Phototransformations of environmental contaminants in models of the aerosol: 2 and 4-Nitropyrene

Article abstract of DOI:10.1016/j.jphotochem.2016.08.018

A comparative photochemical study of 2- and 4-nitropyrene (2- and 4-NO₂Py) in different organic solvents was performed in order to provide information on the fate of these contaminants in models of the atmospheric aerosols. The isomers presented small photodegradation yields, 10^?4–10^?5 for 4-NO₂Py and 10^?5–10^?6 for 2-NO₂Py, demonstrating the low reactivity of the excited states and intermediate species that participate in their photodegradation. Photoproducts such as 4,5-pyrenedione, 4-hydroxypyrene, aminopyrene and pyrene were identified during the irradiation of 4-NO₂Py. Substantial differences were observed in the photodegradation yields, and type and relative yields of the photoproducts of the 2-NO₂Py and 4-NO₂Py when compared to those of 1-NO₂Py. These differences were related with the orientation of the nitro group and with differences in intersystem crossing rates which affect the yields of the pyrenoxy radical (PyO) and of the (π,π*) triplet state, principal precursors in their photodegradation. The smallest photodegradation yield was for 2-NO₂Py due to the lack of interaction between the nitro group and the aromatic moiety thus resulting in a low yield of formation of the PyO radical. In the presence of O₂, the photodegradation quantum yields of 4-NO₂Py were reduced in all solvents due the quenching of the (π,π*) triplet state, and 4-aminopyrene was not observed thus demonstrating that its formation occurs from this state. These results suggest that in atmospheric aerosols containing an organic liquid-like layer, the nitropyrene isomers will show low photoreactivity resulting in an increase in their residence time in the atmosphere. An increase in reactivity is expected when the excited nitropyrenes are nearby substances with hydrogen donor capacities such as phenols. The photoproducts formed in the transformation could increase the toxicity of the particulate matter in the atmosphere.

Full text of DOI:10.1016/j.jphotochem.2016.08.018

Journal of Photochemistry and Photobiology A: Chemistry 332 (2017) 131–140
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Phototransformations of environmental contaminants in models of the
aerosol: 2 and 4-Nitropyrene
Zulma I. García-Berríos, Rafael Arce*, Melanie Burgos-Martínez,
Natalia D. Burgos-Polanco
Department of Chemistry, University of Puerto Rico, Río Piedras Campus, San Juan, 00931-3346, Puerto Rico
A R T I C L E I N F O
A B S T R A C T
Article history:
Received 11 December 2015
Received in revised form 15 July 2016
Accepted 15 August 2016
Available online 16 August 2016
2
A comparative photochemical study of 2- and 4-nitropyrene (2- and 4-NO Py) in different organic
solvents was performed in order to provide information on the fate of these contaminants in models of
ꢀ
4
ꢀ5
the atmospheric aerosols. The isomers presented small photodegradation yields, 10 –10 for 4-NO Py
2
ꢀ5
ꢀ6
2
and 10 –10 for 2-NO Py, demonstrating the low reactivity of the excited states and intermediate
species that participate in their photodegradation. Photoproducts such as 4,5-pyrenedione, 4-
Keywords:
hydroxypyrene, aminopyrene and pyrene were identified during the irradiation of 4-NO Py. Substantial
2
Nitropyrene isomers
Photochemical degradation
Photodegradation quantum yields and
mechanism
differences were observed in the photodegradation yields, and type and relative yields of the
photoproducts of the 2-NO
related with the orientation of the nitro group and with differences in intersystem crossing rates which
affect the yields of the pyrenoxy radical (PyO) and of the ( *) triplet state, principal precursors in their
photodegradation. The smallest photodegradation yield was for 2-NO Py due to the lack of interaction
between the nitro group and the aromatic moiety thus resulting in a low yield of formation of the PyO
radical. In the presence of O , the photodegradation quantum yields of 4-NO Py were reduced in all
solvents due the quenching of the ( *) triplet state, and 4-aminopyrene was not observed thus
2 2 2
Py and 4-NO Py when compared to those of 1-NO Py. These differences were
p,p
2
2
2
p,p
demonstrating that its formation occurs from this state. These results suggest that in atmospheric
aerosols containing an organic liquid-like layer, the nitropyrene isomers will show low photoreactivity
resulting in an increase in their residence time in the atmosphere. An increase in reactivity is expected
when the excited nitropyrenes are nearby substances with hydrogen donor capacities such as phenols.
The photoproducts formed in the transformation could increase the toxicity of the particulate matter in
the atmosphere.
ã 2016 Elsevier B.V. All rights reserved.
1
. Introduction
Nitropyrenes are among the most abundant nitro-PAHs detected
3
in polluted urban air and can be found in concentrations of pg/m
3
Nitropolycyclic aromatic hydrocarbons (nitro-PAHs) are a group
of aromatic molecules found to be major environmental contam-
inants. The study of these molecules is of importance due to their
persistence in ambient air, and carcinogenic and mutagenic effects
on human and animals [1–4]. Analyses of organic extracts of
airborne particulate matter have shown that their mutagenic
activity increases with the presence of nitro-PAHs [5–8].
and ng/m in ambient particulate samples [8–13]. 1-Nitropyrene
(1-NO
atmospheres by incomplete combustion of diesel engines,
automobile exhaust and wood smoke [9,14], while 2-NO Py is
formed in the atmosphere by light initiated reactions of PAHs with
hydroxyl radicals followed by NO addition, or by reaction with
which prevails at night [10,15,16]. The transformation of
2 2
Py) and 4-nitropyrene (4-NO Py) are emitted in ambient
2
2
x y
N O
nitro-PAHs in the atmosphere is still a topic of interest and
knowledge of their fates in the environment is needed to assess
their exposure and risk. Photolytic degradation appears to be one
of the principal transformation routes for nitro-PAHs encountered
in the atmosphere [17–20]. More recently, the atmospheric
2 2 2
Abbreviations: 2-NO Py, 2-nitropyrene; 4-NO Py, 4-nitropyrene; 1-NO Py, 1-
nitropyrene; 1-OHPy, 1-hydroxypyrene; 4-OHPy, 4-hydroxypyrene; PyO, pyrenoxy
radical.
photochemical transformation of 1-NO
2
Py has been documented
*
Corresponding author at: Department of Chemistry, University of Puerto Rico,
Río Piedras Campus, P.O. Box 23346, San Juan, 00931-3346, Puerto Rico.
by Kameda et al. [21] from measurements on the diurnal variations
E-mail address: rafael.arce.upr@gmail.com (R. Arce).
http://dx.doi.org/10.1016/j.jphotochem.2016.08.018
1010-6030/ã 2016 Elsevier B.V. All rights reserved.

