The Photochemical Branching Ratio in 1,6-Dinitropyrene Depends on

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The Photochemical Branching Ratio in 1,6Dinitropyrene Depends on Excitation Energy Matthew M Brister, Luis E Piñero-Santiago, Maria Morel, Rafael Arce, and Carlos E. Crespo-Hernández J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b02549 • Publication Date (Web): 22 Nov 2016 Downloaded from http://pubs.acs.org on November 24, 2016

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The Journal of Physical Chemistry Letters

The Photochemical Branching Ratio in 1,6Dinitropyrene Depends on Excitation Energy Matthew M. Brister,1 Luis E. Piñero-Santiago,2 María Morel,3 Rafael Arce,3,* and Carlos E. Crespo-Hernández1,* 1

Department of Chemistry, Case Western Reserve University, 10900 Euclid Avenue, Cleveland,

Ohio 44106; 2 Department of Chemistry, University of Puerto Rico at Humacao, Humacao Campus, Puerto Rico, 00792; 3 Department of Chemistry, University of Puerto Rico, Río Piedras Campus, San Juan, Puerto Rico, 00931 * E-mail: [email protected] (R.A.); [email protected] (C.E.C.-H.)

ABSTRACT Nitro-polycyclic aromatic hydrocarbons constitute one of the most concerning classes of pollutants. Photochemical degradation is thought to be a primary mode of their natural removal from the environment, but the microscopic mechanism leading to product formation as a function of excitation wavelength is poorly understood. In this letter, it is revealed that excitation of 1,6-dinitropyrene with 425, 415, or 340 nm radiation leads to an increasing amount of radical production through photodissociation at the expense of triplet-state population―the two primary reaction pathways in this class of pollutants. Radical formation requires overcoming an energy barrier in the excited singlet manifold. This activation energy explains the large fraction of the initial singlet-state population that intersystem crosses to a doorway triplet state, instead of leading overwhelmingly to photodissociation. The unforeseen excitation-wavelength dependence of this branching process is expected to regulate the photochemistry of 1,6-dinitropyrene and possibly of other nitro-aromatic pollutants in the environment.

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1,6-Dinitropyrene (1,6-DNP) is a mutagenic and carcinogenic compound,1-7 and a key member of the nitro-polycyclic aromatic pollutants.1-2 Photochemical degradation of this class of ubiquitous pollutants is thought to play a key role in its transformation mechanism. 1,6-DNP is four times more photochemically active than its 1,8-DNP isomer under polychromatic (300 to 500 nm) radiation in acetonitrile.8 Oxidized forms of the chromophore, i.e., hydroxynitropyrenes and pyrenediones, were identified as the major photoproducts, although the presence of molecular oxygen did not affect the photodegradation rate.8 The elementary steps that lead to photoproduct formation are not well understood, particularly with regard to the competition among proposed reaction pathways and predominant intermediate species as a function of excitation wavelength, solvent, and models of particulate matter in the environment.9-10 Recent time-resolved investigations of mono nitropyrenes10-18 and nitronaphthalenes19-28 have shown that ultrafast intersystem crossing occurs through a doorway triplet state (Tn), which leads to the population of the lowest-energy triplet state (T1) in high yield. A stable aryloxy radical has also been characterized,14-15, 20-22, 26 which leads to the formation of several major products.19-21, 29 The excited-state mechanism by which this radical intermediary is formed is uncertain and continues to be a topic of debate.9, 13-15, 19-22, 25-28, 30-31 In this letter, we directly measure the formation of a nitropyrenoxy radical from 1,6-DNP as a function of excitation wavelength. This radical forms in parallel with the population of a Tn state that internally converts to the T1 state. The relative yield of these competitive reaction pathways is shown to vary with excitation energy.


