective absorbed photon fluxes of sunlight (PaCDOM

, PaNO2−

, PaNO3−

).

In particular, it has been found that (Vione et al., 2009d and 2010a):

CDOM

a

CDOM

OH PR ⋅⋅±= −

•

510)4.00.3( (ii)

−−−

• ⋅⋅±= 222 10)3.02.7( NO

a

NO

OH PR (iii)

−−−

• ⋅+

+⋅⋅±= 323

0075.0][25.2

0075.0][10)2.03.4( NO

a

NO

OH PIC

ICR (iv)

where [IC] = [H2CO3] + [HCO3−] + [CO3

2−] is the total amount of inorganic carbon. The calculation

of the photon fluxes absorbed by CDOM, nitrate and nitrite requires to take into account the mutual

competition for sunlight irradiance, also considering that CDOM is the main absorber in the UV

region where also nitrite and nitrate absorb radiation. At a given wavelength λ, the ratio of the

photon flux densities absorbed by two different species is equal to the ratio of the respective

absorbances. The same is also true for the ratio of the photon flux density absorbed by species to the

total photon flux density absorbed by the solution, patot

(λ) (Braslavsky, 2007). Accordingly, the

following equations hold for the different •OH sources (note that A1(λ) is the specific absorbance of

the surface water layer over a 1 cm optical path length, in units of cm−1

, d is the water column depth

in cm, Atot(λ) the total absorbance of the water column, and p°(λ) the spectrum of sunlight):

dAAtot ⋅= )()( 1 λλ (v)

][)()( 333

−

−− ⋅⋅= NOdA NONO λελ (vi)

iv

][)()( 222

−

−− ⋅⋅= NOdA NONO λελ (vii)

)()()()()( 23 λλλλλ totNONOtotCDOM AAAAA ≈−−= −− (viii)

)101()()()(λλλ totAtot

a pp−−⋅°= (ix)

)()]([)()()( 1 λλλλλ tot

atotCDOM

tot

a

CDOM

a pAApp ≈⋅⋅= − (x)

1

2

2 )]([)()()( −

−

− ⋅⋅= λλλλ totNO

tot

a

NO

a AApp (xi)

1

3

3 )]([)()()( −

−

− ⋅⋅= λλλλ totNO

tot

a

NO

a AApp (xii)

An important issue is that p°(λ) is usually reported in units of einstein cm−2

s−1

nm−1

(see for

instance Figure A-SI), thus the absorbed photon flux densities are expressed in the same units. To

express the formation rates of •OH in M s

−1, the absorbed photon fluxes Pa

i should be expressed in

einstein L−1

s−1

. Integration of pai(λ) over wavelength would give units of einstein cm

−2 s

−1 that

represent the moles of photons absorbed per unit surface area and unit time. Assuming a cylindrical

volume of unit surface area (1 cm2) and depth d (expressed in cm), the absorbed photon fluxes in

einstein L−1

s−1

units would be expressed as follows (note that 1 L = 103 cm

3):

∫−=

λ

λλ dpdPCDOM

a

CDOM

a )(10 13 (xiii)

∫−−− =

λ

λλ dpdPNO

a

NO

a )(10 2132 (xiv)

∫−−− =

λ

λλ dpdPNO

a

NO

a )(10 3133 (xv)

Accordingly, having as input data d, A1(λ), [NO3−], [NO2

−] and p°(λ) (the latter referred to a 22 W

m−2

sunlight UV irradiance, see Figure A-SM), it is possible to model the expected R•OHtot

of the

sample. The photogenerated •OH radicals could react either with 2,4-dichloro-6-nitrophenol

(DCNP) or with the natural scavengers present in surface water (mainly organic matter,

bicarbonate, carbonate and nitrite). The natural scavengers have a •OH scavenging rate constant

Σi kSi [Si] = 2×104 NPOC + 8.5×10

6 [HCO3

−] + 3.9×10

8 [CO3

2−] + 1.0×10

10 [NO2

−] (units of s

−1;

NPOC = non-purgeable organic carbon is a measure of DOC, expressed in mg C L−1

, and the other

concentration values are in molarity). Accordingly, the reaction rate between DCNP and •OH can

be expressed as follows:

v

∑

•

•

•

=

i iSi

OHDCNPtot

OH

OH

DCNPSk

DCNPkRR

][

][,

(xvi)

where kDCNP,•OH is the second-order reaction rate constant between DCNP and •OH (2.3×10

