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A Comparative Photophysical and Photochemical Study of Nitropyrene Isomers Occurring in the Environment Rafael Arce,* Eduardo F. Pino,† Carlos Valle, Ideliz Negro´n-Encarnacio´n, and Marı´a Morel Department of Chemistry, UniVersity of Puerto Rico, Rio Piedras Campus, San Juan, Puerto Rico, 00931-3346 ReceiVed: September 10, 2010; ReVised Manuscript ReceiVed: NoVember 12, 2010
Ground state absorption, first excited-singlet state, and properties of reactive intermediates of mononitropyrene isomers encountered in the atmospheric aerosol have been studied under different conditions that could mimic the environment. The nitro group can present different orientations relative to the pyrene ring depending on its geometric location and could induce differences in the photochemistry of the isomers. The 2-NO2Py isomer has the largest red shift and lowest oscillator strength in the UV-visible band associated with the nitro group. The isomers show very low fluorescence yields (10-3-10-4). Only 1-NO2Py and 4-NO2Py have phosphorescence emission (Φp ≈ 10-4), indicating that the lowest triplet state decays mainly through effective radiationless channels. Laser photolysis produces a low-lying triplet state (τT ) 10-5-10-6s), a long-lived pyrenoxy radical, and a PyNO2H radical in solvents in which the triplet can abstract a hydrogen atom. Similar triplet yields were calculated (0.1-0.6) for the isomers, while significant differences in the relative yield of the long-lived species were determined. Differences in the quenching rate constants of the triplet by water and phenols suggest a strong hydrogen-bond interaction with the nitro group in the C-2 position, which provides for radiationless deactivation routes. Introduction Most nitropolycyclic aromatic hydrocarbons (nitro-PAHs), a class of genotoxic environmental pollutants, are formed by incomplete combustion processes and emitted from diesel engine vehicles, wood stoves, fire places, and kerosene heaters.1,2 These can be also encountered in urban air particles, coal fly ash, and grilled foods. Nitropyrenes exist almost entirely in the particle phase under ambient conditions.3 In air and diesel particulate, the three nitropyrene isomers (1-NO2Py, 1-nitropyrene; 2-NO2Py, 2-nitropyrene; and 4-NO2Py, 4-nitropyrene; Scheme 1) are found at levels of ng/g, the 1-NO2Py level (155 ng/g) being at least 3-10 times higher than for 2-NO2Py and 4-NO2Py.4 Of these isomers, 2-NO2Py can be formed through heterogeneous or homogeneous reactions of the parent pyrene with OH radicals and nitrogen oxides under photochemical conditions and detected in ambient air and not in direct combustion emissions.5 The reactions and fate of the four ring nitroaromatics continue to be a topic of high interest due to their mutagenic and possible carcinogenic properties. The nitro group in these nitro-PAHs can have different orientations relative to the arene moiety depending upon its geometric location. The orientation can be described as parallel, if the torsion or dihedral angle, C-C-N-O, is coplanar with the aromatic ring or perpendicular when the group is in an outof-plane rearrangement. Intermediate angles are possible. For the three nitropyrenes, PM3 and density functional calculations have predicted that in 2-NO2Py the nitro group is coplanar with the rings,6,7 while for 1-NO2Py angles of 55°,6 23.15°,7 and 27.5° 8 have been reported. Torsion angles of 12.7° 6 and 26° 7 were reported for 4-NO2Py. More recently, using density functional level theory, Reichardt et al.9 have demonstrated that at room temperature a distribution of nitroaromatic torsion angles
is available for nitronaphthalenes. This orientation of the nitro group has been correlated with direct acting mutagenicity and tumorigenicity10,11 and the photochemical reactivity of these molecules.12,13 In the development of these correlations, different structural, spectroscopic, and electronic properties associated with the orientation of the nitro group, such as half-wave potentials,14 calculated energy of the lowest unoccupied molecular orbital,15 MS,6 high resolution proton NMR data,15,16 and Raman depolarization spectral data,17 have been used to predict the biological activity of nitro-PAHs. A general observation is that nitro-PAHs, with a nitro group planar to the arene moiety, are more mutagenic than their isomers, although in the case of the nitropyrenes this is not sustained.7 The photochemical reactivity of the nitropyrene isomers has been reported in methanol solutions.13 The 1-NO2Py showed a rapid conversion, 4-NO2Py was 3 times more stable than 1-NO2Py in terms of the irradiation time, while 2-NO2Py did not show a clear reaction. Some products were identified,13 and from those it was proposed that 1-NO2Py undergoes the Chapman nitro-nitrite rearrangement,18 typical of nitroarenes, leading to 1-hydroxypyrene in 88% and 1-hydroxy-2-nitropyrene in a 7% yield. For 4-NO2Py, pyrene was the major photoproduct observed.13 More recently, Warner et al.12 demonstrated that some nitro-PAHs showed an association between the torsion angle and photoreactivity in solution, while when adsorbed on a surface, there was no correlation. Some photophysical and SCHEME 1: Nitropyrene Isomers
* To whom correspondence should be addressed. E-mail:
[email protected]. † Present address: Facultad de Quı´mica y Biologı´a, Universidad de Santiago de Chile, Chile.