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Figure 1. Steady-state absorption and fluorescence spectra of 1,6-dinitropyrene in acetonitrile at room temperature, and phosphorescence spectrum in 30% ethyl acetate/1-propanol glass at 77 K. The inset shows the structure and standard numbering of 1,6-dinitropyrene. Figure 1 shows the normalized absorption and emission spectra of 1,6-DNP in acetonitrile. The molar absorption coefficient at 411 nm (1.6  104 M-1 cm-1) is nearly twofold larger than that for 1-nitropyrene (1NP) at 402 nm in acetonitrile.14 The magnitude of this coefficient shows that 1,6DNP absorbs sunlight strongly, which facilitates its photochemical transformation in the environment.32-34 The fluorescence spectrum of 1,6-DNP is broad (ca. 400 to 650 nm) with a zero-zero singlet energy of 273 kJ/mol and a Stokes shift of 3248 cm-1. These are similar to those reported for 1NP (275 kJ/mol and 4876 cm-1) in the same solvent,14 suggesting the nitro group at the C6 position does not significantly affect the photophysics compared to 1NP. In addition, 1,6DNP was shown to have a fluorescence yield on the same order of magnitude as that of 1NP (1.3  0.1  10-3).14 The low fluorescence yield hints at the participation of non-radiative relaxation pathways, as have been shown for 1NP13 and other nitro-PAHs.9, 12, 18, 25-28 Phosphorescence emission of 1,6-DNP was also measured at 77K, exhibiting maxima at 663 and 725 nm (Fig. 1), showing that a fraction of the S1-state population must undergo



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intersystem crossing to populate the T1 state. A triplet energy of 180 kJ/mol was determined from the position of the first emission maximum―a similar value to that of 1NP (187 kJ/mol).14 The ca. 96.5 kJ/mol (1 eV) energy gap between the S1 and T1 states is similar to that of 1NP (ca. 86.9 kJ/mol or 0.9 eV), suggesting the existence of triplet states with intermediate energies between the S1 and T1 states, as has been shown for 1NP.13 A lifetime of 15.5 ± 0.2 ms was determined at the maximum of both phosphorescence bands, similar in magnitude to the T1 states of other nitro aromatic compounds exhibiting ππ* character.14, 19-20, 35 Figure 2a shows the S0-optimized structure for 1,6-DNP in acetonitrile. The structure has nitro-group torsion angles of 34.9° and a planar pyrene chromophore, which is essentially identical to the torsion angle predicted for 1NP in acetonitrile at the same level of theory (Fig. S1). Figure 2b shows the calculated vertical excitation energies (VEEs) in acetonitrile (Table S1). The S1 and T1 states have mostly * character, with vertical excitation energies of 2.85 eV (275 kJ/mol) and 1.72 eV (166 kJ/mol) in acetonitrile, respectively. These values are in excellent agreement with the experimental zero-zero singlet energy, the triplet-state energy, and the S1-T1 energy gap reported above. The S1 state of 1,6-DNP was also optimized in acetonitrile and its structure is shown in Figure 2a. An energy of 2.4 eV was calculated for the S1-state minimum, in agreement with the fluorescence emission reported in Fig. 1. The most significant structural change relative to the S0-state structure is a ca. 15° reduction in the torsion angle of both nitro groups (see Fig. 2a).36


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Figure 2. (a) Optimized S0 structure of 1,6-DNP at the B3LYP/IEFPCM/6-311++G(d,p) and optimized S1 structure of 1,6-DNP at the TD-PBE0/IEFPCM/6-311++G(d,p) levels of theory, both in acetonitrile. (b) Singlet and triplet vertical excitation energies relative to the S0-optimized structure and triplet vertical excitation energies relative to the S1-optimized structure of 1,6-DNP calculated in acetonitrile at the TD-PBE0/IEFPCM/6-311++G(d,p) level of theory (see Tables S1 and S2 for details). Only the S1 state energy is shown for the S1-optimized structure.