9 M

−1

s−1

, as determined in this work) and [DCNP] a molar concentration. Note that, in the vast majority

of the environmental cases it would be kDCNP,•OH [DCNP] « Σi kSi [Si]. The pseudo-first order

degradation rate constant of DCNP is kDCNP = R•OHDCNP

[DCNP]−1

, and the half-life time is tDCNP =

ln 2 kDCNP−1

. The time tDCNP is expressed in seconds of continuous irradiation under sunlight, at 22

W m−2

UV irradiance. It has been shown that the sunlight energy reaching the ground in a summer

sunny day (SSD) such as 15 July at 45°N latitude corresponds to 10 h = 3.6⋅104 s of continuous

irradiation at 22 W m−2

UV irradiance (Minero et al., 2007). Accordingly the half-life time of

DCNP, because of reaction with •OH, would be expressed as follows in SSD units:

OHDCNP

tot

OH

i iSi

OHDCNP

tot

OH

i iSiSSD

OHDCNP kR

Sk

kR

Sk

••••

•

∑∑ −⋅=⋅

=,

5

,

4,

][109.1

106.3

][2lnτ (xvii)

This expression is reported as equation (11) in the manuscript. A difference with equation (25) of

Vione et al. (2010a) is that here R•OHtot

is expressed in mol L−1

s−1

, there in mol s−1

. For this reason,

the volume V = S d is not reported here in equation (xvii). Note that 1.9⋅10−5

= ln 2 (3.6⋅104)−1

.

vi

Modelling the formation and the reactivity of 1O2 in surface waters (Vione et al., 2010b).

The formation of singlet oxygen in surface waters arises from the energy transfer between ground-

state molecular oxygen and the excited triplet states of CDOM (3CDOM*). Accordingly, irradiated

CDOM is practically the only source of 1O2 in the aquatic systems. In contrast, the main

1O2 sink is

the energy loss to ground-state O2 by collision with the water molecules, with a pseudo-first order

rate constant k1O2 = 2.5×105 s

−1. The dissolved species, including the dissolved organic matter that

is certainly able to react with 1O2, would play a minor to negligible role as sinks of

1O2 in the

aquatic systems. The main processes involving 1O2 and DCNP in surface waters would be the

following:

3CDOM* + O2 → CDOM +

1O2 (xviii)

1O2 + H2O → O2 + H2O + heat (xix)

1O2 + DCNP → Products (xx)

In the Rhône delta waters it has been found that the formation rate of 1O2 by CDOM is R1O2

CDOM =

1.25⋅10−3

PaCDOM

(Al-Housari et al., 2010). Considering the competition between the deactivation of

1O2 by collision with the solvent (reaction xix) and the reaction (xx) with DCNP, one gets the

following expression for the degradation rate of DCNP by 1O2:

21

21

21

21

][,

O

ODCNPCDOM

O

O

DCNPk

DCNPkRR

⋅⋅= (xxi)

In a pseudo-first order approximation, the rate constant is kDCNP = RDCNP1O2

[DCNP]−1

and half-life

time is tDCNP = ln 2 kDCNP−1

. Considering the usual conversion (≈ 10 h) between a constant 22 W

m−2

sunlight UV irradiance and a SSD unit, the following expression for τDCNP,1O2SSD

is obtained

(remembering that R1O2CDOM

= 1.25⋅10−3

PaCDOM

and ∫−=

λ

λλ dpdPCDOM

a

CDOM

a )(10 13 ):

∫⋅⋅

==

λ

λλτ

dpk

d

kR CDOM

aODCNPODCNP

CDOM

O

SSD

ODCNP)(

85.381.4

21

21

21

21

,,

, (xxii)

Note that 3.85 = (ln 2) k1O2 (1.25⋅10−3

⋅ 3.60⋅104 ⋅ 10

3)−1

.

vii

Modelling the formation and the reactivity of 3CDOM* in surface waters (Vione et al., 2010b).