132
Z.I. García-Berríos et al. / Journal of Photochemistry and Photobiology A: Chemistry 332 (2017) 131–140
of hydroxynitropyrenes in Osaka, Japan. They demonstrated that
some of the products originate not only from vehicle emissions but
was favored due to the formation of pyrene, its principal product.
Traces of nitrohydroxy-, nitrodihydroxy-, and nitromethoxypyr-
from photochemical reactions of 1-NO
2
Py.
enes were also observed in the photolysis of 4-NO
indicating that the nitro-nitrite rearrangement is also participating
in its degradation. The 4-NO Py was three times less photoreactive
compared to 1-NO Py while the 2-NO Py was unreactive and no
products were reported. A similar trend in the relative photo-
degradation rates in dimethylsulfoxide was observed by Yang et al.
[35] They also identified some of the photoproducts from the
2
Py thus
The photochemistry and the biological activity of the nitro-
pyrenes, and other nitro-PAHs, have been related to the orientation
of the nitro group [22–29]. Chapman and coworkers [30]
correlated the photodegradation of the nitro-PAHs with the
orientation of the nitro group relative to the aromatic plane.
According to the proposed nitro-nitrite rearrangement for nitro-
PAHs, the orientation of the nitro group must be perpendicular to
the aromatic plane to favor the overlap of the half-vacant
nonbonding orbital of the nitro group with the adjacent orbital
of the aromatic plane, and to form an oxaziridine ring which
2
2
2
2
irradiation of 1-NO Py.
Information on the participation of different excited states and
intermediate species during the photoexcitation of the nitro-
pyrenes has been helpful in the elucidation of the different routes
of photodegradation of these contaminants [22,23,33,38–40].
Femtosecond broadband transient absorption and femtosecond
fluorescence up-conversion measurements indicated that the
decay of the first excited singlet state occurs in femtoseconds or
picoseconds for various nitro-PAHs, in this manner, demonstrating
the ultrafast intersystem crossing transition of these molecules
[22,23,40–45]. However, recent time-resolved fluorescence results
[23] indicated that there are differences in the time ranges of the
intersystem crossings for each nitropyrene isomer which could
collapses to a nitrite, and consequently gives an aryloxy radical
ꢁ
(
ArO ) (Fig. 1). In addition, a C ꢀꢀ N bond dissociation-recombina-
tion path can lead to the formation of a nitrite ester which
decomposes into an aryloxy radical [30]. However; there is no
spectroscopic evidence [22] that demonstrates the existence of an
ꢁ
aryl nitrite as an intermediate for the formation of the ArO .
According to femtosecond-resolved experiments, Plaza-Medina
et al. [31] observed that the formation of the aryloxy radical occurs
in picoseconds and the kinetic data indicated a direct dissociation
ꢁ
ꢁ
(
ArO and NO ) from only one intermediate, possibly an
affect the relative yields of the (p,p*) triplet state and of the long-
oxaziridine. The nitrite intermediate could be formed during this
dissociation process but with a moderate stability [31]. An
intramolecular charge transfer state eventually leading to radicals
lived pyrenoxy radical, principal precursors for the photodecom-
position of the nitropyrenes. A remarkable difference between the
S
1
lifetimes of the nitropyrenes was attributed by Plaza-Medina
et al. [23] to changes in the energy coincidence between their
*) state and the T(n, *) receiver state according to TD-DFT
calculations.
ꢁ
ꢁ
(
ArO and NO ) has been proposed as a channel in the photo-
destruction reactions of nitronaphthalene derivatives [32]. Re-
cently, for 1-NO Py, we identified photoproducts such as 1-
hydroxypyrene, 1-hydroxy-x-nitropyrenes and pyrenediones
Fig. 1) demonstrating that two of the photodegradation routes
S
1
(p,
p
p
2
The photophysical and photochemical properties of the nitro-
pyrenes are governed by the position and orientation of the nitro
group in the pyrenic moiety [39]. Furthermore, the nature of the
solvent can allow for hydrogen abstraction reactions by reaction
intermediates and excited states [38,39] and by inducing changes
in the relative order of the electronic states thus, resulting in
changes in the intersystem crossing rates [23]. Motivated by these
results, in this work we have determined phototransformation
(
of this contaminant are through the dissociation-recombination or
the nitro-nitrite rearrangement via the singlet or triplet excited
states [33]. Some of these products had been reported by others
although a discussion on their routes of formation, precursors and
quantum yields have been limited [19,21,34–37]. Herein we
present photochemical studies on the other isomers. Van den
Braken et al. [34] compared the spectroscopic and photochemical
yields and identified some of the principal products for 4-NO
2
Py
properties of the three mononitropyrenes, 1-, 2-, and 4-NO
methanol. According to their results [34], the principal photo-
degradation pathway for 1-NO Py is the nitro-nitrite rearrange-
ment leading to 1-hydroxypyrene and 1-hydroxy-2-nitropyrene,
while for 4-NO Py presumably the C-N bond photodissociation
2
Py in
and 2-NO Py in polar protic, polar aprotic and non-polar solvents
2
in order to establish the effect of the solvents on the role of the
reactive intermediates and routes of photodegradation, thus
providing a better insight on the possible transformation routes
of these pollutants in the atmospheric aerosol.
2
2
Fig. 1. Intramolecular nitro nitrite rearrangement proposed by Chapman and the possible mechanism of the formation of the photoproducts from the nitropyrene isomers.

Z.I. García-Berríos et al. / Journal of Photochemistry and Photobiology A: Chemistry 332 (2017) 131–140
133
2
. Experimental section
irradiation conditions. The irradiation time was 2 min for each of
the filters used in the irradiation, see below. On the average six
solutions were prepared from the irradiated potassium ferrioxalate
actinometer solutions. After adding 1,10-phenanthroline the
absorbance at 510 nm, due to the red colored Fe -phenanthroline
complex was measured with a spectrophotometer. For the
determination of the photodegradation quantum yields of 4-
1-Hydroxynaphthalene (1-naphthol) 99%, 1,4-dihydroxyben-
zene (hydroquinone) ꢂ99% 2,6-dimethoxyphenol (syringol) 99%
and the solvents hexane ꢂ99%, 2-propanol 99.5%, pyrene (99%) and
aminopyrene (99%) were from Sigma-Aldrich Chemical Co. All
these materials were used as received with the exception of the
nitropyrene isomers and hexane. 2-Nitropyrene (99.2%) and 4-
nitropyrene (99.5%) were from Chiron AS and were purified using a
Pasteur pipette filled with activated silica gel as a chromatographic
2
+
2
NO Py and for the experiments of the nitropyrenes with phenols,
the irradiations were done using a 405 ꢅ 5 nm interference filter
from Edmund Optics (stock number G48-640) in order to overlap
with the absorption band corresponding to nitro compounds and
avoid photolysis of the cosolutes or products. At this wavelength
(405 nm), spectra recorded during the irradiations do not show an
isosbestic point (see below Fig. 2) and these decreases in
absorbance at this wavelength during the irradiation interval.