VEEs were calculated for the six lower-energy triplet states of 1,6-DNP at both the S0- and S1optimized structures in order to estimate the singlet-triplet energy gaps and the density of triplet states in the Franck-Condon and at the S1-state minimum (Table S3). Figure 2b shows that there are three triplet states (T2, T3, and T4) nearly isoenergetic to the vertically-excited S1 state within the accuracy of the calculations (0.2 eV),37-40 whereas these triplet states are slightly higher in 5 ACS Paragon Plus Environment


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energy than the relaxed S1 state minimum. The high density of triplet states near the FranckCondon region of the S1 state hints at an increased probability41 of intersystem crossing to the triplet manifold, which can partially explain the small fluorescence yield, and the phosphorescence emission, shown in Figure 1. A strong vibronic coupling between S1 and Tn states has been reported for 1NP13 and other nitro-aromatics,23-28, 42 with intersystem crossing lifetimes to the triplet manifold as short as hundreds of femtoseconds.25, 28, 42

Figure 3. Transient absorption spectra of 1,6-DNP (150 M) in acetonitrile over the first ca. 3 ns following excitation at 425 (a to c), 415 (d to f), or 340 nm (g to h). Panel (i) shows the nanosecond to microsecond transient absorption spectra of 1,6-DNP (30 M) under N2-saturated conditions in acetonitrile following excitation at 355 nm.


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Figure 3 shows the transient absorption spectra of 1,6-DNP in acetonitrile following excitation at 425, 415, and 340 nm. The 425 nm excitation wavelength was chosen to minimize excess vibrational energy in the S1 state, whereas excitation at 340 nm was selected to increase vibrational energy in the S1 state, while simultaneously allowing the ground-state repopulation dynamics to be observed. Upon 425/415 nm excitation (Fig. 3a,d), a broad absorption band is observed within the cross-correlation of the pump and probe pulses that has a maximum at 545 nm and a shoulder around 675 nm (transient species at 545 nm, hereafter). Negative-amplitude signals are also observed at probe wavelengths shorter than ca. 475/485 nm for excitation at 415/425 nm, respectively. These transient signals are largely due to stimulated Raman emission bands originating from the solvent within the cross-correlation of the pump and probe pulses. As the transient species at 545 nm begins to decay, a species with absorption maximum at 445 nm appears (Fig. 3b,e). Importantly, as the 445 nm absorption band increases, an absorption in the spectral range from 450 to 510 nm subsequently appears, increasing at a slower rate and reaching a maximum at ca. 2.5 ps. Transient absorption between 450 to 510 nm then partially decays over 15 ps (Fig. 3c,f), showing more clearly the band at 445 nm, while the transient species at 545 nm fully decays within ca. 30 ps, and a long-lived transient species simultaneously rises in with maxima at ca. 495, 538, and 573 nm (transient species at 573 nm, hereafter). Figure 3g,h shows the transient spectra following excitation at 340 nm. Initially, a band with absorption maximum at ca. 550 nm increases within the cross-correlation of the pump and probe pulses, which is closely followed by the appearance of absorption in the probe region between ca. 445 to 510 nm. Ground-state depopulation and stimulated Raman emission bands from the solvent are also observed at probe wavelengths below 425 nm. As the transient species at 550 nm begins to decay, absorption in the spectral range from ca. 450 to 510 nm continues to increase up 7 ACS Paragon Plus Environment


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to ca. 1 ps. After 1 ps, transient absorption below ca. 550 nm begins to decay and long-lived absorption bands with maxima at ca. 445, 495, 538, and 573 nm are observed, similar to those in Figure 3c,f for excitation at 425 and 415 nm.

Figure 4. Representative kinetic traces (left) and decay associated spectra (right) for 1,6-DNP in acetonitrile at the excitation wavelengths of 425, 415, and 340 nm, respectively. The decay associated spectra were obtained from a target and global fit analysis of the transient data at the selected excitation wavelengths. The black line represents a combination of the excited-state absorption of the S1 excited state and ground-state depopulation (GSD). The red line represents a combination of nitropyrenoxy radical (NO2PyO) and residual S1 excited state. The green line represents the combination of the doorway triplet state (Tn) and nitropyrenoxy radical (NO2PyO). The blue line represents the combination of the T1 state and nitropyrenoxy radical (NO2PyO).