The formation of the excited triplet states of CDOM (3CDOM*) in surface waters is a direct

consequence of the radiation absorption by CDOM. In aerated solution, 3CDOM* could undergo

thermal deactivation or reaction with O2, and a pseudo-first order quenching rate constant k3CDOM* =

5×105 s

−1 has been observed. The quenching of

3CDOM* would be in competition with the reaction

between 3CDOM* and DCNP:

CDOM + hν → 3CDOM* (xxiii)

3CDOM* (O2)→ Deactivation and

1O2 production (xxiv)

3CDOM* + DCNP → Products (xxv)

In the Rhône delta waters it has been found that the formation rate of 3CDOM* is R3CDOM* =

1.28×10−3

PaCDOM

(Al-Housari et al., 2010). Considering the competition between the reaction

(xxv) with DCNP and other processes (reaction xxiv), the following expression for the degradation

rate of DCNP by 3CDOM* is obtained:

*

*,

*

*

3

3

3

3 ][

CDOM

CDOMDCNP

CDOM

CDOM

DCNPk

DCNPkRR

⋅⋅= (xxvi)

In a pseudo-first order approximation, the rate constant is kDCNP = RDCNP3CDOM*

[DCNP]−1

and the

half-life time is tDCNP = ln 2 kDCNP−1

. Considering the usual conversion (≈ 10 h) between a constant

22 W m−2

sunlight UV irradiance and a SSD unit, one gets the following expression for

τDCNP,3CDOM*

SSD (remembering that ∫

−=λ

λλ dpdPCDOM

a

CDOM

a )(10 13 ):

∫⋅⋅

=

λ

λλτ

dpk

dCDOM

aCDOMDCNP

SSD

CDOMDCNP)(

52.7

*,

*,3

3 (xxvii)

Note that 7.52 = (ln 2) k3CDOM* (1.28⋅10−3

⋅ 3.60⋅104 ⋅ 10

3)−1

.

viii

Modelling of direct photolysis processes in surface water (Vione et al., 2009a,b)

The calculation of the photon flux absorbed by DCNP requires taking into account the mutual

competition for sunlight irradiance between DCNP itself and the other lake water components

(mostly Coloured Dissolved Organic Matter, CDOM, which is the main sunlight absorber in the

spectral region of interest, around 300-500 nm).

Under the Lambert-Beer approximation, at a given wavelength λ, the ratio of the photon flux

densities absorbed by two different species is equal to the ratio of the respective absorbances. The

same is also true of the ratio of the photon flux density absorbed by species to the total photon flux

density absorbed by the solution (patot

(λ)) (Braslavsky, 2007). Accordingly, the photon flux

absorbed by DCNP in a water column of depth d (expressed in cm) can be obtained by the

following equations (note that A1(λ) is the specific absorbance of the surface water sample over a 1

cm optical path length, Atot(λ) the total absorbance of the water column, p°(λ) the spectrum of

sunlight, εDCNP(λ) the molar absorption coefficient of DCNP, in units of M−1

cm−1

, and paDCNP

(λ) its

absorbed spectral photon flux density; it is also paDCNP

(λ) « patot

(λ) and ADCNP(λ) « Atot(λ) in the

very vast majority of the environmental cases):

dAAtot ⋅= )()( 1 λλ (xxviii)

][)()( DCNPdA DCNPDCNP ⋅⋅= λελ (xxix)

)101()()()(λλλ totAtot

a pp−−⋅°= (xxx)

1)]([)()()( −⋅⋅= λλλλ totDCNP

tot

a

DCNP

a AApp (xxxi)

Note that the sunlight spectrum p°(λ) in the calculations is referred to a sunlight UV irradiance of

22 W m−2

(see Figure A-SI, which also reports the molar absorption coefficient of the anionic form

of DCNP and the surface water spectrum A1(λ)). Also note, that the quantities relative to DCNP

should be referred to the anionic form that prevails in surface waters. Finally, the absorbed photon

flux PaDCNP

is the integral over wavelength of the absorbed photon flux density:

∫=λ

λλ dpPDCNP

a

DCNP

a )( (xxxii)

