Conformational Control in the Population of the Triplet State and

Article pubs.acs.org/JPCA

Conformational Control in the Population of the Triplet State and Photoreactivity of Nitronaphthalene Derivatives R. A. Vogt and Carlos E. Crespo-Hernández* Department of Chemistry and Center for Chemical Dynamics, Case Western Reserve University, Cleveland, Ohio 44106, United States ABSTRACT: Nitronaphthalene derivatives (NNDs) are among the most abundant volatile nitro-polycyclic aromatic hydrocarbons found in the Earth’s atmosphere. Investigations of the atmospheric degradation processes show that photolysis is the major degradation pathway under ambient conditions. In this contribution, we present photochemical measurements and quantum-chemical calculations of three major NNDs. It is shown that the magnitude of the photodegradation and triplet quantum yields in 1nitronaphthalene (1NN), 2-methyl-1-nitronaphthalene (2M1NN), and 2nitronaphthalene (2NN) are inversely related to each other. In accord with a recent time-resolved and computation study (J. Phys. Chem. A 2013, 117, 6580) and in order to explain this striking observation we propose that these photochemical yields are largely controlled by (1) the conformational heterogeneity of the nitro-aromatic torsion angle, (2) the energy gap (spin− orbit coupling interaction) between the excited singlet state and the receiver triplet state, and (3) the topology of the excited singlet state in the Franck−Condon region of configuration space sampled at the time of excitation. A distribution of torsion angles closer to 90° leads to a higher photoreactivity. Methylation of the ortho position in 1NN to give 2M1NN increases the photoreactivity by 97%, while 2NN is largely photoinert. Conversely, the triplet yield decreases as the distribution of torsion angles gets closer to 90°: 0.93 ± 0.15 in 2NN, 0.64 ± 0.12 in 1NN, and 0.33 ± 0.05 in 2M1NN. These results suggest an important relationship between conformational heterogeneity and the photochemical fate of these NNDs. This structure− photoreactivity relationship is of relevance to current efforts aimed at modeling and understanding the distribution patterns of NNDs in the atmosphere and their overall contribution to air quality.

1. INTRODUCTION 1-Nitronaphthalene (1NN) and 2-nitronaphthalene (2NN) are among the most abundant volatile nitro-polycyclic aromatic hydrocarbons (NPAHs) found in ambient atmospheres.1−4 2methyl-1-nitronaphthalene (2M1NN) and other methylated nitronaphthalene derivatives are also found in the atmosphere as transformation products of 1- and 2-methylnaphthalene.2,5 Sources for these nitronaphthalene derivatives (NNDs) in ambient air include atmospheric formations from naphthalene and methylnaphthalene derivatives and emissions from diesel exhaust.5−9 They have been found to be genotoxic in vitro and in vivo and to account for a significant fraction of ambient air mutagenicity, as determined by employing the Salmonella typhimurium assay.10−20 Investigations of the atmospheric degradation processes of NPAHs show that photolysis is the major degradation pathway in 1NN, 2NN, and 2M1NN in the gas phase under ambient conditions.9,21−25 Therefore, knowledge of the photodegradation mechanisms and of the structure−photoreactivity relationships in these NNDs is essential for modeling and understanding their distribution patterns in the atmosphere and their overall contribution to air quality. Previous investigations have presented evidence that the nitroaromatic torsion angle plays a role in the photochemistry of NPAHs.26−33 Of particular relevance to this work is a recent © 2013 American Chemical Society

investigation using 1NN, 2M1NN, and 2NN as model systems31,34 where it was shown that a distribution of nitroaromatic torsion angles exists in the ground (S0) state at room temperature. It was proposed that this distribution plays a key role in the branching of the excited singlet (S1) state population to two relaxation channels: an intramolecular charge-transfer state with dissociative character Sdiss(CT) and a high-energy receiver triplet state Tn(nπ*) that internally converts to the lowest-energy triplet state.31,34 In this contribution, 1NN, 2M1NN, and 2NN were selected as representatives of NNDs to evaluate this hypothesis further. We provide quantitative evidence that conformational heterogeneity in these NNDs modulates the photoreactivity and the triplet yield of these compounds. We propose that this modulation is due to the steering the initial population in the S1 state toward the Sdiss(CT) state or toward the receiver Tn(nπ*) state. Our strategy relies on the premise that the extent of population branching in the S1 state toward the Sdiss(CT) state or the Tn(nπ*) state is to some extent controlled by the distribution of torsion angles available in the S0 state at the time of excitation. The distribution of torsion angles in nitronaphthalene can be expected to shift Received: October 13, 2013 Revised: November 24, 2013 Published: November 27, 2013 14100