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Figure 4 shows representative kinetic traces and decay associated spectra obtained from a target and global-fit analysis of the transient data at each excitation wavelength, whereas Table 1 collects the globally-fitted lifetimes. Each set of lifetimes can be interchanged with those obtained at any other excitation wavelength without affecting the quality of the fit significantly, within the experimental uncertainties. Hence, for simplicity, generic lifetimes of 0.3, 2, and 10 ps will be used for discussion hereafter. This suggests that the dynamics of 1,6-DNP are not excitation-wavelength dependent within the excitation energy range used. Nonetheless, it is clear from Figures 3 and 4 that the relative intensities of the different transient species change with increasing excitation energy. Specifically, the relative amplitude of the 445 nm transient species increases with increasing excitation energy, whereas the relative amplitude of the two transient species at 545 and 573 nm decrease (Fig. 4). Unfortunately, although the observed trends with excitation wavelength of these transient signals are reproducible, we cannot strictly quantify them because an equal photon density at each excitation wavelength cannot be guaranteed.

Table 1. Lifetimes obtained from a target and global fit analysis of the broadband transient data of1,6-DNP in acetonitrile at three different excitation wavelengths. Excitation Energy 1 / ps 2 / ps 3 / ps 425 nm 0.27  0.01 1.94  0.04 10.0  0.2 415 nm 0.27  0.01 1.52  0.03 11.0  0.2 340 nm 1.7 0.27  0.01a 9.5  0.5b 0.27 (0.3) 1.7 (2) 10 Average  a Uncertainties equal at least two standard deviations obtained from the global fit analysis at each excitation wavelength; b this value was obtained by holding the second lifetime in the global analysis to 1.7 ps.

Nanosecond laser flash photolysis experiments were performed in order to follow the decay of the long-lived transient species. Figure 3i shows that the transient spectrum right after



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the nanosecond pulse is essentially identical to the transient spectrum reported in Figure 3c,f,h at the longest time delay. Hence, the positive-amplitude signals are due to the same long-lived transient species observed in the femtosecond experiments, whereas the negative-amplitude signal is due to ground-state depopulation. Figure 3i further shows that the transient species at 573 nm decays much faster than the transient species at 445 nm (see also Fig. S2a), thus demonstrating that these absorption bands are due to two different transient species that decay on different microsecond time scales. According to the calculations (Fig. 2), excitation of 1,6-DNP at 425 and 415 nm populates the S1(*) state directly, but with an increasing amount of excess vibrational energy. Excitation at 340 nm (ca. 3.6 eV) is predicted to populate the S5 and S6 states simultaneously. However, only the oscillator strengths of the S3 and S1 states are large enough to receive significant population (Table S1). Population in the S3 state is expected to internally convert on ultrafast time scale to populate the vibrationally-excited S1 state.43 Therefore, most of the excited population should reside in the S1 state within the instrument response of our setup, independently of the excitation wavelength used, but with unequal amounts of vibrational energy. Hence, we assign the transient species at 545/550 nm to the S1-state absorption and associate the 0.3 ps lifetime with its vibrational relaxation, whereas the 2 ps lifetime is associated with the population of the transient species at 445 nm. Importantly, the transient species at 445 nm is populated before the S1 state population completely decays and before the transient species at 573 nm is populated, as shown in Figure 3. In fact, a residual fraction of the S1-state decays to populate a transient species in the spectral range from ca. 450 to 510 nm, which then decays to populate the transient species at 573 nm with a lifetime of 10 ps. As mentioned above, the transient absorption species probed at 445 and 573 nm in Figure 3c,f,i are long-lived and decay on different microsecond time scales.


