Photochemical Relaxation Pathways in Dinitropyrene Isomer

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Photochemical Relaxation Pathways in Dinitropyrene Isomer Pollutants Matthew M Brister, Luis E Piñero-Santiago, Maria Morel, Rafael Arce, and Carlos E. Crespo-Hernández J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b04769 • Publication Date (Web): 06 Oct 2017 Downloaded from http://pubs.acs.org on October 7, 2017

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The Journal of Physical Chemistry

Photochemical Relaxation Pathways in Dinitropyrene Isomer Pollutants Matthew M. Brister,1 Luis E. Piñero-Santiago,2,3 María Morel,2 Rafael Arce,2,* 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, Río Piedras Campus, San Juan, Puerto Rico, 00931; 3 Present address: Department of Chemistry, University of Puerto Rico at Humacao, Humacao Campus, Puerto Rico, 00792 * E-mail: [email protected] (R.A.); [email protected] (C.E.C.-H.)

ABSTRACT Dinitropyrenes are polycyclic aromatic pollutants prevalent in the environment. While their transformations by sunlight in the environment have been documented, the effect that the nitrogroup substitution pattern has on the relaxation pathways has not been extensively investigated. In this contribution, the steady-state and femtosecond-to-microsecond excited-state dynamics of 1,3-dinitropyrene and 1,8-dinitropyrene isomers are investigated upon visible light excitation at 425 nm, and compare with those recently reported for the 1,6-dinitropyrene isomer. The experimental results are complemented with ground- and excited-state density functional calculations. It is shown that excitation at 425 nm results in the ultrafast branching of the excited-state population in the S1 state to populate the triplet state in ca. 90% yield and to form a nitropyrenoxy radical in less than 10% yield. In addition, the position of the NO2 group does not affect significantly the excited-state relaxation mechanism, while it does influence the absorption and fluorescence spectra; the fluorescence, triplet, singlet oxygen, and photodegradation yields; as well as the relative yield of radical formation. Radical formation is implicated in the photodegradation of these pollutants, while in the presence of hydrogen donors, direct reactions from the triplet state are also observed.

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1. INTRODUCTION 1,3-Dinitropyrene (1,3-DNP), 1,6-dinitropyrene (1,6-DNP), and 1,8-dinitropyrene (1,8-DNP) (Scheme 1) are mutagenic and carcinogenic pollutants widely found in the environment.1-7 They have been detected in diesel exhaust and airborne particles,3-5 as well as in precipitation and soil.6-7 The transformations of these ubiquitous pollutants in the environment are still a subject of great interest because their fate and degradation mechanisms are not completely understood.

Scheme 1. Structure and standard numbering of the dinitropyrene isomers.

Since these compounds reach the atmosphere, a possible pathway for their transformation is through a photochemical mechanism. However, limited photochemical studies of these isomers have been performed.8-10 Holloway and co-workers8 studied the phototransformation of 1,8-DNP in dimethylsulfoxide or coated on silica. One major product was identified as 1-nitropyrene-8-ol. Reports from our laboratory demonstrated that structural differences (1,6- versus 1,8-DNP) influence their photochemical behavior because 1,6-DNP has at least a fourfold faster photodegradation rate than 1,8-DNP in acetonitrile under continuous photolysis conditions in the presence of N2 or O2.9 Hydroxynitropyrenes, pyrenediones, and dinitrohydroxypyrene were 2


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reported as the major products.9 We have also demonstrated that the phototransformation routes of these isomers depend on the microenvironment in which these are encountered. Not only the chemical properties of the surface (i.e., silica versus acidic alumina), but also the average pore diameter, or whether O2 is present, resulted in significant changes in the type of product, their distribution, and degradation rates. Significant differences were also observed when their photochemical behavior in the surfaces was compared to that in solution.10 Studies on the characterization of the participating excited states and reactive intermediate species in the photochemistry of nitropyrenes are mostly limited to the mononitropyrenes.10-18 The excited-state dynamics and primary photochemical processes of nitronaphthalene19-27 and nitroanthracene28 derivatives have also received prominent attention, whereas investigations focusing on the excited-state dynamics of dinitropyrenes have received less scrutiny.29 For the monosubstituted pollutants, the investigations have shown that ultrafast intersystem crossing occurs through a receiver triplet state (Tn) that then populates the lowest-energy triplet state (T1) in high yield. The formation of an aryloxy radical has also been characterized13-14, 20-22, 28 and is currently thought to lead to the formation of several major products.19-21, 30 However, details about the excited-state mechanism leading to the formation of the aryloxy radical are still a matter of debate.12-14, 19-22, 25-28, 31-33 Using 9-nitroanthracene as a model compound, Chapman et. al.31 originally postulated that the aryloxy radical is photochemically formed because of a nitro-nitrite rearrangement, or through a dissociation-recombination mechanism, leading to the release of the nitrogen (II) oxide (NO ) •

radical. Hamanoue and coworkers19-22 suggested the participation of a Tn state in the nitro-nitrite rearrangement, and as the precursor of the aryloxy radical, while the participation of the T1 state was discarded.12-14, 20-21, 28, 32 Plaza-Medina et. al.28 detected the accumulation of the aryloxy