10.1021/jp108652p 2011 American Chemical Society Published on Web 12/16/2010
Comparison of Nitropyrene Isomers photochemical properties of 1-NO2Py were reported by us.19 Low-temperature phosphorescence and room temperature transient absorption spectra showed the presence of a low-energy 3 (π,π*) state decaying with lifetime in the 10-5-10-4 s range depending on the solvent. Reactions of this triplet state with known quenchers and with substances with hydrogen-donor abilities were also informed. A long-lived transient absorption with maximum at 420 nm was assigned to the pyrenoxy radical. In this work, we have studied the ground, excited singlet, triplet states, and the reactive intermediates and their reactions for the three isomers of nitropyrene in order to understand their photochemical differences and to determine if these can be associated with the nitro group orientation. In addition, because the intermediates formed in the photochemical processes can transform into intermediates that participate in the nitro reduction or ring oxidation reactions, produced by the metabolic activation of nitro-PAHs into genotoxic agents,11 it was of interest to compare the reactivity and yield of these intermediates. Experimental Section Reactants and Solvents. Ferrocene (laboratory grade), perylene (99+%), anthracene, benzophenone (99+%), 1-hydroxynaphthalene (1-naphthol) (99%), 2,6-dimethoxyphenol (syringol) (99%), 4-hydroxy-3-methoxybenzoic acid (vanillic acid) (99%) were from Sigma-Aldrich Chemical Co. and used as received. 1-Nitropyrene (99%) (Aldrich Chemical Co.), 2-nitropyrene (99.2%) (Chiron AS), and 4-nitropyrene (99.5%) (Chiron AS) were purified. 1-Nitropyrene was recrystallized three times from methanol,19 while 2-nitropyrene and 4-nitropyrene, due to the small quantities available and high cost, were purified by dissolving them in acetonitrile and passing the solution through a short activated silica gel column contained in a Pasteur pipet. Their purity was assessed from the emission-excitation and UV-visible absorption spectra. The acetonitrile was evaporated with a nitrogen flow, and the bright yellow solids were then used to prepare solutions in other solvents. Hexane (99%), cyclohexane (99%), 3-methylpentane (99+%), 2-propanol (99.5%), n-propanol (99%), and methylcyclohexane (99+%) were from Aldrich Chemical Co. Methanol and acetonitrile were obtained from Fisher Scientific. The purity of the solvents was verified by HPLC. Nonpolar solvents were purified by passing these through an activated silica gel column, and the purification was corroborated by recording the fluorescence and UV-visible spectra of the fractions collected from the column. Other solvents were used as received. Methods. Absorption spectra of the isomers in different solvents were recorded with a Varian Cary IE UV-visible spectrophotometer. Solutions were purged with N2 for 30 min before the spectrum was recorded. Molar absorption coefficients, oscillator strengths, and bandwidths at half-peak height were calculated as previously described.19 Fluorescence spectra were recorded with a Varian Cary Eclipse fluorescence spectrophotometer. Plots of the area under the emission spectrum as a function of 1 - 10-A, where A is the absorbance at the excitation wavelength, were prepared to determine quantum yields. For these, fluoranthene in cyclohexane (Φf ) 0.35)20 was used as the standard for 1-nitropyrene and biacetyl in cyclohexane (0.27 × 10-3)21 for the other isomers, to facilitate the matching of the absorbance of both solutions at the excitation wavelength. Phosphorescence spectra of 1-nitropyrene and 4-nitropyrene in 3-methylpentane, n-propanol, ethanol, and methylcyclohexane glasses were recorded in a Varian Cary Eclipse spectrophotometer in the phosphorescence mode. Phosphorescence yields