dx.doi.org/10.1021/jp4101587 | J. Phys. Chem. A 2013, 117, 14100−14108

The Journal of Physical Chemistry A

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torsion angle as the variable coordinate by fixing this angle at selected values and allowing all other nuclear coordinates to optimize in the S0 state. The excited-state PECs were obtained at the TD-PBE0/IEFPCM/6-311++G(d,p)∥B3LYP/IEFPCM/6311++G(d,p) level of theory. The energy values for these PECs are reported relative to the energy of the S0-state global minimum at the same level of theory. 2.3. Steady-State Measurements. Steady-state absorbance measurements were performed using a Cary 100 UV−vis spectrometer (Varian, Inc.). Photodegradation experiments were performed using a 150 W Xe lamp (Newport-Oriel, Apex Source Arc, source model 66453, lamp model 6255). The wavelength range of 275−375 nm was selected for irradiation by using a FGUVS11S colored glass filter (Schott). Initial experiments filtered out the IR light using a quartz cell filled with water. However, it was found that filtering the IR had no influence on the results, suggesting that thermal decomposition is insignificant in these molecules. The polychromatic light was focused through a 450 mm lens placed at 5 cm from the front of the lamp source. The sample was placed at 41 cm from the front of the lens. The beam width at the sample was 0.95 cm. Solutions were contained in a 1 cm quartz cuvette (Starna, Inc.) and stirred continuously with a magnetic stir bar (Starna, Inc.) to ensure a homogeneous irradiation of the solutions at all times. The determination of the polychromatic photodegradation quantum yields requires the measurement of the change in concentration of the nitronaphthalene derivatives with irradiation time. We measured the change in concentration with irradiation time using a HPLC (Shimadzu LC-20AD) with an amide column (Ascentis RP-Amide, 5 μm, 25 cm × 4.6 mm) to separate the parent compounds from the photoproducts. This method was used instead of the traditional chromophore-loss method as the former provides more accurate determinations of photodegradation yields.51,52 Calibration curves were obtained for each nitronaphthalene derivative. At least 5 data points were used for each of the calibration curves of area under the chromatographic fraction versus concentration. An isocratic elution was used with a solvent composition of 80% acetonitrile and 20% water. A photodiode array detector (Shimadzu SPDM20A) was used to measure the absorbance of the eluting compounds. No degradation was observed for 2NN after a total irradiation of 4 hours under the experimental conditions. Thus, it was concluded for the scope of this work that 2NN is photoinert under the low-intensity, continuous irradiation conditions used. Potassium ferrioxalate was used as an actinometer to measure the lamp intensity in photons per second.53 Polychromatic photodegradation yields were measured in N2-saturated conditions using the method recently developed by Dodson et al.54 2.4. Measurements of Triplet Absorption Spectra, Quantum Yields, and Absorption Coefficients. The transient absorption spectrometer used for measuring the triplet−triplet absorption spectra, triplet yields, and triplet absorption coefficients of 2NN, 1NN, and 2M1NN has been described in detail elsewhere.31,55 Briefly, the output of a Quantronix Integra-i/e 3.5 laser (100 fs fwhm centered at 800 nm) was fed into an optical parametric amplifier (TOPAS, Quantronix/Light Conversion) that generates the femtosecond pulses used for excitation of the samples. Excitation intensities at the sample were 0.5 μJ in all cases to avoid multiphoton processes under the experimental conditions used in this work. The measurements were performed under N2-saturated conditions in a 2 mm optical path length cell using an Eos spectrometer (Ultrafast Systems, LLC), as described recently.55 The probed

toward 0 or 90° by changing the position of the nitro-group in the naphthalene moiety or by methylation of nitronaphthalene in the ortho position, respectively. In the case of 1NN, the peri hydrogen atom is expected to force the distribution of torsion angles away from 0°. Addition of a methyl group in the ortho position of 1NN, as in 2M1NN, is expected to force the distribution of torsion angles to even greater values. 2NN, on the other hand, does not have peri hydrogen atoms that can interact with the nitro group, and the distribution of nitro-aromatic torsion angles is expected to be closer to 0°.