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radical from the photodissociation of 9-nitroanthracene and proposed a mechanism in which the initially-excited S1 state or a rapidly formed Tn state leads to an intermediary that subsequently forms the radical species. In studies using nitronaphthalene derivatives,25-27 a mechanism in which the singlet state branches into two reaction pathways was proposed—ultrafast population of T1 through a receiver Tn state and conformational relaxation which populates an intramolecular charge-transfer state. The latter state was proposed to dissociate into the aryloxy and nitrogen (II) oxide radicals. In a recent investigation from our laboratories, the formation of a nitropyrenoxy radical from 1,6-DNP was measured as a function of excitation wavelength using transient absorption spectroscopic detection.29 It was shown that the nitropyrenoxy radical forms in parallel with the population of a receiver Tn state that internally converts to the T1 state, and that the relative yields of these parallel reaction pathways vary with excitation energy.29 Herein, we extend these spectroscopic investigations to include both the 1,3-DNP and 1,8-DNP isomers. Steady-state absorption and emission spectra, singlet and triplet vertical excitation energies, photodegradation and singlet oxygen yields, and transient absorption dynamics from femtosecond to microsecond timescales are reported for both isomer, and compared to those for 1,6-DNP. It is shown that the position of the NO2 group does not affect significantly the excited-state relaxation mechanism in these dinitropyrene isomers, while it does influence the absorption and fluorescence spectra, the fluorescence, triplet, singlet oxygen, and photodegradation yields, as well as the relative yield of radical formation. 2. METHODS 2.1. Experimental Methods.

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Chemicals. 1,3-Dinitropyrene (1,3-DNP) (99% purity), 1,8-Dinitropyrene (1,8-DNP) (98% purity), perylene (99+% Purity) and acetonitrile (HPLC Grade ≥99.93% purity) were purchased from Sigma-Aldrich and used as received. Fluoranthene was purchased from AccuStandard. High-purity nitrogen was purchased from Linde (Humacao, PR). Steady-State Absorption Spectroscopy. Absorption spectra were recorded with a HP 8453 UV-Vis photodiode array spectrophotometer (CA). Fluorescence Spectroscopy. Fluorescence spectra were recorded in a SLM-4800 or a Varian Cary Eclipse spectrometer. Emission and excitation slits were set at 10 and 70 nm, respectively. Relative fluorescence quantum yield calculations were determined from plots of the area under the emission spectra as a function of 1-10-A, where A is the absorbance at the excitation wavelength. Fluoranthene (Φf = 0.35)34 was used as a standard.13, 29 The following equation was used to calculate the emission yields where the ratios Area/1-10-A were obtained from the slope of the plots. Φfunk/Φfstand = η2unk Areaunk (1-10-A)stand / η2stand Areastand (1-10-A)unk

(1)

Femtosecond Broadband Transient Absorption Spectroscopy (fs-TAS). Femtosecond broadband pump-probe spectroscopy utilizing an 800 nm, 1 kHz laser source (4.0 W, 100 fs pulse, Libra-HE from Coherent Inc.) is described in detail elsewhere.25,

29, 35

Briefly, the

fundamental beam at 800 nm was split into the pump and probe beams using a 98/2 beam splitter, respectively. The pump beam was tuned to 425 nm using an optical parametric amplifier (TOPAS, Light Conversion). The probe beam was attenuated using an optical filter wheel and focused into a continuously translating 2 mm CaF2 crystal used to generate a white light continuum in the spectral window from ca. 320 to 700 nm. The maximum overlap of the crosscorrelated pump and probe beams was determined from the stimulated Raman emission bands of

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acetonitrile and defined as time zero in the transient absorption spectra. The instrument response function was estimated to be 200 ± 50 fs from the two-photon absorption of pure methanol.36 A 2 mm fused silica cuvette with a magnetic stir bar was used to ensure homogeneity of the solutions. Samples were monitored for potential photodegradation using steady-state absorption spectroscopy (Cary 100 Bio Instrument, Agilent Technologies). A fresh solution was used if the absorbance of the solutions at the absorption maximum of each compound decreased by 5%. Concentrations of 100 and 86 µM for 1,3-DNP and 1,8-DNP, respectively, were used for these experiments. The fs-TAS kinetic data were modeled through global fit analyses using Igor Pro 6.32A and a selection of equidistant probe wavelength in the spectral range from 445 to 700 nm for each sample. The global and target analysis method37 based on a sequential kinetic model plus a constant offset, and convoluted with an instrument response function of 200 ± 50 fs, was used to extract the lifetimes and corresponding evolution associated difference spectra (EADS). The uncertainties reported for the lifetimes of the fs-TAS are twice the standard deviation obtained from the global fit analyses (residuals are shown in Figure S1). Nanosecond laser flash spectroscopy. The 355 nm harmonic output of a Nd:YAG Surelite II laser (Continuum, Santa Clara, CA) was used to measure the long-lived transient absorption species by laser flash photolysis, as previously described elsewhere.13, 29 The methodology and procedures used for determining the triplet state absorption coefficients, the triplet yields, the triplet lifetimes at infinite dilution, and Stern-Volmer quenching plots have been described in detail elsewhere.29 Briefly, the triplet molar absorption coefficient of the dinitropyrenes (εT) was determined using the energy transfer method38 with perylene as the energy acceptor. The dinitropyrenes solution were excited at 355 nm in the absence or presence of perylene. The absorption changes produced by the dinitropyrenes triplet (ODT) were measured at the triplet 6