ꢃ
column. The silica was heated in an oven at 400 C for 10 h. A
nitropyrene acetonitrile solution was prepared and passed through
the activated silica column. The purity was assessed from the
emission spectrum of the purified substance in acetonitrile using a
Varian Cary Eclipse fluorescence spectrophotometer. In the case of
4-NO
2
Py, its purity was confirmed by the decrease of intensity of
2
The relative yields of formation of the 4-NO Py photoproducts
the emission bands at 395 and 415 nm (excitation 350 nm),
characteristic of 4-hydroxypyrene, its major impurity. The
acetonitrile was then evaporated with a nitrogen flow. Methanol
(Table 2) were determined as the ratio of the chromatographic
peak area of the product to the chromatographic area of the
photodegraded 4-NO
photodegradation depending of the solvent). For the determina-
tion of these formation yields, the irradiation of 4-NO Py was
2
Py after 4 h of irradiation (25–60% of
(
99.9%) and acetonitrile (99.9%) were obtained from Fisher
Scientific.
Stock solutions were prepared by standard procedures. The
2
performed using a broad band filter (300–420 nm) to achieve a
concentrations of the nitropyrenes were calculated from the molar
relative yield for the principal photoproducts greater than 5%. The
absorption coefficients of the longest wavelength band (350–
rate of photodegradation of 2-NO
ence filter was very small to produce significant amounts of
photodegraded 2-NO Py and of the photoproducts to be detected
by HPLC. To increase the rate of degradation, irradiations were
performed using a broad band Corning 7–59 filter to obtain a
higher irradiance than with the interference filter and to achieve at
least 2–7% photodegradation in 8 h. The number of molecules
destroyed was determined from the area of the chromatographic
peak. Because the amount of incident light passing through the
broad band filter and measured by the ferrioxalate actinometer
2
Py using the 405 nm interfer-
450 nm), associated with a
2
p,p* transition of the NO group and
the pyrene's ring system [22] determined from Beer's plots. For 4-
2
ꢀ
5
NO
of 2-NO
2
Py the concentrationwas 7.0 ꢄ10 M, while the concentration
ꢀ
5
2
Py was three times higher, 20 ꢄ10 M. The photolysis
set-up consisted of a Xe-Hg 1000W Oriel short arc lamp (ozone
free), collimating lenses and a cylindrical cell containing water to
filter the infrared radiation and to control the temperature to
minimize thermal degradation.
The quantum yields (f) for the photodegradation of the
nitropyrenes or for the photoproducts formation were determined
using the area of the chromatographic peaks of the corresponding
compounds to calculate the number of molecules destroyed or
formed during the irradiation [33]. The molar absorption coeffi-
cient of each nitropyrene was calculated from a calibration curve at
2
was not entirely absorbed by the 2-NO Py it was estimated as
follows. The relative intensity of the incident light, passing through
the filter at the different wavelength intervals of 5 nm, was
measured using a photomultiplier coupled to a monochromator.
These intensities were plotted as a function of wavelength. The
area under this curve was divided in wavelength intervals of 5 nm.
254 nm using HPLC data [33]. Quantum yield values reported in
Tables and represent the average of at least three
1
3
For each interval the intensity of the absorbed light by 2-NO
I (l )(1–10
0 i
the light intensity as determined by the chemical actinometer by
the fraction of light in the wavelength interval (rectangular
2
Py,
) (photons s ), was calculated by multiplying
ꢀOD(
l
i)
ꢀ1
determinations. The number of photons absorbed by the sample
was determined using potassium ferrioxalate as a chemical
actinometer to measure the incident light intensity [46]. For this
measurement a solution of potassium ferrioxalate was prepared.
Two aliquots of 3 mL of this solution were irradiated before and
after the photolysis of the nitropyrene sample under the same
ꢀ
1
segments of 5 nm/total area), I (li), in photons s . This result was
0
ꢀOD(li)
then multiply by (1–10
absorbance of the nitropyrene during the irradiation interval and it
was measured at 0 and at time t of irradiation (OD = OD ).
) where OD(li) is the average of the
t
ꢀ OD
0
Table 1
The following equation was used to calculate the photodegradation
Photodegradation Quantum Yieldsa of the Mononitropyrenes in Different Solvents.
ꢀ1
quantum yields (molecules photons ) of 2-NO
2
Py:
3
solvent
f
(ꢀ1-
f
(ꢀ4-
f
(ꢀ2-NO2Py) ꢄ 10ꢀ
D
½2 ꢀ NO Pyꢆ
2
NO2Py) ꢄ 10ꢀ
3
NO2Py) ꢄ 10ꢀ3
f
¼ P
I
0
ð
liÞð1 ꢀ 10ꢀ
ODð
l
i
ÞÞðtÞ
N
2
2
-propanol 1.4 ꢅ 0.2
methanol
1.1 ꢅ 0.1
acetonitrile 0.77 ꢅ 0.05
hexane
0.16 ꢅ 0.04
0.05 ꢅ 0.02
0.009 ꢅ 0.002
not measured
0.005 ꢅ 0.001
no observable
degradation
where
2
D[2-NO Py] was the change of concentration of the
0.036 ꢅ 0.002
0.12 ꢅ 0.04
nitropyrene before and after the photolysis determined from the
area of its chromatographic peak and t was the irradiation time (s).
Using the change of nitropyrene concentration and the calibration
curve, obtained from the HPLC data, the number of molecules of 2-
0.012 ꢅ 0.002
O
2
NO
OD = OD
calculated after irradiation times in which less than 7% of the
nitropyrene isomers were destroyed. For 4-NO Py, this low percent
2
Py that have been degraded was determined. The average OD
2
-propanol 1.2 ꢅ 0.1
0.010 ꢅ 0.003
0.013 ꢅ 0.002
0.08 ꢅ 0.03
0.019 ꢅ 0.002
not measured
0.003 ꢅ 0.001
not measured
(
t
ꢀ OD ) and the reported photodestruction yields were
0
methanol
0.8 ꢅ 0.1
acetonitrile 0.7 ꢅ 0.1
hexane
0.13 ꢅ 0.06
0.010 ꢅ 0.001
2
a
reduces the absorption of light by photoproducts absorbing at
405 nm. In addition, at 405 nm there is no isosbestic point which
Quantum yield values represent the average of at least three experiments. The
Py and 4-NO Py was performed using an interference filter
irradiation of 1-NO
405 ꢅ 5 nm) and irradiance of 10 W/cm , while for the irradiation of 2-NO
2
2
ꢀ
3
2
(
2
Py a
means that
DOD is not zero and the photodegradation quantum
broad band Corning 7–59 filter (300–500 nm) was used and the irradiance was
yield could be determined. A similar method was used by Crespo-
3
2
>
5 ꢄ10ꢀ W/cm .