J. Phys. Chem. A, Vol. 115, No. 2, 2011 153 were calculated from plots of the areas under the emission spectra as a function of 1 - 10-Av, where Av is an average absorbance of the solution in a 4 mm i.d. diameter quartz tube. Biacetyl in n-propanol (Φp ) 0.23)22 was used as a standard to calculate the relative quantum yield. The transient absorption spectrokinetic system has been described elsewhere.19,23 It consists of a Continuum Surelite ll Nd:YAG Q switched laser delivering up to 70 mJ at 266 nm and 150 mJ at 355 nm. Time-resolved absorption measurements were made perpendicular to the excitation beam by using a conventional Xe-300 W arc lamp (Oriel Co.), a monochromator, and an R 928 five-stage wired photomultiplier. The output signal was recorded on a digital oscilloscope (Le Croy 9360, 600 MHz, 5 Gs/s) and transferred to a personal computer. Signals were averaged to improve the signal-to-noise ratio. The N2 degassed solutions were changed after 300 laser shots to avoid for the excitation of photoproducts and photodegradation of the sample. The energy of the laser was measured with a Newport 1835 high power meter or using benzophenone as an actinometer. The molar absorption coefficients of the triplet-triplet absorption of the isomers were measured by the energy transfer method.24 Perylene was used as a triplet energy acceptor in the cases of 1-NO2Py and 4-NO2Py, while anthracene was used for the determination of the molar absorption coefficient of 2-NO2Py. Briefly, a solution of the nitropyrene isomer in the specific solvent was excited in the absence and in the presence of the energy acceptors. Kinetic corrections were applied for less than 100% energy transfer and decay of the triplets.25 Using the molar absorption coefficient of the triplet state of the isomers and benzophenone as an actinometer, triplet quantum yields were obtained from power dependence plots of the triplet absorption signals for the isomers and of the benzophenone at the wavelength, where the molar absorption coefficients are known, and using optically matched solutions at the excitation wavelengths.26 In quenching experiments using phenols, the concentration of the isomer was in the range of 10-5 M, while that of the cosolute was in the range of 10-3-10-4 M. Results and Discussion Ground State Properties. The UV-visible spectra of the three nitropyrene isomers show three major bands typical of other nitro-PAHs (Figure 1). The long-wavelength band observed in the region of 345-475 nm, depending on the isomer and solvent and extending the absorption of these molecules into the visible, thus increasing the possibility of photochemical transformation in the environment, is typical of nitropyrenes and has contributions from π,π* transitions from the NO2 group and the pyrene’s ring system.8,13,27 The intensity of this band decreases in the order 1-NO2Py > 4-NO2Py . 2-NO2Py and the oscillator strengths associated with this band are 0.2-0.3, 0.09-0.15, and 0.01-0.07, respectively (Table 1). Vibronic structure on this band is more clearly observed in nonpolar solvents and it is more clearly seen in the 1-NO2Py band. Larger bandwidths and red shifts (5-10 nm) were observed in polar solvents. For comparison, the absorption spectrum of pyrene, the parent PAH compound, is included in Figure 1. It has four absorption bands in the region from 26 000 (384.6) to 47 000 cm-1 (212.8 nm). On the basis of theoretical calculations, absorption, and fluorescence spectral data, the first band (very weak, not seen in Figure 1) has been attributed to a short-axis polarized transition, the second (∼30 000 cm-1, 333.3 nm) to a long-axis polarized transition, the third (37 500 cm-1, 266.7 nm) to a short-
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Figure 2. Normalized fluorescence spectra of (1) 1-NO2Py, (2) 2-NO2Py, and (3) 4-NO2Py in acetonitrile.