2. EXPERIMENTAL AND COMPUTATIONAL METHODS 2.1. Chemicals. Cyclohexane (99.9%) was obtained from Fisher Scientific. Acetonitrile (99.6%) was obtained from Acros. Both solvents were used as received. 2NN, 1NN, and 2M1NN were obtained from Sigma-Aldrich (99.7%, 99%, and 99%, respectively). 2NN, 1NN, and 2M1NN are moderately toxic compounds. Proper safety precautions were taken at all times to limit health risks. The purity of the NNDs was verified by high-performance liquid chromatography and fluorescence spectroscopy. None of the nitronaphthalene compounds showed fluorescence emission within the sensitivity of the instrument (Cary Eclipse, Varian, Inc.), except for 2NN, where a small fluorescence emission was observed after excitation at 340 nm in acetonitrile when a solution for this compound was prepared as received from Sigma-Aldrich. However, the excitation spectrum of the emitting species did not match the absorption spectrum of 2NN; therefore, 2NN was recrystallized using methanol as the solvent. After recrystallization, the intensity of the fluorescence spectrum was reduced almost completely, suggesting that the fluorescence signal was due to a very small amount of impurity present in the commercial sample. Recrystallized samples of 2NN were used in all the experiments reported in this work. 2.2. Quantum Mechanical Calculations. All calculations were performed using the Gaussian 03 suite of programs.35 Solvent effects were modeled by using self-consistent reaction field calculations (SCRF) with the polarizable continuum model (PCM)36 with the integral equation formalism (IEFPCM).37 Ground-state optimizations were performed using density functional theory (DFT) at the B3LYP/IEFPCM/6-311+ +G(d,p) level of theory.38−40 The procedure used was similar to the one reported by Reichardt et al.31 Briefly, a gradient procedure was used starting with the 6-31G(d,p) basis set and increasing in size until the changes in geometry and energy were insignificant. Ground-state bond lengths converged to within 0.008 Å while nitro-group torsion angles converged to within 0.1 degrees at the B3LYP/IEFPCM/6-311++G(d,p) level of theory. To confirm that the optimized geometries were at a local minimum on the potential energy surface, vibrational frequencies were calculated at the B3LYP/IEFPCM/6-31G(d,p) level of theory, and it was verified that all were positive. Vertical excitation energies were calculated using the TDPBE0/IEFPCM/6-311++G(d,p) level of theory,41,42 with the gradient procedure described previously.31 Excitation energies converged to 0.07 eV or better in all cases. It has been shown that the PBE0 functional provides accurate excited-state energies (within ∼0.2 eV),43−46 singlet−triplet energy gaps,46,47 and excited-state ordering,43,46,48 particularly when solvent effects using the PCM solvation model are included in the calculations.43,46,48−50 Potential energy curves (PECs) were calculated for the S0 and the excited singlet and triplet states using the nitro-aromatic 14101



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volume of the samples was continuously renewed using a Tefloncoated stir bar and a magnetic stirrer. It was verified that the low magnetic field at the cell induced by the magnet in the magnetic stirrer (∼2 × 10−3 Tesla) had no effect on the transient absorption signals and triplet yield measurements. To avoid potential interference by photoproduct formation, samples were replaced with fresh solutions if a decrease of 5% or more was observed in the steady-state absorbance of the solutions. The triplet yields of the NNDs were measured in acetonitrile using the energy-transfer method, with pyrene as the triplet standard.56 The reported triplet yield of pyrene is 0.38.57 Solutions of the NNDs with an optical density of 0.5 in a 2 mm optical path length cell were prepared with various concentrations of pyrene. The excitation wavelength used for the energy-transfer experiments was 355 nm. The ratio of the absorption coefficients of the NNDs to that of the pyrene was found using the following expression: ε AP = P AN ΦET εN (1)

Figure 1. Optimized geometries of 2-nitronaphthalene (left), 1nitronaphthalene (center), and 2-methyl-1-nitronaphthalene (right) at the B3LYP/6-311++G(d,p) level of theory. Nitro-aromatic torsion angles are marked with asterisks.