134
Z.I. García-Berríos et al. / Journal of Photochemistry and Photobiology A: Chemistry 332 (2017) 131–140
Table 2
Photoproducts Formed during the Photolysis of 4-Nitropyrene in Different Solvents.
Solvent
lmax (nm)
Compounda
Relative yieldb
%)
(
N
2
2
-propanol
218,238,254,292,309,323,433
4,5-pyrenedione
9.39 min
4-aminopyrene
11.17 min
4-hydroxypyrene
pyrene
18.70 min
33.49
1.92
14.75
1.84
9.38
7.66
2.69
5.53
2.40
2
2
2
2
2
2
2
2
02,275
09,242,257,284,350
02,250,275,285,375
02,240,279,342,378
04,231,240,261,272,305,318,334
02,244,285,342,380
42,284,345
18.97min
19.39 min
02,242,284,345,378
methanol
acetonitrile
hexane
220,275
18,238,254,292,309,323,433
7.65 min
4,5-pyrenedione
10.34 min
11.17 min
4-hydroxypyrene
17.32 min
1.33
19.17
4.34
1.65
1.62
5.20
2
2
2
2
2
02,244,260,300,335
02,250,275,285,355,370
02,240,279,342,378
03,232,267,327,421
218,238,254,292,309,323,433
4,5-pyrenedione
4-hydroxypyrene
13.80 min
15.38 min
18.10 min
44.82
2.67
2.34
3.18
2.25
6.32
2
2
2
2
2
02,240,279,342,378
02,293
02
02
03,284
18.30 min
216,252–270
20,275
18,238,254,292,309,323,433
6.56 min
7.65 min
4,5-pyrenedione
11.02 min
16.34 min
4.20
24.95
5.88
2.75
5.77
2
2
2
2
02,252,275,285,354,370
02,235,275,337
O
2
2
-propanol
207
19,273
18,238,254,292,309,323,433
5.08 min
6.75 min
4,5-pyrenedione
9.50 min
11.17 min
10.95
2.84
2.75
5.76
28.54
1.20
2
2
2
2
2
02,275
02,235,252,275,285,355,374
02,240,279,342,378
4-hydroxypyrene
methanol
207
5.08 min
4,5-pyrenedione
11.17 min
4-hydroxypyrene
13.76 min
17.00 min
4.38
42.64
0.69
4.90
2.30
4.83
2
2
2
2
2
18,238,254,292,309,323,433
02,235,252,275,285,355,374
02,240,279,342,378
02,290,433
03,232,273,318
acetonitrile
218,238,254,292,309,323,433
4,5-pyrenedione
13.80 min
15.38 min
18.10 min
18.30 min
34.66
1.84
1.99
1.26
2.72
2
2
2
2
02,290,433
02
03,295
03,283
hexane
220,275
18,238,254,292,309,323,433
02,235,252,275,285,355,371
6.90 min
4,5-pyrenedione
11.02 min
27.44
6.64
11.33
2
2
a
Products not identified are characterized by their retention times in minutes.
Relative yield = (product chromatographic peak area/photodestroyed nitropyrene area) ꢄ 100. Relative yield values represent the average of at least three experiments.
b
Table 3
Effect of Phenols in Acetonitrile.a
(ꢀ1-NO2Py) ꢄ 10ꢀ
3
f
(ꢀ4-NO2Py) ꢄ 10ꢀ3
f
(ꢀ2-NO2Py) ꢄ 10ꢀ3
quencher
no phenol
f
0.77 ꢅ 0.05
1.5 ꢅ 0.3
1.9 ꢅ 0.6
1.5 ꢅ 0.7
0.12 ꢅ 0.04
0.8 ꢅ 0.3
0.005 ꢅ 0.001
1
-naphthol
0.16 ꢅ 0.02
hydroquinone
syringol
0.9 ꢅ 0.1
no observable degradation
no observable degradation
0.27 ꢅ 0.01
a
atmosphere and using an interference filter (405 ꢅ 5 nm) with an irradiance of 1 ꢀ5 ꢄ10ꢀ3 W/cm2.
The irradiation of all the nitropyrenes were performed under a N
2

Z.I. García-Berríos et al. / Journal of Photochemistry and Photobiology A: Chemistry 332 (2017) 131–140
135
A
2.0
2
1
0
.5
2
1
1
1
1
1
0
0
0
0
0
.8
.6
.4
.2
.0
.8
.6
.4
.2
.0
.5
1
.5
0
2
00
25 0
300
350
400
45 0
50 0
Wavele ng th ( nm)
2
00
250
300
350
400
450
500
550
Wavelength (nm)
0
0
0
0
0
0
0
.30
.25
.20
.15
.10
.05
.00
B
345
395
445
495
545
595
645
695
745
Wavelength (nm)
Fig. 2. A Photodegradation of 4-NO
1
2
Py in 2-propanol under anaerobic conditions at different irradiation times (broad band Corning 7–51 filter: 300–420 nm): 0 min (black),
h (blue), 3 h (green) and 5 h (red). Inset: Photodegradation of 4-NO Py in hexane under anaerobic conditions at different irradiation times. Fig. 2B Photodegradation of 2-
Py in 2-propanol under anaerobic conditions at different irradiation times (broad band Corning 7–59 filter: 300–500 nm): 0 min (black) and 8 h (blue). (For interpretation
2
NO
2
of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Hernández [47] to calculate the quantum yield of photodegrada-
tion of salbutamol in aqueous solutions. Compared to the method
used for calculating the number of photons absorbed per second
using the 405 nm interference filter, this is less accurate.
Nonetheless, this method provided us with an order of magnitude
To follow the photolysis, absorption measurements were
carried out with a Varian CARY 1E double-beam spectrophotome-
ter. Each analysis was preceded by a baseline measurement to
correct for lamp intensity fluctuations. Chromatographic separa-
tions of unirradiated and irradiated nitropyrene samples and the
identification of photoproducts were done using a LC-10AD
2
for the photodestruction yield of 2-NO Py.

136
Z.I. García-Berríos et al. / Journal of Photochemistry and Photobiology A: Chemistry 332 (2017) 131–140
Shimadzu HPLC equipped with a photodiode array SPD-M10AVP
and fluorescence detectors RF-10AXL. For these nitropyrenes,
a gradient mode was used starting with methanol/water (70:30,
v/v), at a flow rate of 1 mL/min, and changing linearly to 100:0 in
absorption bands were observed when changing the irradiation
conditions. In the photochemical studies of van den Braken et al.