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absorption maximum of dinitropyrenes in the absence of perylene. With perylene present, the rate constant for the energy transfer process was measured from either the decay of the dinitropyrenes triplet or from the growth of the absorbance of the perylene triplet (λmax = 485 nm, εT = 13,400 M-1 cm-1, ET = 36 kcal mol-1).39 The maximum value of the unknown triplet molar absorption coefficient (εT) was then calculated according to the following equation: 𝜀!!"# = 0.95𝜀!!"#$%"&"

∆!"!"# ∆!"!"#$%"&"

(2)

The intersystem crossing quantum yield (ϕT) was obtained by the comparative method described by Bensasson et al.40 using benzophenone (BP) in acetonitrile as the actinometer. The BP triplet has a maximum at 525 nm with ϕT = 1.0 and εT = 6500 M-1cm-1.39 Optically matched samples of the standard and dinitropyrenes were irradiated at 355 nm in N2-saturated solutions. The corresponding triplet-state absorbance of dinitropyrenes and BP (525 nm) were measured at different laser intensities (less than 5 mJ/pulse). The ϕT value for dinitropyrenes were calculated according to the following equation: 𝜑 !"# = 𝜑 !" ! !

∝!"# !!" ! ! !" !"# !! !!

(3)

where εT is the triplet molar absorption coefficient and αT is the triplet absorbance per mJ, i.e., it is the slope of a plot of the triplet transient OD’s as a function of the laser energy. The intrinsic decay rate constant of the triplet and the self-quenching rate constant (kQ, with Q = dinitropyrenes) were obtained from Stern–Volmer analyses, according to Eq. (4). The reported quenching triplet lifetimes correspond to the reciprocal of the k0 values. 𝑘!"# = 𝑘! + 𝑘! 𝑄

(4)

2.2. Computational Methods. All calculations were performed using the Gaussian 09 suite of programs.41 An integral equation formalism of the polarized continuum model (IEFPCM) 7



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coupled with a self-consistent reaction field (SCRF) was used to simulate solvent effects.42-43 The level of theory used to optimize the ground-state structures was B3LYP/IEFPCM/6311++G(d,p). The vibrational frequencies were calculated to ensure that no imaginary frequencies were observed. Time-dependent density functional theory (TD-DFT) was used to estimate the vertical excitation energies at the PBE0/IEFPCM/6-311++G(d,p) level of theory. The excited state character was estimated through visual inspection of the Kohn-Sham orbitals and the magnitude of the oscillator strengths. 3. RESULTS AND DISCUSION 3.1. Steady-State Absorption and Emission. The steady-state properties of the 1,3-DNP, 1,6DNP, and 1,8-DNP isomers are summarized in Table 1. Figure 1a shows that the DNP isomers absorb visible radiation efficiently, as evidenced by the magnitude of their absorption coefficients in acetonitrile. The extension of the long-wavelength band into the visible is a common feature of nitropyrene derivatives,13-14 which facilitates their photochemical transformations under solar radiation in the environment. The long-wavelength band is significantly red shifted in 1,3-DNP relative to the other two isomers, with that for 1,6-DNP showing an intermediate shift.

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ε (±0.5 × 104 M-1cm-1)

7

(a)

Absorption

6

1,3-DNP 1,6-DNP 1,8-DNP

5 4 3 2 1 0 200

250

300

350

400

450

500

650

700

Wavelength (nm) (b)

Normalized Intensity

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Emission

1.0

0.5

0.0

450

500

550

600

Wavelength (nm)

Figure 1. (a) Absorption and (b) normalized fluorescence emission spectra of the dinitropyrene isomers in acetonitrile. A concentration of 3.0 × 10-5 M was used and the emission spectra were collected exciting at 415 nm for 1,3-DNP, 378 nm for 1,6-DNP,29 and 396 nm for 1,8-DNP.

The fluorescence emission spectra of the DNPs show a broad band extending from 400 to 700 nm in acetonitrile (Figure 1b). The low fluorescence quantum yields (ca. 10-3) suggest that one or more nonradiative decay pathways govern the excited-state relaxation mechanism, such as intersystem crossing often observed in other nitro-polycyclic aromatic hydrocarbons.11-12, 15, 18, 25, 32

The emission yields for these DNP isomers are 5 to 29 times larger than that for 1-nitropyrene 9



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in the same solvent.13 The additional nitro group at the C3 position increases the fluorescence yield by at least 6-fold in 1,3-DNP, compared to the nitro group at the C6 or C8 position in 1,6and 1,8-DNP, respectively (see Section 3.2 and discussion below). The redshift in the longwavelength absorption band, and the increase in the fluorescence yield in going from 1,8-DNP, to 1,6-DNP, to 1,3-DNP (Table 1), can be rationalized in terms of the resonance effects by the electron withdrawing nature of the nitro groups. As the torsion angles of the nitro group becomes, on average, closer to 0º (i.e., almost co-planar to the pyrene chromophore), the conjugation increases, decreasing the energy gap between the ground state and lowest-energy singlet state and increasing the fluorescence emission.