acting between the oxygen atoms on the nitro group and their neighboring atoms on the naphthalene moiety. In the case of 1NN, the peri hydrogen atom forces the torsion angle away from 0°. In 2M1NN, a methyl group in the ortho position, in addition to a peri hydrogen atom, forces the torsion angle to even greater values. 2NN does not have peri hydrogen atoms that can interact with the nitro group, explaining its almost planar conformation. The calculated torsion angles for 1NN and 2NN are in good agreement with calculations found in the literature.31,58 Experimentally, a similar trend is predicted for the torsion angles using UV−vis, mass, and H NMR spectroscopic techniques.27 The aim of these calculations is to assist in the interpretation of the experimental results. Notably, the photodegradation and triplet quantum yields reported in this work were obtained at room temperature. Thus, it is important to examine the effect that temperature has on the nuclear degrees of freedom in these NNDs in the S0 state and, in particular, the effect on the nitroaromatic torsional angle. Figure 2 shows that a distribution of torsion angles exists in the S0 state with energies lower than the thermal energy (kBT) for both 2NN and 2M1NN at room temperature. Similar results have been obtained previously for 1NN.31 Consequently, to gather deeper insights about the structure−photoreactivity relationships in these NNDs, excitedstate calculations must be performed taking into account the distribution of nitro-aromatic conformations thermodynamically available in the S0 state at the time of excitation in each molecule. Excitation of a distribution of nitro-aromatic conformations can affect the photochemistry in these NNDs. This is because each nitro-aromatic conformation will sample a different region of configuration space in the S1 state, which in principle can open up different electronic relaxation pathways in the S1 state potential energy surface (PES). Therefore, vertical excitation energies were calculated as a function of the nitro-aromatic torsion angle for these NNDs. Figure 2 shows PECs generated by using the nitro-aromatic torsion angle as the variable coordinate, while allowing all other nuclear coordinates to optimize in the S0 state. These calculations show that the S1 PEC has a minimum at 0° in the case of 2NN, while it has a minimum at ∼90° in the case of 1NN31 and 2M1NN. Importantly, as the nitro-aromatic torsion angle approaches 90°, the magnitude of the S1 oscillator strength decreases and the state acquires a more dissociative, charge-transfer character in both 2NN and 2M1NN, as found previously in 1NN.31 The vertical excitation energies, the character of the electronic states, and the S1 − T3 energy gaps calculated using the fully optimized ground-state geometries are reported in Table 1. These calculations show that the S1(ππ*) state is almost isoenergetic to the T3(nπ*) in these NNDs and at the TDDFT level of theory, independent of the solvent used. The character and proximity of the S1 and T3 states is expected to

where AP is the triplet−triplet absorbance maximum of pyrene at 412 nm and AN is the triplet−triplet absorbance maximum for the NND (473 nm for 2NN, 570 for 1NN, and 560 for 2M1NN); εP and εN correspond to the triplet molar absorptivity coefficient for the T1 state of the pyrene and NND, respectively, and ΦET is the yield of energy transfer from the NND triplet to the pyrene triplet. The energy-transfer yield is calculated using the following relationship: ΦET =

kET kET + kN

(2)

where kET is the rate constant for the energy transfer from the NND to the pyrene and kN is the rate constant for the decay of the NND’s triplet without pyrene, which can be obtained from the transient absorption experiments. The ratio of the absorption coefficients is obtained from the slope of a graph of AP versuss ANΦET, as shown in eq 1. Transient absorption data was collected using an excitation wavelength of 334 nm to calculate the triplet yield of the NNDs (as opposed to the 355 nm excitation wavelength used in the energy-transfer step of the measurement). At this wavelength both the NNDs and the pyrene absorb. Samples of the NNDs and the pyrene with optical densities of 0.5 at 334 nm were studied back-to-back under the same experimental conditions. The triplet yield is then found by using the following expression: ΦT(N) =

εP AN ΦT(P) εN AP

(3)

where εP/εN is the absorption coefficient’s ratio obtained from eq 1 and ΦT(P) is the triplet yield of pyrene; AN and AP are the triplet absorbance maximum for NNDs and pyrene, respectively.

3. RESULTS AND DISCUSSION 3.1. Ground- and Excited-State Calculations. The ground-state optimizations show that the nitro-aromatic torsion angle in these NNDs, as defined in Figure 1, increases from 0.1° in 2NN to 56.8° in 2M1NN in cyclohexane; 1NN shows an intermediate torsion angle of 33.1° at the B3LYP/IEFPCM/6311++G(d,p) level of theory. Changing the solvent to acetonitrile gives torsion angles of 0.3° in 2NN, 33.9° in 1NN, and 55.4° in 2M1NN for the optimized geometries. As expected, the torsion angle in these NNDs is modulated by steric forces 14102



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Figure 2. Potential energy curves for 2NN (left panel) and 2M1NN (right panel) as a function of the nitro-aromatic torsion angle in cyclohexane (top) and acetonitrile (bottom). Legend: S0 (◇), S1 (□), T1 (△), T3 (▽), and S1 oscillator strength (○). The torsion angle distribution populated at room temperature is highlighted in gray. Vertical excitation energies were calculated at the TD-PBE0/IEFPCM/6-311++G(d,p) level of theory following ground-state optimizations at the B3LYP/IEFPCM/6-311++G(d,p) level of theory in each solvent. The nitro-aromatic torsion angle was constrained to the specified values during the ground-state optimizations.