Figure 1. Absorption spectra of 1-NO2Py (red), 2-NO2Py (green), 4-NO2Py (blue), and pyrene (black) at room temperature in (A) hexane and (B) acetonitrile.
axis polarized transition, while the fourth has been associated with a transition dipole moment polarized to a long axis.28 In general, the substitution of an electron-withdrawing nitro group is known to have a particularly strong stabilizing resonance effect, inducing a red shift in the pyrene bands,29 as observed in Figure 1. Furthermore, a significant loss of the vibronic structure in each pyrene band is also induced. Substitution in the C-2 position with a nitro group, which would affect the longaxis transition moment of pyrene, shows the largest bathochro-
mic shift and effect on the intensity of the third band of pyrene (37 500 cm-1). This substitution also induces a significant reduction in the intensity of the first and third bands of the nitroPAH. Other C-2-substituted pyrene derivatives with electrondonating or -withdrawing groups show similar effects.29,30 These observations have been explained in terms of a poor interaction between the substituent at the C-2 position and the pyrene’s orbitals, which results in a UV-visible absorption spectrum that closely resembles the parent’s spectrum.13 Substitution at the C-1 or C-4 positions should perturb the short-axis polarized transition to a larger extend than the long-axis. In this case, minor effects are seen on the fourth band of pyrene, while the third band decreases in intensity and is red-shifted in both 1-NO2Py and 4-NO2Py relative to pyrene. Singlet Excited State. The three isomers show a broad, very low intensity band with a maximum in the 425-585 nm wavelength region depending on the solvent (Figure 2). A singlet state energy, ranging from 250 to 290 kJ/mol, was calculated from the intersection of the excitation and emission spectra. In general, the 1-NO2Py fluorescence state is stabilized to a greater extend than for the other two isomers in polar solvents and the energy of this state is approximately 15-20 kJ/mol higher. The largest Stokes shifts were observed for 4-NO2Py followed by 2-NO2Py and 1-NO2Py (Table 2). These shifts increased with solvent polarity, suggesting a large dipole moment in the excited state and its π,π* character. Indeed, plots of the Stokes shifts
TABLE 1: Ground State Properties Associated with the Long-Wavelength Band of Nitropyrenes, at Room Temperature nitropyrene 1-NO2Py 2-NO2Py 4-NO2Py 1-NO2Py 2-NO2Py 4-NO2Py 1-NO2Py 2-NO2Py 4-NO2Py 1-NO2Py 2-NO2Py 4-NO2Py 1-NO2Py 2-NO2Py 4-NO2Py
solvent hexane cyclohexane 2-propanol methanol acetonitrile
λmax/ nm
εmax/M-1 cm-1
fnfm
Γ/ cm-1
347s, 370 389, 405 378, 399, 421 347, 370, 389s, 405 347s, 371 391, 407 381, 401,424 347, 371, 391s, 407 375, 397 410 375, 397 375, 399 410 375, 399 377, 402 415 377, 402
8986s/13 226/11 565/9692 1928/2324/2135 6433/6375/4478s/2925 8207s/12 630/11 392/9348 857/1231/1045 8715/8829/6494s/4509 11 494/11 572 1010 5656/4663 10 511/10 698 1155 7474/6143 10 276/10 834 807 5556/4632
0.322 0.0608 0.107 0.240 0.0214 0.150 0.227 0.0249 0.089 0.265 0.0249 0.132 0.249 0.0167 0.107
4833 4500 5613 4833 4500 5581 4750 5000 5670 4750 5000 6390 4750 7500 6390
Comparison of Nitropyrene Isomers
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TABLE 2: Comparison of 1-NO2Py, 2-NO2Py, and 4-NO2Py First Singlet Excited State Parameters in Solution nitropyrene 1-NO2Py 2-NO2Py 4-NO2Py 1-NO2Py 2-NO2Py 4-NO2Py 1-NO2Py 2-NO2Py 4-NO2Py
solvent cyclohexane
2-propanol
CH3CN
Stoke λmax/nm shifts/cm-1 E0/kJ mol-1 Φfluo/10-3 425 465 550 500 585 570
1041 3681 8845 4876 7000 7456
291 277 271 275 254 258