The zero-zero singlet state energy

reported for the DNP isomers in Table 1 is similar to that for 1-nitropyrene (275 kJ/mol)13 in the same solvent, but increases in going from 1,3-DNP to 1,6-DNP to 1,8-DNP.

Table 1. Fluorescence emission properties of dinitropyrene isomers in acetonitrile Isomer

λ emmax (± 5 nm)

1,3-DNP 490 1,6-DNP 470 1,8-DNP 460 a 1σ standard error.

E0,0 (kJ/mol) [eV] 265 [2.75] 273 [2.83] 276 [2.86]

Stokes Shift (cm-1) 3,781 3,248 3,514

Φ Fl (10-3)a 7.8 ± 0.3 1.3 ± 0.2 1.2 ± 0.2

3.2. Electronic-Structure Calculations. Figure 2 depicts the S0-optimized structure for 1,3DNP and 1,8-DNP in acetonitrile, together with that reported for 1,6-DNP recently.29 The pyrene chromophore in both DNP isomers is planar, whereas out of plane conformations are obtained for the nitro groups with torsional angles of ca. 32° and 35° for 1,3-DNP and 1,8-DNP, respectively. The values of these torsion angles are similar within the expected accuracy of these calculations,44 to those reported for 1-nitropyrene and 1,6-DNP at the same level of theory.29 The 10


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magnitude of the torsion angles in the ground state decreases in going from 1,8-DNP ≈ 1,6-DNP < 1,3-DNP. Vertical excitation energies (VEE) were also calculated for both 1,3-DNP and 1,8-DNP in acetonitrile at the TD-PBE0/IEFPCM/6-311++G(d,p) level of theory, and compared to those calculated for 1,6-DNP29 at the same level. As previously observed for 1,6-DNP,29 the S1 and T1 states for both 1,3-DNP and 1,8-DNP are predicted to have ππ* character, and there exist highenergy triplet states of mixed character that are nearly isoenergetic with the S1(ππ*) state in the Franck-Condon region for both isomers (Table 2; see also Tables S1 and S2, and Figures S2 and S3, for additional details). The negligible energy gap between the high-energy triplet states and the S1 state is expected to increase the singlet-triplet spin-orbit coupling interactions, increasing the probability of intersystem crossing from the singlet to the triplet manifold.12,

23-29, 45-46

Efficient intersystem crossing to the triplet manifold due to increased spin-orbit coupling interactions between the singlet and triplet manifolds can also explain the low fluorescence yield measured for these dinitropyrene isomers in acetonitrile (Table 1).

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Figure 2. From left to right: optimized ground state (S0) structures of 1,3-DNP, 1,6-DNP,29 and 1,8-DNP at the B3LYP/IEFPCM/6-311++G(d,p) level of theory in acetonitrile (top panel). Bottom panel: singlet and triplet vertical excitation energies (VEE) relative to the S0-optimized structure in acetonitrile at the TD-PBE0/IEFPCM/6-311++G(d,p) level of theory (Table 2; see also Tables S2 and S3, and Figures S1 and S2, for additional details).

Table 2. Vertical excitation energies of 1,3-DNP and 1,8-DNP at the TD-PBE0/IEFPCM/6311++G(d,p) level of theory in acetonitrile 1,3-DNP

1,8-DNP

Character Energy (eV) Character Energy (eV) 1 1 2.85 (0.46) 2.85 (0.48) ππ* ππ* 3 3 1.77 1.72 ππ* ππ* 3 3 a 3 3 2.73 2.78 ππ*/ nπ* CT ππ*/ nπ* CT 3 3 3 3 2.83 2.83 ππ*/ nπ* CT ππ*/ nπ* CT 3 3 3 3 2.84 2.84 ππ*/ nπ* CT ππ*/ nπ* CT a These states exhibit mixed character (see Tables S1 and S2, and Figures S2 and S3, for additional details); CT stands for charge transfer character; oscillator strengths in parentheses.

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3.3. Nanoseconds to Microsecond Transient Absorption Studies. The nanosecond laser flash photolysis experiments of 21 µM 1,3-DNP and 30 µM 1,8-DNP isomers in acetonitrile under N2saturated conditions are shown in Figure 3. The transient spectra exhibit a band with negative amplitude in the 325-425 nm region, which is assigned to the depopulation of the ground state (Figure 1). A broad absorption band is observed with maxima around 485 and 530 nm for 1,3DNP, and 450 and ca. 515 nm for 1,8-DNP, respectively. For 1,3-DNP, the band at 530 nm decays, while a longer-lived species with absorption between 460 to 510 nm persists (Figure 4a). Similarly for 1,8-DNP, the intensity of the broad absorption band with maximum at ca. 515 nm decreases (Figure 4b), while a longer-lived transient absorption species with maximum around 450 nm remains after ca. 7 µs time delay. Figure 4a depicts the normalized kinetic traces of 1,3DNP at 485 and 530 nm, while Figure 4b shows the normalized kinetic decay traces recorded at 450 and 515 nm for 1,8-DNP. Both graphs demonstrate that two different transient absorption species contribute to the nanosecond-to-microsecond transient data for 1,3-DNP and 1,8-DNP, respectively. Two different transient species were also observed for 1,6-DNP29 and for mononitropyrene isomers14 in acetonitrile. The shorter-lived species were assigned to the triplet state,14, 29 while the longer-lived species absorbing at 445 nm and 430 nm were assigned to a nitropyrenoxy, NO2PyO●,29 and a pyrenoxy radical, PyO● radical,14 respectively. Hence, in analogy to those previous experiments,14, 29 we assign the shorter-lived species to the triplet state and the longer-lived species to a nitropyrenoxy radical, NO2PyO● in both the 1,3- and 1,8-DNP isomers. The transient absorption results suggest that the relative yield of these radical depends on the dinitropyrene isomer studied, with 1,6-DNP > 1,8-DNP ≈ 1,3-DNP. Further evidence that the shorter-lived transient species can be assigned to the triplet state is provided next.