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Nanosecond laser experiments under N2- and O2-saturated conditions were performed in order to characterize the long-lived transient species observed at 445 and 573 nm. Figure S2a shows that the transient species at 573 nm decays within 1 s under N2-saturated conditions, whereas the transient species at 445 nm persists for more than 7 s. The decay rate constant of the transient species at 573 nm increases when O2 is added to the solution (Fig. S2b), whereas the decay rate of the transient species at 445 nm does not change (Fig. S2c). Quenching rate constants of 4.7  108 M-1 s-1 and 1.6  109 M-1 s-1 were determined for the transient species at 573 nm when either O2 or ferrocene (the latter not shown) was added to the solution, respectively. Hence, the transient species at 573 nm is assigned to the T1 state, since both O2 and ferrocene act as efficient triplet quenchers.14-15 This assignment suggests that the transient species absorbing in the spectral region from ca. 450 to 510 nm during ca. 1-30 ps time delay in Fig. 3 should be assigned to a doorway Tn state (T4, T3, and/or T2 according to the VEEs in Fig. 2). A triplet quantum yield of (87  15)% (573 nm = 11,200 M-1cm-1) was determined at 355 nm, with an intrinsic triplet decay lifetime of 83 s, and a triplet-self-quenching rate constant of 7.8  0.5  1010 M-1s-1 in acetonitrile under N2-saturated conditions (Figure S3). The high triplet yield demonstrates that intersystem crossing to the triplet state is one of the principal relaxation pathways. Similar high triplet yields have been reported for other nitro-PAHs.11, 14-15, 27, 28 Attention is now shifted to the transient species at 445 nm. As shown in Figures 3i and S3, this transient species persists after the triplet state signal has decayed, regardless of whether N2, O2, or ferrocene are added to the solution. A similar transient species was observed in several nitropyrene isomers in polar and non-polar solvents,15 which was assigned to a pyrenoxy radical (PyO●). Likewise, we assign the transient species at 445 nm to a nitropyrenoxy radical



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(NO2PyO●). This radical has been shown to decay slowly to form 6-hydroxy-1-nitropyrene, 1,6pyrenedione, and dinitrohydroxypyrene,10 as observed in several nitropyrene isomers.29 Taken together, the experimental and computational results lend strong support to the mechanism shown in Scheme 1. Excitation of 1,6-DNP with 425 to 340 nm radiation populates the S1 state with increasing amounts of vibrational energy. As shown in Fig. 3, the S1-state population bifurcates to form a nitropyrenoxy radical (transient at 445 nm) or to populate the T1 state (transient at 573 nm) through a doorway Tn state (transient absorbance between ca. 450 to 510 nm). The absorption spectrum of this radical and its formation rate (ca. 5  1011 s-1) are reported herein for the first time. The central finding is that the relative yield of this radical increases with an increase in excitation energy, whereas that of the T1 decreases (as can be judged from the relative amplitudes of the transient absorption in Figs. 3c,f,h and 4).This suggests that an energy barrier needs to be overcome for the photodissociation process to ensue from the singlet manifold. This energy barrier explains the large fraction of the singlet population that intersystem crosses to the triplet manifold instead of overwhelmingly leading to radical formation. Nanosecond experiments exciting at 355 nm (Fig. 3i) demonstrate that this radical species participates in the formation of the primary photoproducts of 1,6-DNP.8,

10

Importantly, the unforeseen excitation-wavelength dependence of both primary reaction pathways (i.e., photodissociation versus intersystem crossing) is expected to regulate the photochemistry of 1,6-DNP and potentially of other nitro-polycyclic aromatic pollutants in the environment.


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Scheme 1. Proposed branching relaxation mechanism for 1,6-DNP leading to the formation of a nitropyrenoxy radical (NO2PyO●) or to intersystem crossing to the triplet (T1) state on an ultrafast time scale in acetonitrile.

Acknowledgements M.M.B. and C.E.C.-H. acknowledge the partial support from the donors of the American Chemical Society Petroleum Research Fund and from the NSF CAREER Program (Grant CHE1255084). This work made use of the High Performance Computing Resource in the Core Facility for Advanced Research Computing at Case Western Reserve University. R.A., M.M., and L.E.P-S., acknowledge support by NIH SCoRE (SC1ES017352) from the National Institute of Environmental Health Sciences and the Dept. of Chemistry, UPR- Río Piedras. 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. Supporting Information. Experimental and computational methodologies; vertical excitation energies and oscillator strengths for 1,6-DNP; optimized ground state structures of 1,6-DNP; nanosecond