[34], limited to methanol as the solvent in the presence or absence
of oxygen,1-NO
less than 3 h of irradiation, while 4-NO
the same conditions and 2-NO Py did not show an evident
2
Py showed a rapid photodegradation (95%) during
15 min and returning to 70:30 after an additional 16 min, for a total
2
Py was more stable under
of 31 min (column: Luna 5
enex).
m
C18(2) reverse phase from Phenom-
2
photodegradation after 7 h of irradiation [34].
The characterization of photoproducts was performed using
HPLC/mass chromatography (Waters HPLC/MS; quadrupole time-
of-flight (Micromass UR limited Q-TOF Micro)). Samples of 4-
The photodegradation quantum yields for the mononitropyr-
2
enes in different solvents were calculated. For 4-NO Py and 2-
ꢀ
4
ꢀ5
ꢀ5
ꢀ4
ꢀ6
NO
2
Py these yields were lower, 10 –10
and 10 –10
,
NO
2
Py/2-propanol/N
2
or 4-NO
2
Py/acetonitrile/N
2
were irradiated
respectively (Table 1), compared to a yield of 10ꢀ3–10 for 1-
for 1 h in a Pyrex container, preconcentrated from 60 to 5 mL, and
injected into the MS using the same HPLC gradient mode described
NO
NO
2
Py. [33] These small photodegradation yields for the 2- and 4-
2
Py isomers reflect that radiationless deactivation routes and
above. For 4-NO
2
Py photoproducts, APCI (Atmospheric Pressure
radical recombination processes inside the solvent cage which
regenerate the ground state [33] are the major channels in the
photochemical processes of these isomers. These channels
predominate even though the isomers have a high yield of their
Chemical Ionization) in the negative and positive ionization modes
were used. The characterization of 4,5-pyrenedione was achieved
using APCI in the positive mode (Supporting information).
Reserpine (m/z = 609.2812) was used as the mass corrector. Also,
the characterization was corroborated with the UV–vis absorption
properties reported by Fatiadi. [48] For 4-hydroxypyrene, the
detection was achieved using APCI in the negative mode and using
sulfadimethoxine (m/z = 309.0658) as the mass corrector. The
characterization of 4-aminopyrene and pyrene was made using
GC/MS and the fragmentation patterns of the samples were
compared with the product standards. For the mass spectra, a
Thermo Finnigan Trace GC/Polaris Q chromatograph with a
capillary column model Restek 12623 (stationary phase RTX-
lowest triplet state (0.33–0.45 for 2-NO
NO Py) and a low phosphorescence yield on the order of 10 [39].
A similar trend was observed for 1-NO Py [38].
The differences in quantum yields between 2- and 4-NO
2
Py, and 0.10–0.26 for 4-
ꢀ
4
2
2
2
Py
demonstrated that their photodecomposition strongly depends on
the position of the nitro group in the pyrene moiety (Table 1) as
observed by van den Braken et al. in methanol [34] and by Yang
et al. in DMSO [35]. For 4-NO
2
Py, the torsion angle between the
ꢃ
nitro group and the aromatic plane is ꢇ30 in the ground state due
2
to the presence of a peri hydrogen [22,23,27] while for 2-NO Py in
5
MS: Crossbond_5% diphenyl/95% dimethyl polysiloxane, nominal
the absence of a peri hydrogen the nitro group lies in the same
plane as the aromatic system [23,27,34]. These results imply that
the photodegradation of this nitro-PAH is more favorable when the
nitro-group is out-of-plane of the aromatic moiety to initiate the
nitro-group rearrangement (Reaction 1, Scheme 1) or the
dissociation-recombination mechanism (Reactions 2 and 3). This
out plane conformation favors the formation of pyrenoxy radical,
PyO. Indeed, we have reported that the relative yields of formation
length = 30 m) was used. The oven conditions were set to: initial
temp = 150 C, final temp = 250 C and ramp = 10 C/min. The
characterization of the 1-NO
Ref. [33].
ꢃ
ꢃ
ꢃ
2
Py photoproducts was described in
3
. Results and discussion
3.1. Steady state photolysis and quantum yields under anaerobic and
2 2
of the PyO radical for 4-NO Py and 1-NO Py are higher than for 2-
aerobic environments
NO Py [39]. Thus, one could correlate the increment in the yield of
2
the pyrenoxy radical with the increase in the photo destruction
yield.
To determine the effect of the orientation and position of the
nitro group in the pyrene ring on the photoreactivity of the
isomers, photodegradation quantum yields were determined.
Our results demonstrated that in polar protic solvents (Table 1)
the photodegradation quantum yield for 4-NO
smaller than that obtained for 1-NO Py [33]. This difference in
yield can be correlated to the formation yields of the PyO radicals
(Scheme 1) and of the T( *) state, the principal intermediates for
2
Py was 30–35 times
3
*
Because the lowest triplet state (
p,
p
) and the pyrenoxy radical
2
intermediate of 1-NO Py were shown [33] to react with the
2
solvent, and to control the magnitude of the photodegradation
yields, the relative probability of different reaction paths and
product nature, similar photochemical studies were performed
p,p
the photodegradation of the nitropyrenes (Schemes 2 and 3). These
schemes are based on the identified products (see below) and
intermediates [38,39]. Moreover, the relative yields of the photo-
products (discussed below, Table 2) provide information on the
relative probability of the different reaction channels included in
the Schemes. We have reported [39] that the relative yield of the
with the 2-NO
-propanol), polar aprotic (acetonitrile) and non-polar (hexane)
solvents and compared to the 1-NO Py results [33] (Table 1).
Irradiation of solutions of 4-NO Py resulted in an initial decrease in
2 2
Py and 4-NO Py isomers in polar protic (methanol,
2
2
2
the absorbance of the three major bands in the UV–vis region as a
function of time (Fig. 2A) indicating its photodegradation in the
PyO radical was two times higher for 1-NO
polar protic solvent (methanol). Also, the yield of the T(
was higher (5 times) for 1-NO Py [38]. Thus, the higher formation
yield of PyO radical and T( *) results in a larger photo-
degradation yield for 1-NO Py in comparison with 4-NO Py.
Moreover, the identified photoproducts demonstrated that the PyO
radical (Scheme 2) and the T( *) (Scheme 3) are the principal
2
Py versus 4-NO
2
Py in
p,p
*) state
different solvents. For 2-NO
observed more noticeably in the long-wavelength band (350–
20 nm region) (Fig. 2B). The appearance of new absorption bands
2
Py, the decrease of the absorbance was
2
p,p
4
2
2
and isosbestic points indicate the formation of photoproducts. An
increase in absorbance at 275 and 460 nm was observed for 4-
p,p
NO
when 4-NO
and where only <10% of this isomer was photodegradaded in order
to determine its photodegradation quantum yield. After
prolonged irradiation, an increase in absorbance between 490–
00 nm was barely noticeable in the case of 2-NO Py. For 4-NO Py,
as well as for 2-NO Py the changes in the absorption spectra as
2
Py. The increase of these absorption bands was not remarkable
precursors of their photodegradation.
