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

Solvent Effects on the Singlet-Triplet Couplings in Nitroaromatic Compounds

Nitrated polycyclic molecules can present the largest singlet-triplet crossing rates among organic molecules. This implies that most of these compounds have no detectable steady-state fluorescence. In addition, some nitroaromatics undergo a complex series of photoinduced atom rearrangements that result in nitric oxide dissociation. The overall photochemistry of these systems depends critically on the competition between the rapid intersystem crossing channel and other excited-state pathways. In this contribution, we sought to characterize the degree of stabilization of the S₁ state due to solute-solvent interactions, and to quantify the effect of such stabilization on their photophysical pathways. We studied 2- and 4-nitropyrene (2-NP and 4-NP), which are atypically emissive nitroaromatics in a series of solvents. From steady-state and time-resolved measurements, the S₁ state of these molecules shows significant stabilization as the solvent polarity is increased. On the other hand, specific triplet states that are iso-energetic with the emissive singlet (T₃ for 2-NP and T₂ for 4-NP) in nonpolar solvents become slightly de-stabilized upon increasing the solvent polarity. These combined effects result in rapid singlet-triplet population transfer in nonpolar solvents for both molecules. In contrast, for solvents with even slightly higher polarities, the first excited singlet is stabilized in relation to the specific triplet states, leading to much longer S₁ lifetimes. These effects can be summarized as a highly solvent-dependent coupling/decoupling of the manifolds. Similar effects are also likely to be present in other nitroaromatics where there is a dynamic competition between nitric oxide dissociation and intersystem crossing. The drastic effects of the solvent polarity in the manifold crossing pathway should be taken into consideration in both theoretical and experimental studies of nitroaromatics.

Effect of electron-donating substitution on the triplet state reactivities of 1-nitronaphthalene

Nitro-polycyclic aromatic hydrocarbons (nitro-PAHs), often found in polluted air, are carcinogenic and mutagenic. The nitro group increases the spin-orbit coupling and results in the lowest excited triplet (T₁) on the picosecond time scale with a high yield. The electron-donating substituents have a significant influence on the photophysics and photochemistry of nitro-PAHs. We used transient absorption spectroscopy and kinetic analysis to investigate the reactivities of the T₁ state 1-methoxy-4-nitronaphthalene (³MeO-NN) and 1-methyl-4-nitronaphthalene (³Me-NN). The results show that the methoxy and methyl substitutions have a minor effect on their hydrogen abstraction and electron accepting abilities. The main distinction is their reaction rates towards protons. The second order rate constant of ³MeO-NN towards protons is three orders of magnitude greater than that of ³Me-NN, indicating that ³MeO-NN has a stronger hydrogen bond accepting ability. The kinetic analysis reveals that the dimer of 2,2,2-trifluoroethanol participates in the reaction with ³MeO-NN. These results suggest that the formation of the hydrogen-bonded complex is responsible for the unusually short lifetime of ³MeO-NN in methanol solution and the lack of hydrogen abstraction radicals during the decay of ³MeO-NN in methanol.

Excited state dynamics and photochemistry of nitroaromatic compounds

Nitrated aromatic molecules have unique photoinduced channels. Due to the presence of oxygen-centered non-bonding orbitals, they can undergo sub-picosecond intersystem crossing showing one of the strongest couplings between the singlet and triplet manifolds among organic molecules. Several nitroaromatic compounds also have a distinctive nitric oxide photodissociation channel which occurs through a complex sequence of atom rearrangements and state changes. These remarkable processes have stimulated the attention of several research groups over the last few years who have applied modern femtosecond spectroscopies and new computational methods to these topics. Nitroaromatic molecules also have demonstrated their value as case-studies, where they can serve to understand the influence of torsional motions between the nitro substituent and the aromatic system in the conversions between states. In this contribution we highlight several of the recent results in this area. Due to the importance of the atmospheric photochemistry of nitrated compounds and their accumulating applications as nitric oxide release agents, continued research about the effects of the different state orderings, substitution patterns, and solvent effects is central to the development of future applications and for a better understanding of their environmental pathways. From this analysis, several pending issues are highlighted, which include the nature of the dominant singlet state involved in intersystem crossing, the role of the formation of charge-transfer states, the yield of the internal conversion channel to the electronic ground state, and a more generalized understanding of the sequence of steps which lead to nitric oxide dissociation.

Modeling the Formation, Degradation, and Spatiotemporal Distribution of 2-Nitrofluoranthene and 2-Nitropyrene in the Global Atmosphere

Polycyclic aromatic hydrocarbons (PAHs) are common atmospheric pollutants and known to cause adverse health effects. Nitrated PAHs (NPAHs) are formed in combustion activities and by nitration of PAHs in the atmosphere and may be equally or more toxic, but their spatial and temporal distribution in the atmosphere is not well characterized. Using the global EMAC model with atmospheric chemistry and surface compartments coupled, we investigate the formation, abundance, and fate of two secondarily formed NPAHs, 2-nitrofluoranthene (2-NFLT) and 2-nitropyrene (2-NPYR). The default reactivity scenario, the model with the simplest interpretation of parameters from the literature, tends to overestimate both absolute concentrations and NPAH/PAH ratios at observational sites. Sensitivity scenarios indicate that NO₂-dependent NPAH formation leads to better agreement between measured and predicted NPAH concentrations and that photodegradation is the most important loss process of 2-NFLT and 2-NPYR. The highest concentrations of 2-NFLT and 2-NPYR are found in regions with strong PAH emissions, but because of continued secondary formation from the PAH precursors, these two NPAHs are predicted to be spread across the globe.