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ΔΑ (10-2)

1

(a)

1,3-DNP

0 Delay Time (µs) 0.01 0.30 0.70 9.90

-1 (b)

1,8-DNP

10

ΔΑ (10-2)

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5 0 -5

-10

Delay Time (µs) 0.00 0.60 0.90 1.30 7.3 400 450 500 550 600 650 Wavelength (nm)

Figure 3. Nanosecond transient absorption spectroscopy for (a) 1,3-DNP and (b) 1,8-DNP excited at 355 nm in acetonitrile under N2-saturated conditions.

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(a)

1,3-DNP

485 nm 530 nm

Normalized

1.0

0.5

0.0

0 20 40 60 80 100 120 140 160

Time (µs)

1.0

(b)

1,8-DNP

455 nm 520 nm

Normalized

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0.5

0.0

0

1

2

3

4

5

6

7

Time (µs)

Figure 4. Transient absorption kinetic decay curves for (a) 21 µM 1,3-DNP and (b) 30 µM 1,8DNP in acetonitrile under N2-saturated conditions.

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Table 3. Triplet-state properties for the dinitropyrene isomers in acetonitrile Isomer

λ maxa / nm

ε T / M-1cm-1

τ b / 10-4 s

kSQc,d / 109 M-1s-1

ΦTe

1,3-DNP 530 8800 ± 1000 1.2 ± 0.1 0.60 ± 0.02 0.92 ± 0.2 29 1,6-DNP 570 11200 ± 1000 0.8 ± 0.1 0.78 ± 0.05 0.87 ± 0.2 1,8-DNP 515 8800 ± 1000 1.2 ± 0.4 1.05 ± 0.07 0.89 ± 0.2 a b -4 Triplet-Triplet absorption, infinite dilution lifetimes (10 s) under N2-saturated conditions obtained from Stern-Volmer plots, c triplet self-quenching (SQ) rate constant, d the reported error is one standard deviation, e error reported represents the standard deviation from the average of three independent experiments.

Addition of O2 or ferrocene to the 1,3-DNP or the 1,8-DNP solutions leads to an increase in the decay rate of the absorption band with maximum around 515/530 nm (Table 4), while no appreciable effect on the decay rate of the transient species with maximum around 450 nm was detected. This supports the idea that the former species should be assigned to the triplet state in both isomers. In addition, we investigated the effect of adding 1-naphthol, an efficient triplet hydrogen-donor,47-50 to the decay rate of triplet state in both dinitropyrene isomers. As the concentration of 1-naphthol is increased, the intensity of the transient species with maximum around 515/530 nm decreases (Figure S4). Similar results were recently reported by one of our group for the reactions of a series of phenols with the triplet state of mononitropyrene isomers,1314

where Stern-Volmer quenching rate constants of ca. 109 M-1 s-1 were determined,13-14

comparable to those reported in this work in Table 4. Taken all these results together, the transient species with broad absorption around 515/530 nm can be confidently assigned to the lowest-energy triplet state in both 1,3-DNP and 1,8-DNP isomers. Molecular oxygen, ferrocene, and 1-naphthol have been shown to act as efficient triplet quenchers for both 1-nitropyrene13-14 and 1,6-DNP29 in acetonitrile solutions. A close to 90% triplet quantum yield was determined for all three isomers using the energy transfer method,39-40 as shown in Table 3. The high intersystem crossing yields demonstrate that population of the triplet state is one of the principal 16


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deactivation pathways for both 1,3-DNP and 1,8-DNP, as have been shown for 1,6-DNP29 and for other nitro-polycyclic aromatic compounds.10, 13-14, 27, 51

Table 4. Quenching rate constants for the triplet state of the dinitropyrene isomers by different triplet quenchers Isomer kq (O2) / 108 M-1s-1 kq (Ferrocene) / 109 M-1s-1 kq (1-naphthol) / 109 M-1s-1 1,3-DNP 2.92 ± 0.01 ---1.01 ± 0.09 1,6-DNP 4.67 ± 0.01 1.6 ± 0.1a 0.60 ± 0.04 a 1,8-DNP 0.73 ± 0.2 1.5 ± 0.1 0.61 ± 0.01 a in air. Reported errors are the standard error of the slope of plots of kobs versus the quencher concentration; kobs was determined from the best fit of graphs of ln(Abs) versus time.