2
Py was irradiated using a 405 nm interference filter
a
7
2
2
2
function of irradiation time under anaerobic and aerobic con-
ditions showed a similar pattern in terms that no noticeable new
Scheme 1. Possible excited state precursors to the pyrenoxy radical.

Z.I. García-Berríos et al. / Journal of Photochemistry and Photobiology A: Chemistry 332 (2017) 131–140
137
Scheme 2. Possible reaction channels for the PyO radical.
2
Scheme 3. Possible photochemical channels from T(p,p*) of NO .
Py33
For 4-NO
2
Py, some of the identified principal photoproducts
2
contribution of the T(p,p*) state, the precursor of the PyNO H
were 4,5-pyrenedione, 4-hydroxypyrene (4-OHPy), 4-aminopyr-
ene and pyrene. The observation of these products demonstrated
radical (Reactions 7–9), only observed in methanol and 2-propanol
solutions [38,39]. Indeed, the absence of aminopyrene in the
that the major photodegradation pathways for 4-NO
2
Py were the
2
photolysis of 4-NO Py in hexane is evidence of the triplet not
nitro-nitrite rearrangement reactions (or the dissociation-recom-
bination mechanism), and the hydrogen abstraction reaction by
reacting with this solvent (Reaction 7) despite that this state forms
in yields of the order of 0.26 in cyclohexane [39].
the T(
p
,p
*) state. In the former, the PyO radical formed from the
The photodegradation yield was 6 times smaller for 4-NO
2
Py
S
1
(p
,p
*) or T(n, *) states abstracts an hydrogen atom to form
p
versus 1-NO Py [33] in a polar aprotic solvent (acetonitrile). In this
2
hydroxypyrene (Scheme 2, Reaction 4), or it can react with the NO
radical and oxidizes to form hydroxynitropyrenes in the case of 1-
solvent, the yield of formation of the PyO radical was similar for
both nitropyrenes, in contrast to the results obtained in a polar
protic solvent in which the yield of PyO radical was higher for 1-
NO
2
Py [33]. While, the T(
p
,
p
*) state reacts with hydrogen donors
H radical via an
(
in this case, a protic solvent) to form the PyNO
2
NO
during the photolysis of 4-NO
some insight into the effect of this solvent in the different
photodegradation routes of each nitropyrene. For 4-NO Py, the
principal products were 4-OHPy and 4,5-pyrenedione (Reactions
4–6), and products formed from reactions of the T( *) state such
2
Py [38]. The distribution of the different photoproducts formed
electron transfer within a hydrogen-bonded triplet exciplex
followed by a proton-transfer mechanism [38] (Scheme 3, Reaction
2
Py and 1-NO Py in acetonitrile gives
2
7). This radical in turn, abstracts another hydrogen atom from the
2
solvent to transform into hydroxylamine (Scheme 3, Reaction 8),
which transforms into aminopyrene [33,49,50] (Scheme 3, Reac-
tion 9). The pyrene product was formed through the dissociation of
p,p
as aminopyrene and nitrosopyrene were not observed. This
indicates that the hydrogen abstraction reaction involving the T
2
the NO group from the pyrene system leading to a pyrenyl radical
(
Scheme 1, Reaction 2) which then abstracts a hydrogen atom from
(
p
,
p
*) has a low participation in the photodegradation of 4-NO
in acetonitrile. The hydroxynitropyrene products identified in the
irradiation of 1-NO Py [33] in this solvent were not observed.
2
Py
the solvent (Reaction (10)).
ꢁ
2
Pyꢁ + RH ! R + Py (pyrene)
(10)
Nonetheless, the nature of the products characterized for both
isomers indicates that the nitro-nitrite rearrangement or dissocia-
tion-recombination processes are common to both isomers.
Comparing with a polar protic solvent (methanol), the photo-
degradation yield for 4-NO Py in hexane was reduced by 67%
Table 1). This decrease can be accounted by the lower formation
yield of pyrenoxy radicals in non polar solvents (cyclohexane ꢈ
acetonitrile < CCl < methanol)[38], as well as the absence of a
contribution of an hydrogen atom abstraction reaction (Reaction 7)
by the T( *) state in this type of solvent [39]. The low yield in
PyO radicals can be explained in terms of the short lifetime of their
possible S precursor in hexane. According to Plaza-Medina et al.
23], a destabilization of the S *) state of 4-NO Py was
observed when the polarity of the solvent was changed. The iso-
energetic coincidence that exists between the S state and the
2
(
2
In the case of 2-NO Py in polar protic and aprotic solvents, the
photodegradation quantum yields were the lowest in comparison
with the other nitropyrene isomers. We have reported [39] that for
4
2-NO
than that of 4-NO
CCl . This lack of interaction between the nitro group and the
aromatic moiety results in a reduced yield of the PyO radical, and
thus, in a smaller photodegradation yield for 2-NO Py.
Under aerobic conditions, the photodegradation quantum
yields of 4-NO Py were reduced in all solvents, 33% in acetonitrile,
64–80% in methanol and 2-propanol and 18% in hexane. In general,
this decrease in the photodegradation yields for 4-NO Py was
larger than those obtained for 1-NO Py, except in hexane solutions.
This could be explained in terms of a larger participation of the T
*) state in these polar protic solvents in the photodegradation
of 4-NO Py in comparison with 1-NO Py through Reaction 7 within
2
Py the yield of the PyO radical was about five times smaller
p,
p
2
Py and it was observed only in methanol and
4
1
[
1
(p
,p
2
2
1
2
receiver triplet state in non polar hexane results in an increase in
the intersystem crossing rate which is reflected in a decrease in the
2
S
1
state lifetime [23]. This iso-energetic coincidence is lost in polar
solvents due to an energy separation between these two states
resulting in an increase in the S
lifetime by a factor of ꢇ100 [23].
Thus, if the principal precursor of the PyO radical is the S state or a
charge transfer state with dissociative character which can lead to
geminate radicals [51], then the ultrafast decay of the S state in
2
(p,p
1
1
2
2
the solvent cage before the triplet state is by effectively quenched
by a diffusion controlled reaction with O
9
ꢀ1 ꢀ1
2
(k ffi 10 M
s
) [39].
1
cyclohexane reduces the formation yield of this radical and of other
intermediate species that participate in the photodegradation
paths resulting in a net decrease in the photodestruction yield.