Table 1. Vertical Excitation Energies and Selected Energy Gaps in Electron Volts (eV) Obtained from the Fully-Optimized Ground-State Structures Determined at the TD-PBE0/IEFPCM/6-311++G(d,p)∥B3LYP/IEFPCM/6-311++G(d,p) Level of Theorya cyclohexane state S1(ππ*) T1(ππ*) T2(nπ*) T3(nπ*) ΔE(S1 − T3) a

1NN

31

3.41 (0.133) 2.34 2.82 3.21 0.202

acetonitrile

2M1NN

2NN

3.28 (0.051) 2.44 2.79 3.10 0.179

3.40 (0.093) 2.40 3.27 3.27 0.136

1NN

31

3.23 (0.132) 2.28 2.91 3.20 0.030

2M1NN

2NN

3.09 (0.054) 2.38 2.85 3.04 0.043

3.16 (0.089) 2.30 2.86 3.14 0.021

Oscillator strengths are shown in parentheses.

favor intersystem crossing between these two states by increasing the vibronic and spin−orbit coupling interactions, lending support to the idea that intersystem crossing in this molecules occurs in the strongly nonadiabatic limit.34 This prediction is in

good agreement with the triplet quantum yields reported for these NNDs below. 3.2. Steady-State Results. The ground-state absorption spectra of 1NN, 2NN, and 2M1NN in cyclohexane and 14103



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acetonitrile are shown in Figure 3. A red shift is seen in the lowest-energy absorption band when going from cyclohexane to acetonitrile. This red shift suggests that the S1 state of these NNDs has ππ* character, in agreement with the calculations shown in Table 1. In addition, Figure 3 shows that the S1 absorption band shifts to the blue in going from 2NN to 1NN to 2M1NN in both solvents and the magnitude of the extinction coefficients in 2NN is intermediate between that in 1NN and 2M1NN; 2M1NN shows the extinction coefficients with the lowest magnitude. The experimental features of the S 1 absorption band are captured by the average vertical excitation energies and oscillator strengths predicted for the distribution of nitro-aromatic conformations thermodynamically available for each compound in the S1 state (shown in Figure 2 for 2NN and 2M1NN and in Figure 3 of ref 31 for 1NN). The agreement between the experimental and computational results provides further support to the idea that these NNDs exist in a distribution of nitro-aromatic conformations in solution at room temperature.

Figure 4. Molar absorptivity coefficients for the triplet−triplet absorption band of the nitronaphthalene derivatives in acetonitrile.

the spectral region from 375 to 650 nm in acetonitrile. The absorption coefficients of the triplet−triplet absorption band were estimated to be (6400 ± 800) M−1cm−1 for 2NN at 473 nm, (6400 ± 1000) M−1cm−1 for 1NN at 570 nm, and (4500 ± 500) M−1cm−1 for 2M1NN at 560 nm in acetonitrile. The magnitude of the triplet absorption coefficients suggests that the T1 state has significant ππ* character, in agreement with the predictions shown in Table 1. We also determined the triplet quantum yields of these NNDs by using the energy-transfer method, with the triplet yield of pyrene as the reference standard. The magnitudes of the triplet quantum yields for 2NN, 1NN, and 2M1NN are 0.93 ± 0.15, 0.64 ± 0.12, and 0.33 ± 0.05, respectively, in acetonitrile under N2-saturated conditions. The high magnitudes of the triplet yields support the results from the calculations that show a small energy gap between the S1(ππ*) and the T3(nπ*) states and thus predict an efficient population of the triplet state. The triplet yield of 1NN in benzene and of 2NN in EPA (ether/isopentane/ ethyl alcohol glass) has been reported to be 0.63 and 0.82, respectively,59,60 in excellent agreement with our measurements in acetonitrile. To our knowledge, the triplet yield of 2M1NN has not previously been reported in the literature. 3.4. Relationship between the Triplet and Photodegradation Quantum Yields. The most important finding of this work is the observation that there is an inverse relationship between the magnitude of the photodegradation and of the triplet yields in these NNDs (Figure 5). This observation does not necessarily imply that the triplet state does not participate in the photochemistry of these NNDs under other experimental conditions. It simply shows that the participation of the triplet state is not the main photochemical degradation pathway in these NNDs in acetonitrile solution under N2-saturated conditions. In the next sections we attempt to rationalize this striking

Figure 3. Steady-state absorption spectra of 2NN, 1NN, and 2M1NN in cyclohexane (top) and acetonitrile (bottom).