3.4. Femtosecond Transient Absorption Studies. Figure 5 shows the femtosecond transient spectra of 1,3-DNP and 1,8-DNP in acetonitrile, together with those reported for 1,6-DNP29 for completeness. Excitation at 425 nm populates the S1 state for both 1,3-DNP and 1,8-DNP (see Tables 2 and 3) with a small excess vibrational energy, as previously observed for 1,6-DNP.29 The transient absorption spectra of the three isomers show marked differences in term of the absorption bands. The early transient spectrum of 1,3-DNP shows an absorption band with maximum around 480 nm, a broad shoulder in the spectral probe region from ca. 600 to 700 nm, and a negative-amplitude transient band around 525 nm. The amplitude of these bands grows and the spectra blueshift during the initial 0.8 ps to ca. 515 to 460, and 700 to 575 nm, respectively (Figure 5a). Then, the band with maximum at 460 nm decays with the simultaneous growth of a long-lived species that has shoulder around 460 and a maximum at 530 nm (Figure 5b). An apparent isosbestic point is observed at ca. 485 nm during this relaxation process. This long-lived transient species is basically identical to that observed in the nanosecond transient absorption experiments for 1,3-DNP in Figure 3a, and hence it is assigned to the combination of the transient spectra of the lowest-energy triplet state (530 nm) and that of a nitropyrenoxy radical of 17



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1,3-DNP (460 nm). The early relaxation process that occurs within the initial 0.8 ps time delay is thus assigned to a conformational relaxation of the S1 state (Figure 5a), whereas the negativeamplitude band with maximum around 515 nm is assigned to stimulated emission from the S1 state. The latter band is in agreement with the fluorescence emission spectrum of 1,3-DNP shown in Figure 1. The femtosecond-to-nanosecond transient absorption spectra of 1,8-DNP in Figure 5f-h resemble that of the 1,6-DNP reported recently by our groups,29 which are reproduced in Figure 5c-e for completeness. An absorption band with maximum at ca. 550 nm, and with a broad shoulder at longer wavelengths, grows for up to a time delay of ca. 0.2 ps (i.e., within the crosscorrelation of the pump-probe pulses; Figure 5f). The transient species then decays to populate an intermediate transient species with absorption maximum at ca. 490 nm (Figure 5g). An apparent isosbestic point is observed at 516 nm. This intermediate species decays to populate a long-lived transient species with maximum at ca. 515 nm, as shown in Figure 5h. The latter transient species mirrors very closely that observed in the nanosecond transient experiments in Figure 3b, and hence, it is assigned to the combination of the transient spectra of the lowestenergy triplet state and that of a nitropyrenoxy radical of 1,8-DNP. The transient species with maximum at ca. 550 nm is assigned to the absorption spectrum of the S1 state, as previously done for 1,6-DNP.29 We assign the intermediate absorption with maximum at ca. 490 nm to a combination of absorption bands from a nitropyrenoxy radical and a high-energy receiver triplet state, which we assume are formed in a competitive process from the S1 state, as recently shown for 1,6-DNP.29 It was shown in that earlier work that an ultrafast branching relaxation mechanism occurs in 1,6-DNP, in which the initial S1 state population bifurcates to form a nitropyrenoxy radical and to populate the T1 state through an intermediate Tn state.29 Similar 18


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branching dynamics have been reported for other nitro-polycyclic aromatic hydrocarbons.25-29 This kinetic model is also consistent with the transient absorption results shown here for 1,8DNP. The assignment of the transient species for 1,8-DNP is further supported by the calculations shown in Figure 2 (see also Table 2) and by the nanosecond transient absorption results presented in Figure 3 in Section 3.3. Taken together, the transient absorption data presented in Figures 3 and 5 show that the electronic relaxation pathways of these dinitropyrene isomers are similar, further supporting the reaction mechanism recently proposed for 1,6-DNP.29

25

-0.3 -0.1 0.0 0.1 0.3 0.8

20

ΔA (10-3)

40 (c)

Time delay (ps)

15 10 5

1,6-DNP

15 Time delay (ps) -0.4 -0.2 -0.1 -0.05 0.0 0.2

30

0

20

1,8-DNP

(f)

Time delay (ps) -0.4 -0.1 0.0 0.1 0.2

10

ΔA (10-3)

1,3-DNP

ΔA (10-3)

30 (a)

10 0

5 0 -5

-5 0.8 1.9 3.5 6.3 15.1 25.8 60.1 104.1 688.5 2802.1

20

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(g)

0.2 0.24 0.4 0.6 0.8 1.4 2.5 2.9

35 30 25 20 15 10

0.2 0.4 0.6 0.7 0.9 1.4 1.6 3.8

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(b)

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30

ΔA (10-3)

5

5

0

0

0

-5 500

550

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-5 2.9 3.4 4.0 5.6 7.6 10.1 13.0 16.3 26.7 2810.8

14 12 10 8 6 4

(h)

6

ΔA (10-3)

450

-10 16 (e)

ΔA (10-3)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3.8 7.7 13.6 99.4 1200.2 2961.6