The sunlight spectrum p°(λ) is referred to a unit surface area (units of einstein s−1

nm−1

cm−2

, see

Figure A-SM), thus PaDCNP

(units of einstein s−1

cm−2

) represents the photon flux absorbed by

ix

DCNP inside a cylinder of unit area (1 cm2) and depth d. The rate of photolysis of DCNP, expressed

in M s−1

, can be approximated as RateDCNP = 103 ΦDCNP Pa

DCNP d

−1, where ΦDCNP is the multi-

wavelength, average photolysis quantum yield of DCNP in the relevant wavelength interval, and d

is expressed in cm (also note that 1 L = 103 cm

3). This approximated expression of RateDCNP can be

adopted if the detailed wavelength trend of ΦDCNP is not known, provided that ΦDCNP is referred to

the same wavelength interval where the spectra of DCNP and sunlight overlap. The pseudo-first

order degradation rate constant of DCNP is kDCNP = RateDCNP [DCNP]−1

, which corresponds to a

half-life time tDCNP = ln 2 (kDCNP)−1

. The time tDCNP is expressed in seconds of continuous

irradiation under sunlight, at 22 W m−2

UV irradiance. It has been shown that the sunlight energy

reaching the ground in a summer sunny day (SSD) such as 15 July at 45°N latitude corresponds to

10 h = 3.6×104 s continuous irradiation at 22 W m

−2 UV irradiance (Minero et al., 2007).

Accordingly, the half-life time expressed in SSD units would be given by: τSSDDCNP = (3.6×10

4)−1

ln

2 (kDCNP)−1

= 1.9×10−5

[DCNP] d 10−3

(ΦDCNP PaDCNP

)−1

= 1.9×10−5

[DCNP] d 10−3

(ΦDCNP

∫λ

λλ dpDCNP

a )( )−1

= 1.9×10−5

[DCNP] d 10−3

(ΦDCNP ∫−⋅⋅

λ

λλλλ dAAp totDCNP

tot

a

1)]([)()( )−1

=

∫−

−

−°Φ

×

λ

λ λλ

λελ d

Ap

d

DCNPdA

DCNP)(

)()101()(

109.1

1

)(

8

1

(xxxiii)

Note that 1.9⋅10−8

= 10−3

(ln 2) (3.6⋅104)−1

. This expression for τSSDDCNP is also reported as equation

(10) in the manuscript. A few additional considerations are made here to aid the comparison

between this equation and equation (14) in Vione et al. (2009d). The numerator in Vione et al.

(2009d) contains V = S d, expressed in litres, and at the denominator the sunlight spectrum i°(λ) has

units of einstein s−1

nm−1

. Dividing i°(λ) by V one obtains units of [einstein L−1

s−1

nm−1

]. Here the

sunlight spectrum is p°(λ), in units of einstein cm−2

s−1

nm−1

(see Figure A-SI), and at the numerator

there is d, expressed in cm. Dividing p°(λ) by d one obtains units of [einstein cm−3

s−1

nm−1

]. This is

the reason why here S is not present at the numerator, and the numerical coefficient is 1.9×10−8

and

not 1.9×10−5

as in Vione et al. (2009d) (1 L = 103 cm

3). Therefore, despite the slightly different

format, the two equations are exactly equivalent. The equation reported here is easier to be handled

when the sunlight spectrum has units of [einstein cm−2

s−1

nm−1

], as is often reported in the literature

(see for instance Frank and Klöpffer, 1988, and Figure A-SI).

x

Modelling the formation and the reactivity of CO3−−−−•••• in surface waters (Vione et al., 2009e).

The radical CO3−•

can be produced upon oxidation of carbonate and bicarbonate by •OH, upon

carbonate oxidation by 3CDOM*, and possibly also from irradiated Fe(III) oxide colloids and

carbonate. However, as far as the latter process is concerned, there is still insufficient knowledge

about the Fe speciation in surface waters to enable a proper modelling. The main sink of the

carbonate radical in surface waters is the reaction with DOM, which is considerably slower than

that between DOM and •OH.