We have also measured the polychromatic photodegradation quantum yields of 1NN, 2NN, and 2M1NN in acetonitrile solutions under N2-saturated conditions. The photodegradation quantum yields decrease in the following order: 2M1NN (0.123 ± 0.008) > 1NN (0.0035 ± 0.0004) > 2NN (∼0), all in molecules per photon. Analogous experiments using cyclohexane as the solvent were impractical because of its higher volatility versus acetonitrile. However, the calculations shown in Figure 2 suggest that the photodegradation yields in cyclohexane should follow a trend similar to that shown in acetonitrile. Furthermore, the trend in photoreactivity observed in solution is in excellent agreement to that observed in the gas phase by Arey and co-workers from both indoor and outdoor photolysis experiments.24 The good agreement between the solution-phase and gas-phase photolysis experiments lends support to the idea that solution-phase experiments can provide important mechanistic insights about the photochemistry of these NNDs in the atmosphere. 3.3. Triplet Yield Measurements. Figure 4 shows the triplet−triplet absorption spectra for 2NN, 1NN, and 2M1NN in

Figure 5. Relationship between the magnitude of the triplet and photodegradation quantum yields. Horizontal and vertical bars represent the uncertainty of the yields from three independent measurements. 14104



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at which the distribution of torsion angles is located in the S0 state (i.e., the region of configuration space sampled in the S1 state PES at the time of light absorption, as shown by the gray area in Figure 2) and the photochemistry of these NNDs. Even though previous works have provided evidence for the importance of the nitro-aromatic torsion angle on the photochemistry of NPAHs,26−33 to the best of our knowledge, this is the first report showing that the magnitude of the triplet yield also correlates with the distribution of nitro-aromatic torsion angles in the S0 state, albeit in an inverse fashion relative to the photodegradation yield. 3.6. Plausible Mechanism Explaining the Inverse Correlation between the Photoreactivity and the Triplet Quantum Yields. The results presented in this work, together with the experimental and computational results presented previously,31,34,61 provide definitive clues regarding a mechanism that can explain the structure−photoreactivity relationship observed in these NNDs. The region of the PECs highlighted in gray in Figure 2 represents the distribution of 2M1NN conformations that can be excited to the Franck−Condon region of the S1 PES. According to these PECs and recent time-resolved results for 1NN and 2M1NN,31,34 the Franck−Condon population can decay by two parallel relaxation pathways. One of the relaxation pathways is intersystem crossing to the receiver Tn(nπ*) state (i.e., the T3(nπ*) state in this work at the TDPBE0/IEF/6-311++G(d,p)∥B3LYP/IEFPCM/6-311++G(d,p) level of theory). This relaxation pathway has been proposed to take place before complete conformational relaxation of the S1 state,34 in excellent agreement with recent CASPT2//CASSCF calculations for 1NN.61 The receiver Tn(nπ*) state subsequently undergoes internal conversion (IC) to the T1(ππ*).34 The second decay pathway is dissociation to form the aryloxy (ArO) and nitrogen(II) oxide (NO) radicals. Direct spectroscopic evidence of the formation of the ArO and NO radical intermediates comes from electron spin resonance and transient absorption studies.62−65 Excited-state calculations suggest that conformational relaxation of the S1 state populates an intramolecular charge-transfer state Sdiss(CT).31,34 The conformational relaxation in the S1 state is due primarily to the rotation of the nitro group toward a close-to-perpendicular torsion angle. This Sdiss(CT) state is thought to eventually lead to the dissociation of the NNDs to form the ArO and NO radicals.31,34 The dissociation reaction possibly originates from a direct dissociation−recombination mechanism31,32,34,66 or from a nitro-nitrite intramolecular rearrangement mechanism.26 Herein we propose that the distribution of torsion angles thermodynamically available in the ground state for 1NN and 2M1NN controls the population branching in the S1 state toward the Sdiss(CT) state or toward the receiver Tn(nπ*) state and therefore the photochemistry in these NNDs (see below). For 2NN, the highlighted gray area in the PECs shown in Figure 2 represents the distribution of conformations in the ground state that populates the Franck−Condon region of configuration space in the S1 PEC. In this region, the T3(nπ*) state is almost isoenergetic to the S1 state. Figure 2 also shows that there is an energy barrier of ca. 5.8 kcal/mol that must be surmounted to access the region of configuration space leading toward the Sdiss(CT) state in the case of 2NN (i.e., the region where the torsion angle is close to 90° in the S1 PEC). Hence, the calculations shown in Figure 2 predict that the S1-state population in 2NN will favorably intersystem cross to the triplet manifold, which should result in a triplet yield close to unity. This is in excellent agreement with the 0.93 triplet yield measured in