4

2

2 0

450

500

550

600

650

700

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0

450

500

550

600

650

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Figure 5. Transient absorption spectra of 1,3-DNP, 1,6-DNP,29 and 1,8-DNP in acetonitrile excited at 425 nm. Negative-amplitude absorption bands observed within the cross-correlation of the pump-probe pulses at ca. 450, 470, and 485 nm are due to stimulated Raman emission from the solvent. 19



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Top panels of Figure 6 shows representative kinetic traces and curve fits obtained from a global fit analysis of the transient data for 1,3-DNP and 1,8-DNP in acetonitrile. The lifetimes and the corresponding EADS obtained from the target and global analyses are shown in Table 5 and in the bottom panels of Figure 6, respectively. Excitation of 1,3- and 1,8-DNP at 425 nm populates the S1 state with some excess vibrational energy, as shown previously for 1,6-DNP.29 For 1,3-DNP, the S1 state relaxation occurs simultaneously with stimulated emission with a lifetime of 0.23 ps. The high triplet yield reported in Table 3 evidences that most of the population in the S1 state intersystem crosses to the triplet manifold, while only a small fraction of the initial population (ca. ≤ 8%) leads to radical formation. It is unclear from Figure 5a,b whether the radical is formed directly from the vibrationally-excited or the relaxed S1 state because both spectra overlap strongly in the probe region near 450 nm. On the other hand, the apparent isosbestic point in Figure 5b suggests that the T1 state is populated from the relax S1 state within a lifetime of ca. 19 ps. It is likely that intersystem crossing to a high-energy Tn state occurs faster than this lifetime suggest, as observed previously for 1,6-DNP,29 but the strong spectral overlap of the transient absorption bands in 1,3-DNP does not permit the extraction of the intrinsic lifetimes.

20


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15

30 (a)

445 nm 457 nm 534 nm

1,3-DNP

25

(c)

445 nm 492 nm 552 nm

1,8-DNP

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(b)

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2

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τ1: [1ππ*(unrelaxed) + SE] τ3: [1ππ* (relaxed) + SE]

offset: [T1 + NO2PyO•]

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24 τ1: [1ππ* + SE] 22 (d) 20 τ2: [1ππ* + NO2PyO•] 18 τ3: [Tn + NO2PyO•] 16 14 offset: [T1 + NO2PyO•] 12 10 8 6 4 2 0 -2 -4 -6 -8 450 475 500 525 550 575 600 625 650 675 700

ΔΑ (10-3)

35

ΔΑ (10-3)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


450 475 500 525 550 575 600 625 650 675 700

Wavelength (nm)

Wavelength (nm)

Figure 6. Representative kinetic decay traces (a) and evolution associated difference spectra (b) for 1,3-DNP and for 1,8-DNP (c) and (d), respectively (see Table 5), following excitation at 425 nm in acetonitrile. SE stands for stimulated emission.

Table 5. Lifetimes obtained from a target and global fit analysis of the broadband transient data of the dinitropyrene isomers upon excitation at 425 nm in acetonitrile Isomer τ1 / ps τ2 / ps τ3 / ps 1,3-DNP 0.23 ± 0.10 19.3 ± 0.08b 1,6-DNP 0.27 ± 0.10 1.94 ± 0.04 10.0 ± 0.2 1,8-DNP 0.22 ± 0.10 0.97 ± 0.03 9.4 ± 0.38 a Uncertainties equal at least two standard deviations obtained from the global fit analysis (see Methods section for details); b this lifetime is likely a combination of two relaxation processes, as observed in 1,6-DNP29 and 1,8-DNP.

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For 1,8-DNP, as the population in the S1 state relaxes, two other transient species begin to emerge (Figure 5g), which we have assigned to a nitropyrenoxy radical species and to a receiver Tn state, in analogy to the results for 1,6-DNP.29 From the global fit analysis, we were able to deconvolute the two transient species. As the S1 state initially relaxes with a lifetime of ca. 0.22 ps, the formation of the nitropyrenoxy radical is observed in the EADS (Figure 6d). Simultaneously, the S1 state relaxes fully leading to the population of the receiver Tn state with a lifetime of ca. 0.97 ps (Figure 5g and 6d). The receiver Tn state internally converts to the T1 state with a lifetime of 9.4 ps (Figure 5h). As previously inferred in other nitro aromatic compounds, 25, 27, 29, 52-54

the slow internal conversion process in the triplet manifold seems to be due to the

large triplet-triplet energy gaps and small Franck-Condon factors for internal conversion in these dinitropyrene isomers. Conclusions In this study, we have investigated the steady-state properties and the femtosecond-tomicrosecond excited-state dynamics of 1,3-DNP and 1,8-DNP, and compared the results with those of 1,6-DNP isomer. The experiments are complemented with ground- and excited-state calculations at the density functional level of theory. Excitation of these dinitropyrene isomers with visible light radiation at 425 nm results in the ultrafast branching of the excited-state population in the S1 state to populate a high-energy triplet state and to form a nitropyrenoxy radical. About 90% of the excited-state population intersystem crosses to reach the T1 state in all three isomers, while a fraction of the residual 10% of the initial population form the radical. The overwhelming population of the T1 state is proposed to be due to an energy barrier that needs to be surmounted in order to form the radical, as recently shown for 1,6-DNP.29