•OH + CO3

2− → OH

− + CO3

−• [kxxxiv = 3.9×10

8 M

−1 s

−1] (xxxiv)

•OH + HCO3

− → H2O + CO3

−• [kxxxv = 8.5×10

6 M

−1 s

−1] (xxxv)

3CDOM* + CO3

2− → CDOM

−• + CO3

−• [kxxxvi ≈ 1×10

5 M

−1 s

−1] (xxxvi)

DOM + CO3−•

→ DOM+•

+ CO32−

[kxxxvi ≈ 102 (mg C)

−1 s

−1] (xxxvii)

The formation rate of CO3−•

in reactions (xxxiv,xxxv) is given by the formation rate of •OH times

the fraction of •OH that reacts with carbonate and bicarbonate, as follows:

][CO103.9][HCO108.5][NO101.0NPOC10.02

][CO103.9][HCO108.5RR

2

3

8

3

6

2

104

2

3

8

3

6tot

OHOH)(3CO −−−

−−

••−⋅⋅+⋅⋅+⋅⋅+⋅⋅

⋅⋅+⋅⋅⋅= • (xxxviii)

The formation of CO3−•

in reaction (xxxvi) is given by:

CDOM

a

2

3

3

(CDOM)3CO P][CO106.5R ⋅⋅⋅= −−•− (xxxix)

The total formation rate of CO3−•

is RCO3−•tot

= RCO3−•(•OH) + RCO3−•(CDOM). The transformation rate

of DCNP by CO3−•

is given by the fraction of CO3−•

that reacts with DCNP, in competition with

reaction (xxxvii) between CO3−•

and DOM:

NPOCk

[DCNP]kRR

xxxvii

3CODCNP,

tot

3CO

3CODCNP,⋅

⋅⋅=

•−•−

•− (xl)

where kDCNP,CO3−• is the second-order reaction rate constant between DCNP and CO3−•

. In a pseudo-

first order approximation, the rate constant is kDCNP = RDCNP,CO3−• [DCNP]−1

and the half-life time is

xi

tDCNP = ln 2 kDCNP−1

. Considering the usual conversion (≈ 10 h) between a constant 22 W m−2

sunlight UV irradiance and a SSD unit, the following expression for τDCNP,CO3−•SSD

is obtained:

⋅

⋅⋅⋅=

•−•−

−•−

3COP,

tot

3CO

5

3,kR

NPOCk109.1 xxxviiSSD

CODCNPτ (xli)

Note that 1.9⋅10−5

= ln 2 (3.6⋅104)−1

.

Surface-water absorption spectrum

It is possible to find a reasonable correlation between the absorption spectrum of surface waters and

their content of dissolved organic matter, expressed as NPOC (Non-Purgeable Organic Carbon).

The following equation holds for the water spectrum (Vione et al., 2010a):

( ) ( ) λλ ⋅±−⋅⋅±= 0.0020.015

1 e0.040.45)(A NPOC (xlii)

This equation was used as the basis for the light-absorption calculations to generate Figure 7a of the

manuscript, where the half-life time of DCNP is reported also as a function of the NPOC.

Literature Cited

Al-Housari, F., Vione, D., Chiron, S., Barbati, S., 2010. Reactive photoinduced species in estuarine

waters. Characterization of hydroxyl radical, singlet oxygen and dissolved organic matter

triplet state in natural oxidation processes. Photochem. Photobiol. Sci. 9, 78-86.

Braslavsky, S.E., 2007. Glossary of terms used in photochemistry, 3rd

edition. Pure Appl. Chem. 79,

293-465.

Chiron, S., Minero, C., Vione, D., 2007. Occurrence of 2,4-dichlorophenol and of 2,4-dichloro-6-

nitrophenol in the Rhône river delta (Southern France). Environ. Sci. Technol. 41, 3127-

3133.

Frank, R., Klöpffer, W., 1988. Spectral solar photon irradiance in Central Europe and the adjacent

North Sea. Chemosphere 17, 985-994.

xii

Minero, C., Chiron, S., Falletti, G., Maurino, V., Pelizzetti, E., Ajassa, R., Carlotti, M.E., Vione, D.,

2007. Photochemical processes involving nitrite in surface water samples. Aquat. Sci. 69,

71-85.

Vione, D., Feitosa-Felizzola, J., Minero, C., Chiron, S., 2009a. Phototransformation of selected

human-used macrolides in surface water: Kinetics, model predictions and degradation

pathways. Wat. Res. 43, 1959-1967.