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transient absorption kinetic decay curves of 1,6-DNP in acetonitrile under O2 and N2-saturated conditions; triplet self-quenching for 1,6-DNP. References (1) Yu, H. Environmental Carcinogenic Polycyclic Aromatic Hydrocarbons: Photochemistry and Phototoxicity. J. Environ. Sci. Health C 2002, C20, 149-183. (2) Selected Nitro- and Nitro-Oxy-Polycyclic Aromatic Hydrocarbons. Environmental Health Criteria Monographs; WHO: Geneva, 2003. (3) Hayakawa, K.; Murahashi, T.; Butoh, M.; Miyazaki, M. Determination of 1,3-, 1,6- and 1,8-Dinitropyrenes and 1-Nitropyrene in urban air by High-Performance Liquid Chromatography using chemiluminescence detection. Environ. Sci. Technol. 1995, 29, 928-932. (4) Murahashi, T.; Miyazaki, M.; Kakizawa, R.; Yamagishi, Y.; Kitamura, M.; Hayakawa, K. Diurnal Concentrations of 1,3-, 1,6-, 1,8-Dinitropyrenes, 1-Nitropyrene and Benzo [a] Pyrene in air in downtown Kanazawa and the Contribution of Diesel-Engine Vehicles. Jpn. J. Toxicol. Environ. Health 1995, 41, 328-333. (5) Kuo, C.-T.; Chen, H.-W. Determination of 1,3-, 1,6-, 1,8-Dinitropyrene and 1Nitropyrene in Airborne Particulate by Column Liquid Chromatography with Electrochemical Detection. J. Chromatogr. A 2000, 897, 393-397. (6) Hayakawa, K.; Butoh, M.; Hirabayashi, Y.; Motoichi, M. Determination of 1,3-, 1,6-, 1,8-Dinitropyrenes and 1-Nitropyrene in Vehicle Exhaust Particulates. Jpn. J. Toxicol. Environ. Health 1994, 40, 20-25. (7) Lübcke-von Varel, U.; Bataineh, M.; Lohrmann, S.; Loffler, I.; Schulze, T.; FlückigerIsler, S.; Neca, J.; Machala, M.; Brack, W. Identification and Quantitative Confirmation of Dinitropyrenes and 3-Nitrobenzanthrone as Major Mutagens in Contaminated Sediments. Environ. Int. 2012, 44, 31-39. (8) Morel, M.; Alers, I.; Arce, R. Photochemical Degradation of 1,6- and 1,8-dinitropyrene in Solution. Polycycl. Arom. Compd. 2006, 26, 207-219. (9) Crespo-Hernández, C. E.; Vogt, R. A.; Sealey, B. On the Primary Reaction Pathways in the Photochemistry of Nitro-Polycyclic Aromatic Hydrocarbons. Mod. Chem. Appl. 2013, 1, 106. (10) Arce, R.; Morel, M. Phototransformations of Dinitropyrene Isomers on Models of the Atmospheric Particulate Matter. Atmos. Environ. 2013, 75, 171-178. (11) Scheerer, R.; Henglein, A. The Triplet State of 3-Nitropyrene. Ber. Bunsen-Ges. 1977, 81, 1234-1239. (12) Morales-Cueto, R.; Esquivelzeta-Rabell, M.; Saucedo-Zugazagoitia, J.; Peon, J. Singlet Excited-state Dynamics of Nitropolycyclic Aromatic Hydrocarbons: Direct Measurements by Femtosecond Fluorescence Up-Conversion. J. Phys. Chem. A 2007, 111, 552-557. (13) Crespo-Hernández, C. E.; Burdzinski, G.; Arce, R. Environmental Photochemistry of Nitro-PAHs: Direct Observation of Ultrafast Intersystem Crossing in 1-Nitropyrene. J. Phys. Chem. A 2008, 112, 6313-6319. (14) Arce, R.; Pino, E. F.; Valle, C.; Agreda, J. Photophysics and Photochemistry of 1Nitropyrene. J. Phys. Chem. A 2008, 112, 10294-10304.


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