22


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Interestingly, the relaxation pathways and triplet yields of these dinitropyrene isomers are practically the same, but the magnitude of the photodegradation rate reported for 1,6-DNP is larger than that of 1,8-DNP in acetonitrile under both N2- and O2-saturared conditions.9 This suggests that the triplet state is not the primary pathway for photodegradation, and that the formation of the radical species likely plays a key role. Similarly, the low photodegradation yields of 4 × 10-4, 1.2 × 10-3, and 3 × 10-4 for 1,3-DNP, 1,6-DNP, and 1,8-DNP in acetonitrile, respectively (Arce & Piñero-Santiago, unpublished results), and the type of identified photoproducts (hydroxynitropyrenes, and pyrenediones),9 also suggest a minor participation of the T1 state in the photochemistry of these isomers, unless H-donor compounds are added to the solution. Furthermore, the T1 state of these DNP isomers exhibit high efficiency in generating singlet oxygen (Arce & Piñero-Santiago, unpublished results). Quantum yields for singlet oxygen generation of 0.54, 0.84, and 0.60 for 1,3-DNP, 1,6-DNP, and 1,8-DNP, respectively, in acetonitrile under O2-saturated conditions were determined using 2,5-dimethylfuran and 2,5diphenylfuran as chemical scavengers of singlet oxygen using UV-vis detection55 or gas chromatography56 methods. These singlet oxygen yields also show variation with the isomer investigated. No reaction of singlet oxygen with these dinitropyrene isomers was detected, although reactions of this oxygen reactive species with other components of the aerosol are possible. The observed products can also originate from subsequent reactions of the initially formed nitropyrenoxy and nitrogen (II) oxide radical pair. The nitropyrenoxy radical can transform into a pyrenedione with the release of the NO● radical or can abstract an H atom from a suitable H atom source to produce a hydroxynitropyrene.17 The radical pair could also lead to a hydroxydinitropyrene by the attack of the NO● radical to another carbon position of the nitropyrenoxy radical inside a solvent cage.30

23



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Supporting Information Example of the residual components obtained from target and global analyses of the femtosecond transient absorption data for 1,3-DNP and 1,8-DNP. Tables containing the vertical excitation energies, percentage of state character, and oscillator strengths for 1,3-DNP and 1,8DNP in acetonitrile. Figures showing the Kohn-Sham Orbitals for the primary single-electron vertical transitions for 1,3-DNP and 1,8-DNP in acetonitrile. This information is available via the Internet free of charge at http://pubs.acs.org 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. 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, 928932. 24


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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. Holloway, M. P.; Biaglow, M. C.; McCoy, E. C.; Anders, M.; Rosenkranz, H. S.; Howard, P. C., Photochemical Instability of 1-Nitropyrene, 3-Nitrofluoranthene, 1,8Dinitropyrene and Their Parent Polycyclic Aromatic Hydrocarbons. Mutat. Res. 1987, 187, 199207. 9. Morel, M.; Alers, I.; Arce, R., Photochemical Degradation of 1,6- and 1,8-Dinitropyrene in Solution. Polycycl. Arom. Compd. 2006, 26, 207-219. 10. Scheerer, R.; Henglein, A., The Triplet State of 3-Nitropyrene. Ber. Bunsen-Ges. 1977, 81, 1234-1239. 11. 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. 12. 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. 13. Arce, R.; Pino, E. F.; Valle, C.; Agreda, J., Photophysics and Photochemistry of 1Nitropyrene. J. Phys. Chem. A 2008, 112, 10294-10304. 14. Arce, R.; Pino, E. F.; Valle, C.; Negrón-Encarnación, I.; Morel, M., A Comparative Photophysical and Photochemical Study of Nitropyrene Isomers Occurring in the Environment. J. Phys. Chem. A 2011, 115, 152-160. 15. Plaza-Medina, E. F.; Rodríguez-Córdoba, W.; Peon, J., Role of Upper Triplet States on the Photophysics of Nitrated Polyaromatic Compounds: S1 Lifetimes of Singly Nitrated Pyrenes. J. Phys. Chem. A 2011, 115, 9782-9789. 16. Murudkar, S.; Mora, A. K.; Singh, P. K.; Nath, S., Origin of Ultrafast Excited State Dynamics of 1-Nitropyrene. J. Phys. Chem. A 2011, 115, 10665-10822. 17. Arce, R.; Morel, M., Phototransformations of Dinitropyrene Isomers on Models of the Atmospheric Particulate Matter. Atmos. Environ. 2013, 75, 171-178. 18. Mora, A. K.; Murudkar, S.; Singh, P. K.; Gowthaman, N. S. K.; Mukherjee, T.; Nath, S., Ultrafast Excited State Dynamics of 1-Nitropyrene: Effect of H-Bonding. J. Photochem. Photobiol. A 2013, 271, 24-30. 19. Hirayama, S.; Kajiwara, Y.; Nakayama, T.; Hamanoue, K.; Teranishi, H., Correct Assignment of the Low-Temperature Luminescence from 9-Nitroanthracene. J. Phys. Chem. 1985, 89, 1945-1947. 25



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TOC Graphic

Radical Species

Triplet States

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TOC 50x50mm (96 x 96 DPI)


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Photochemical Relaxation Pathways in Dinitropyrene Isomer

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