experimental observation by making use of the time-resolved and computational evidence gathered thus far for these NNDs. 3.5. Relationship between the Ground-State Conformational Heterogeneity and the Triplet and Photodegradation Quantum Yields. Figure 2 shows that there is a distribution of conformations with a range of nitro-aromatic torsion angles in the ground state of 2NN and 2M1NN at room temperature. Similar results have been reported for 1NN.31 Interestingly, Figure 6a shows that there is a correlation between

Figure 6. (a) Photodegradation and (b) triplet quantum yields for 2NN (▲), 1NN (■), and 2M1NN (●) in acetonitrile under N2-saturated conditions plotted as a function of the nitro-aromatic torsion angle of the fully optimized ground-state geometry in each molecule. Vertical bars represent the uncertainty of the yields from three independent measurements.

the position at which the distribution of torsion angles is located and the experimentally measured photodegradation yield for each NND in acetonitrile. 2M1NN, which has a distribution of torsion angles in the range of 36.5 to 81.0°, shows the highest photoreactivity of the three NNDs investigated in this work, whereas 2NN, which has a distribution of torsion angles in the range of 0.0 to 25.8°, is nearly photoinert. 1NN has an intermediate distribution of torsion angles in the range of 10.5 to 53.5° and shows intermediate photoreactivity compared to that of 2M1NN and 2NN. The closer the distribution of torsion angles is to 90° for a given NND, the higher its photodegradation yield. These results are in good agreement with the structure− reactivity relationship proposed originally by Chapman et al. for 9-nitroanthacene26 and further validate the computational and experimental approaches used in this work. Figure 6b also shows that there is a correlation between the position at which the distribution of torsion angles is located and the experimentally measured triplet quantum yields. Strikingly, however, this correlation is inversely proportional to that found for the photodegradation quantum yields in Figure 6a. 2NN, which is practically photoinert, shows the highest triplet yield, while 2M1NN, which is the most reactive of the three NNDs, shows the lowest triplet yield. Taken together, these two correlations show that there is a strong link between the position 14105



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population dynamics originating from the different Franck− Condon regions of configuration space in the S1 PES of each NND that ultimately control the fraction of the S1-state population that intersystem crosses to the triplet manifold or that populates the Sdiss(CT) state. As shown in Figure 2 (and Figure 3 of ref 31 for 1NN), the topology of the Franck−Condon region in the S1 PEC for 2NN (gray area in Figure 2) is different from that in 2M1NN (or 1NN31). In particular, the slope of the S1 PEC has a gradient toward a 0° torsional angle in 2NN, whereas it has a gradient toward 90° in both 2M1NN and 1NN. In addition, PECs for the optimized S1 state as a function of the nitro-aromatic torsion angle show a steeper gradient toward 90° in 2M1NN than in 1NN in the region of configuration space sampled at the time of excitation.34 Hence, it is the topology of the S1 PEC in the region of configuration space sampled by each subgroup of conformations at the time of excitation that controls the population branching. It is in this sense that we propose that the distribution of torsion angles thermodynamically available in the ground state in each NND controls the dynamics and photochemistry of these NNDs. Furthermore, we propose that rotation of nitro-aromatic torsion angle is the reaction coordinate that plays the primary role in the eventual population of the dissociation channel by competing effectively with ultrafast intersystem crossing to the triplet state.34 The actual reaction coordinate that leads to dissociation to form the ArO and NO radicals remains elusive, but the experimental and computational results presented herein and elsewhere31,32,34 suggest that the dissociation possibly originates from the Sdiss(CT) state, where the nitro group acquires a pyramidal conformation with a torsion angle of 106° in 1NN and 104° in 2M1NN and with an elongated C−N bond.34 We remark that all the evidence for the putative population of the Sdiss(CT) state relies on indirect experimental data and on the computational results performed at the TD-DFT level of theory. Direct spectroscopic evidence of the reactive state is currently lacking and needed. Our results do show that the triplet state is not the reactive state under the experimental conditions used in this work (i.e., acetonitrile solutions in N2-saturated conditions). In principle, the PECs shown in Figure 2 and elsewhere for 1NN31 can be used to predict the distribution of torsion angles that is available at the time of light absorption at other temperatures. In fact, the PECs calculated in cyclohexane agree satisfactorily with those obtained in vacuum (not shown). Thus, these PECs could be used to make predictions about the photochemistry of these NNDs in the atmosphere at different temperatures.