Moreover, the low photodegradation quantum yields for the
nitropyrenes in non polar solvents is also due to the low
3.2. Product identification and relative yields
A very revealing result, in terms of the differences in photo-
reactivity of the nitropyrene isomers, is that for 4-NO Py the
2

138
Z.I. García-Berríos et al. / Journal of Photochemistry and Photobiology A: Chemistry 332 (2017) 131–140
product with the highest relative yield in the different solvents was
a pyrenedione (Table 2), while the yield of the 4-OHPy was much
lower and hydroxynitropyrenes were not detected unlike 1-NO Py
2
where 1-OHPy and hydroxynitropyrenes were the photoproducts
with higher quantum yields of formation [33]. Furthermore, a
significant difference in the photochemistry was the presence of
pyrene (Reaction (10)) and aminopyrene (Reaction 9) formed in the
incomplete degassing process. According to the kinetic behavior of
the photoproducts in these polar protic solvents, the formation
yield of 4,5-pyrendione decreased after a prolonged irradiation,
thus indicating the photodegradation of this dione and the
formation of secondary products. In 2-propanol, the highest
formation yield was for an unidentified product while in
acetonitrile and hexane solutions, 4,5-pyrenedione and the
unidentified product absorbing at 220 nm and 275 nm remain as
the main products, respectively. Another product that was affected
photolysis of 4-NO
solvent. The kinetic curves (data not shown) for the 4-NO
2
Py but not in 1-NO
2
Py in 2-propanol as the
Py
2
photoproducts demonstrated that their concentrations were
increasing with irradiation time at the time when relative yields
were determined. Because the formation of pyrenediones and
hydroxypyrene can be explained in terms of the reactions of the
by the presence of O
2
was 4-aminopyrene which disappeared
3
under these conditions thus demonstrating its formation from a
*
(p
,p
2
) state of 4-NO Py.
For 2-NO Py in 2-propanol, an unidentified product with a
2
pyrenoxy radical, the observed differences between 4-NO
2
Py and
relative yield of 20% was observed under anaerobic conditions. This
photoproduct appeared at shorter retention times in comparison
with the parent compound thus indicating that is more polar, and
presented absorption bands at 205, 263, 286 and 357 nm. The
relative yield of this photoproduct increased to 34.4% when the 2-
1
-NO Py in the type of products and relative yields are explained in
2
terms of differences in the reactivity of the 4-PyO and 1-PyO
radicals. Our results suggested that the 4-PyO radical is less
reactive than the 1-PyO in the H-atom abstraction reaction, thus
ꢀ
4
preferring to transform into a pyrenedione. The low yields (ꢈ10
39] of these radicals, which absorb in the same wavelength region
as the lowest triplet state and the PyNO H radical and decay very
), excluded the possibility of comparing the
)
NO
2 2
Py solution was exposed to O .
[
2
3.3. Reactions with phenols
ꢀ
3 ꢀ1
slowly (k ꢈ 10
s
absolute reactivity of these radicals using transient spectroscopic
methods. The formation of the pyrenedione in the photolysis of 4-
Hydroxy-aromatics such as 1-naphtol and hydroquinone are
present in diesel exhaust and wood smoke (syringol) particles or
can be formed in the atmosphere by the slow oxidation of
components of the aerosol [53,54]. These are known to react with
2
NO Py in methanol was not reported by van den Braken et al. [34]
These authors [34] reported the formation of pyrene in methanol,
not detected in our work in this solvent, but seen in 2-propanol.
3
*
the lowest triplet state (
p,p ) of nitropyrenes with rate constants
7
9
ꢀ1 ꢀ1
The presence of pyrene implies that for the 4-NO
dissociation of the excited state into a NO and a pyrenyl radical
Reaction 2), although not the major channel, is a photodestruction
2
Py isomer the
[38,39] of the order of 10 –10 M
s . Because some of the
2
products from the photodegradation of these nitropyrene isomers
resulted from reactions of the triplet state, the effect of phenols on
the photodegradation yields was studied by incorporating them in
the irradiated solutions at concentrations one order of magnitude
higher than that of the nitropyrenes. In general (Table 3), the
presence of phenols resulted in an increase in the photodestruction
(
2
route not observed for 1-NO Py. This difference can be explained
because the dissociation occurs from different carbons of the
aromatic moiety (carbon 4 vs. carbon 1) which have different
orders of nodes [52]. The pyrenyl radical further reacts by
abstracting a hydrogen atom to form pyrene (Reactions 2 and
yields except for 2-NO
presence of hydroquinone and syringol. Nonetheless these phenols
quenched the lowest triplet state of 2-NO Py with a rate constant
[39]. Moreover, it is interesting to note
2
Py where no effect was observed in the
10). It is possible that the longer lifetime of the excited singlet state
of 4-NO Py in polar solvents [23] allows for the dissociation
2
2
8
ꢀ1 ꢀ1
mechanism which can only be indirectly detected in solvents in
which the pyrenyl radical can abstract an H atom. Van den Braken
et al. [34] also reported traces of photoproducts such as
nitrohydroxy-, nitrodihydroxy- and nitromethoxypyrenes after
of the order of 10 M
s
that although the triplet yields of the isomers are of the order of
0.1–0.6 [39], the net photodestruction yields in the presence of the
ꢀ3
ꢀ4
phenols were of the order of 10 –10 , much lower. The proposed
mechanism for the quenching of the nitropyrene triplet state by
phenols goes through an electron transfer within an H-bonded
exciplex followed by a proton transfer to produce a protonated
2
the photolysis of 4-NO Py in methanol. Analysis of irradiated
samples by HPLC reverse phase showed that the unidentified
photoproducts had retention times that are smaller or larger than
their parent nitropyrene indicating that these correspond to more
or less polar compounds, respectively (Table 2). The majority of
these photoproducts present spectroscopic properties similar to
aromatic compounds with functional groups (nitro and/or
hydroxy) absorbing above 330 nm, thus, it is possible that some
of the unidentified photoproducts, formed in very low yields, could
be nitrohydroxy-, methoxy-, or dihydroxypyrene compounds.
Another significant difference is in the type of final product that
results from a hydrogen atom abstraction reaction of the lowest
2
radical (PyNO H) [33,38,39] (Reaction (11)). If this reaction would
have proceeded to completion, then photodegradation yields of the
order of the triplet yields should have resulted.
3NO
Py + Ar-OH $ triplet exciplex ! PyNO
H + ArO
ꢁ
ꢁ
(11)
2
2
3
The low photodestruction yields mean that once the
NO PyArOH exciplex forms, it deactivates efficiently before the
2
electron-proton transfer reaction is completed (back electron or
ISC processes).
3
*
energy triplet state (
from this reaction (Reaction (8)) transforms via a PyN(OH)
intermediate into 1-nitrosopyrene in the case of 1-NO Py while for
-NO Py it reduces into 4-aminopyrene implying a less stable
nitropyrene intermediate product (Reaction 9).