Vione, D., Minella M., Minero, C., Maurino, V., Picco, P., Marchetto, A., Tartari, G., 2009b.

Photodegradation of nitrite in lake waters: role of dissolved organic matter. Environ.

Chem. 6, 407-415.

Vione, D., Lauri, V., Minero, C., Maurino, V., Malandrino, M., Carlotti, M. E., Olariu, R. I.,

Arsene, C., 2009c. Photostability and photolability of dissolved organic matter upon

irradiation of natural water samples under simulated sunlight. Aquat. Sci. 71, 34-45.

Vione, D., Khanra, S., Cucu Man, S., Maddigapu, P. R., Das, R., Arsene, C., Olariu, R. I., Maurino,

V., Minero, C., 2009d. Inhibition vs. enhancement of the nitrate-induced

phototransformation of organic substrates by the •OH scavengers bicarbonate and

carbonate. Wat. Res. 43, 4718-4728.

Vione, D., Maurino, V., Minero, C., Carlotti, M. E., Chiron, S., Barbati, S., 2009e. Modelli the

occurrence and reactivity of the carbonate radical in surface freshwater. C. R. Chimie 12,

865-871.

Vione, D., Das, R., Rubertelli, F., Maurino, V., Minero, C., Barbati, S., Chiron, S., 2010a.

Modelling the occurrence and reactivity of hydroxyl radicals in surface waters:

Implications for the fate of selected pesticides. Intern. J. Environ. Anal. Chem. 90, 258-

273.

Vione, D., Das, R., Rubertelli, F., Maurino, V., Minero, C., 2010b. Modeling of indirect

phototransformation processes in surface waters. In: Ideas in Chemistry and molecular

Sciences: Advances in Synthetic Chemistry, Pignataro, B., ed., Wiley-VCH, Weinheim,

Germany, pp. 203-234.

xiii

Figure A-SI. Absorption spectrum (molar absorption coefficient ε) of the anionic form of DCNP,

and specific absorbance spectrum of water from the Rhône delta (A1(λ)). Spectral

photon flux density of sunlight (p°(λ)), corresponding to 22 W m−2

UV irradiance

(Frank and Klöpffer, 1988), as can be found on 15 July at 45°N latitude, under clear-

sky conditions, at 10 am or 2 pm.

xiv

Reaction between DCNP and 1O2 in the presence of Rose Bengal (RB) under irradiation

Irradiated RB also produces other reactive species than 1O2 (Rózanowska et al., 1995) (30). To

demonstrate that DCNP really undergoes degradation by 1O2, an additional competition experiment

was carried out with NaN3 as 1O2 scavenger. Figure B-SI reports the time evolution under blue light

of 10 µM DCNP + 10 µ RB, with and without 0.32 mM NaN3. We have adopted 0.32 mM N3−,

which has a second-order rate constant of 7.8×108 M

−1 s

−1 with

1O2 (Wilkinson and Brummer,

1981), to obtain a reaction rate of 1O2 with N3

− that is equal to the expected deactivation rate upon

collision with the solvent (reaction 7). Therefore, with 0.32 mM NaN3 the consumption rate of 1O2

would be around double than without NaN3, and both [1O2] and the rate of DCNP degradation by

1O2 would be about halved. Figure B-SI shows that RDCNP is (1.58±0.08)×10

−9 M s

−1 without azide

and (9.98±1.04)×10−10

M s−1

with 0.32 mM NaN3. The ratio of the rates is 0.63±0.10, compatible

with 1O2 as playing the main role into the degradation of DCNP.

Rózanowska, M., Ciszewska, J., Korytowski, W., Sarna, T., 1995. Rose-bengal-photosensitized

formation of hydrogen peroxide and hydroxyl radicals. J. Photochem. Photobiol. B-Biol. 29,

71-77.

Wilkinson, F., Brummer, J., 1981. Rate constants for the decay and reactions of the lowest

electronically excited singlet-state of molecular oxygen in solution. J. Phys. Chem. Ref. Data

10, 809-1000.

xv

Figure B-SI. Time evolution of DCNP 10 µM upon irradiation of RB 10 µM under the blue lamp.

It is reported the DCNP time trend with RB alone and with RB + NaN3 0.32 mM.

The solution pH was 8, adjusted by addition of NaOH.