this work for 2NN in acetonitrile. The prediction that the S1 PEC for 2NN has an energy minimum closer to 0°, instead of 90° as observed for 2M1NN and 1NN, supports the idea that population of the Sdiss(CT) state should be insignificant in 2NN. This is in excellent agreement with the nearly zero photodegradation yield reported in this work in acetonitrile under N2-saturated conditions, under the assumption that the Sdiss(CT) state is the precursor of the dissociation channel. Scheme 1 shows a pictorial representation of a kinetic model that can explain the experimental and computational results Scheme 1. Pictorial Illustration Displaying a Plausible Kinetic Mechanism That Can Explain the Structure−Photoreactivity Relationship Observed for the NNDs in Solution under N2Saturated Conditionsa

a

Excitation of the distribution of nitro-aromatic conformations available in the ground state to the S1 state is proposed to result in bifurcation to two primary relaxation pathways: (1) intersystem crossing to a receiver triplet (Tn) state with nπ* character (left) followed by internal conversion (IC) to the lowest-energy triplet (T1) state and (2) population of an intramolecular charge-transfer state, which eventually leads to dissociation, forming the aryloxy (ArO) and nitrogen(II) oxide (NO) radicals through a yet unknown reaction coordinate (right). This dissociation channel is not observed in 2NN because of a sizable energy barrier (ca. 6 kcal/mol) that must be surmounted to populate the intramolecular charge-transfer state. The generic yellow Gaussian-shaped symbol represents different distributions of torsion angles available for each molecule in the ground state, the location of which is not the same in each molecule (see Figure 2 and Figure 3 in ref 31). The red, green, and orange Gaussian-shaped symbols represent the fraction of population that bifurcates to each of the main relaxation channels (not shown at scale). It is difficult to convey a mechanism that includes multiple excited states and requires multiple reaction coordinates in a generic two-dimensional scheme. Thus, caution should be exercise to avoid over interpretation of this pictorial illustration.

4. CONCLUSIONS It is shown that a correlation exists between the magnitude of the photodegradation and triplet quantum yields and the region of configuration space initially accessed in the S1 PES, which is controlled by the distribution of nitro-aromatic torsion angles thermodynamically available in 2NN, 1NN, and 2M1NN at the time of light absorption. Strikingly, we found an inverse relationship between the magnitude of the photodegradation and the triplet yields: whereas the photodegradation quantum yield increases when the distribution of torsion angles is closer to 90°, the triplet quantum yield decreases in a corresponding and dramatic fashion. The results presented in this work show that there is an important relationship between conformational heterogeneity before light absorption and the photochemistry of these NNDs. It is expected that other electronic−structure parameters may also play a role in the observed structure−

presented in this work regarding the S1-state population branching and photochemistry of these NNDs. Excitation of 2M1NN, which has a ground-state distribution of torsion angles closer to 90°, results in an increase in population transfer toward the charge-transfer channel, while excitation of 2NN, which has a distribution of torsion angles closer to 0°, results in an increase in population transfer toward the triplet manifold. Excitation of 1NN, which has an intermediate distribution of torsion angles compared to that of 2M1NN and 2NN, results in an intermediate population branching to the triplet manifold and to the Sdiss(CT) state compared to that in 2M1NN and 2NN. The conformational heterogeneity in the ground state of these NNDs results in the excitation of a subgroup of molecules with different torsional angles (highlighted in gray in Figure 2) to different regions of configuration space in the S1 PES. It is the 14106



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photoreactivity relationships observed in this work;67 however, it is clear that the distribution of the nitro-aromatic torsion angle available at room temperature is one of such important factors. Interestingly, a correlation also seems to exist between the mutagenic and carcinogenic structure−activity relationship of these NNDs68 and the structure−photoreactivity relationship presented in this work: the higher the triplet yield, the more mutagenic the NNDs seem to be. Similar correlations have been observed previously between the photolysis rates and the toxicity of NNDs.1,24,28 If these correlations can be generalized to other NNDs and NPAHs, simple photochemical experiments could be used as an affordable and convenient method to screen and prioritize NPAHs before more costly and involved experimental analyses using laboratory animals are performed to test for the biological activity of these compounds.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 1-(216)-368-1911. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was partially supported by the donors of the American Chemical Society Petroleum Research Fund. The authors thank the Mississippi Center for Supercomputer Research and the Ohio Supercomputer Center for generous allotment of computer time.

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