In the presence of O , the distribution and the yields of the
photoproducts varied for both nitropyrenes in comparison with N
saturated solutions. For 4-NO Py in polar protic solvents, the yields
p
,
p
). The PyNO
2
H radical that results [37,38]
A slightly larger effect on the photodegradation yield was
observed for hydroquinone and 1- naphthol compared to syringol.
These phenols show somewhat larger acid ionization constants
than syringol [55–57] and in the case of hydroquinone the two
phenolic hydrogens ( ꢀꢀ OH) bond dissociation energies (82 and
64 kcal/mol) [58] are lower in comparison with the corresponding
phenolic hydrogen in 1-naphthol (84.0 kcal/mol) [59], which could
favor the hydrogen transfer process within the triplet exciplex.
Another factor that can be considered is the statistical condition of
the nitro group approaching either of the two phenolic hydrogens
in hydroquinone when forming the exciplex compared to only one
OH group in 1-naphthol. Steric effects introduced by the presence
2
2
4
2
2
2
2
of 4,5-pyrenedione and 4-hydroxypyrene decreased by 12 and 8
times in 2-propanol, respectively, while in methanol their yields
increased (2–3 times). The formation of the pyrenediones in N
saturated solutions is possibly due to traces of O in the solution
which could have slowly leaked into the irradiation cell or to an
2
2

Z.I. García-Berríos et al. / Journal of Photochemistry and Photobiology A: Chemistry 332 (2017) 131–140
139
of a methoxy group in the ortho position relative to the OH group in
syringol, as well as its smaller acid dissociation constant could
explain the smaller effect on the photodegradation of this phenol
compared to the others. The methoxy group could impede the
hydrogen transfer by the nitro group hindered by one peri
hydrogen.
arrangement will affect the possibility of forming an oxaziridine
cyclic intermediate postulated in the nitro- nitrite rearrangement
[30,34] or the postulated intramolecular charge transfer state [32].
A significant solvent effect on the phototransformations was
observed when comparing photodestruction yields of 4-NO
2
Py and
2-NO Py with the 1-NO Py isomer in alcohols, acetonitrile, and
2
2
The larger increase in the photodestruction yield of 2-NO
the presence of 1-naphthol in comparison to the other nitropyrene
isomers is noteworthy. This was associated with the observation
2
Py in
hexane. Because the solvent influences the yield of the lowest
triplet state and of the PyO intermediate [39] this effect is reflected
in the photodestruction yields as observed. Moreover, the yield of
these intermediates could be related to the effect of the solvent on
[
1
39] that the fluorescence of 2-NO
-naphthol, a phenomenon not observed for the other isomers.
This behavior was explained in terms of the favorable formation of
a hydrogen bond between 2-NO Py and 1-naphthol but not in the
2
Py is quenched by alcohols and
1
the coupling between the S and the triplet receiver state [23]
which in turn determines the relative contributions of an ISC
process into the receiver state versus a dissociation channel,
possible paths to the formation of the intermediates.
Despite the similar angle of orientation of the nitro group with
2 2
respect to the aromatic ring for the 4-NO Py and 1-NO Py isomers
there are contrasting differences in their photodestruction yields
(Table 1) and on the type of principal products (pyrenediones for 4-
2 2
NO Py versus 1-OHPy in 1-NO Py) in the same solvent. These
2
other isomers due to the presence of a peri hydrogen which hinders
this type of interaction. Of the tested phenols, 1-naphthol presents
the largest acid ionization which will favor both a stronger H bond
formation and the H atom tranfer process in the triplet exciplex.
This H ꢀꢀ bond formation enhances the possibility of the hydrogen
atom abstraction reaction by the 2-NO
2
Py triplet state. The
imperceptible effect of syringol and hydroquinone on the photo-
degradation yield of 2-NO Py, but not in the quenching of the
differences have been interpreted in terms of variations in the
lowest triplet state yields [33,39] and the reactivity of this state
with the solvent in hydrogen abstraction reactions. Still another
interpretation is that even if the 1-PyO and 4-PyO radicals are
formed in equal yields the reactivity of the 1-PyO is greater to
account for these observations. Thus, these isomers despite having
similar ground state torsion angle of 300 and possible initial
excited states dynamics, would have different reactivity of the
lowest triplet state and pyrenoxy radicals.
2
lowest excited triplet state [39], is understood in terms of the
instability of the triplet exciplex formed with these phenols which
showed a larger probability in deactivating by non-radiative
processes (ISC) instead of undergoing the electron transfer process
followed by proton transfer.
3.4. Atmospheric implications
The small photodegradationyields of these nitropyrene isomers
Acknowledgments
in solvents of different polarity, that could mimic the surrounding
environment of these pollutants in the atmospheric aerosol, will
favor long residence times in the environment. Furthermore our
results imply that the contribution of the excited nitropyrenes to
the chemistry of the aerosol is through reactions of the pyrenoxy
and of the lowest triplet state with neighboring molecules with
abstractable hydrogen atoms such as phenols. The distinct large
We would like acknowledge financial support provided by NIH
SCoRE (1SC1ES017352). The content is solely the responsibility of
the authors and does not necessarily represent the official views of
the National Institute of Environmental Health Sciences or the
National Institutes of Health. We also thank Luis Piñero and
Salvador Gavaldá for their helpful assistance with the GC–MS and
HPLC–MS instruments, respectively, and to Dr. Jorge Peón of
Universidad Nacional Autónoma de México and Dr. Carlos Crespo-
Hernández of Case Western Reserve University for their helpful
comments.
2 2
increase in photodegradation for 2-NO Py and 4-NO Py as well as
for 1-NO Py in the presence of phenols suggest that the reaction of
2
the lowest triplet state could be a significant photodegradation
path of these atmospheric contaminants in the aerosol. Recently,
we determined singlet oxygen yields of the same order of
magnitude as their triplet yields [60]. This reactive intermedi-
1
ate, O
2
, can react with other aerosol components resulting in an
Appendix A. Supplementary data
increase of the oxidation of the aerosol and formation of potential
toxic intermediates. If the initially formed excited states
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.
jphotochem.2016.08.018.
Mass spectrum by APCI+ and absorption spectra of 4,5-
pyrenedione. This material is available free of charge via the
Internet at http://www.elsevier.com.
[22,23,44,45] rapidly decay through channels that eventually lead
to formation of a pyrenoxy radical, reactions of this intermediate
could result in the formation of hydroxypyrenes, hydroxynitropyr-
enes, and pyrenediones, some of these already identified in low
yields, in polluted atmospheres [21]. These products can enhance
the toxic properties of the aerosol resulting in a significant risk to
human health.
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