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Polycyclic aromatic hydrocarbons (PAHs) and nitro-PAHs comprise a significant fraction of the mass composition of the emitted particulate matter. They have been identified as mutagenic or carcinogenic agents representing a potential human health risk. While the fate of PAHs is relatively well understood much less is known about the fate of nitro-PAHs in the environment. Photochemical transformation is one of the primary routes of natural removal of nitro-PAHs in the environment. Studies suggest that the composition of the liquid organic layer surrounding the elemental carbon core of an aerosol significantly influences their photochemistry. However, there is limited knowledge regarding the kinetics and photo-induced relaxation pathways of nitro-PAHs in the environment; primarily because it requires large expenditures of money, time, and equipment. Consequently, development of laboratory models enabling accurate and quantitative measurements of these photochemical properties is essential for modeling the concentration and persistence of nitro-PAHs in the environment. This is because at the heart of a photochemical reaction are competing relaxation pathways that alter the extent to which the absorption of light is effective in producing a chemical transformation. Such deactivation pathways (i.e., radiative and nonradiative transitions) take the excited-state population back to the ground state and hence are expected to play a central role in the photochemical outcome of nitro-PAHs.

Our group has initiated a research program aimed at understanding the photochemistry of nitro-PAHs in different solvents and co-solute mixtures used as simple models of the organic liquid layer in aerosol particles. Of particular interest is to provide fundamental knowledge about the environmental, physicochemical, and structural factors that modulate the photochemistry of nitro-PAHs at the molecular level. This background information is essential to develop kinetic models and to predict the role that sunlight can have on the degradation of nitro-PAHs in the environment. During the first year of this grant, we have investigated the steady-state and time-resolved photochemistry of 1-nitronaphthalene (1NN), 2-nitronaphthalene (2NN) and 2-methyl-1-nitronaphthalene (2M1NN) upon UVA (320 to 400 nm) radiation in non-polar, polar aprotic and protic solvents. The experiments have been complemented with quantum chemical calculations that have been instrumental in guiding the interpretation of the experimental results.

Ground-state optimizations show that the nitro-aromatic torsion angle in these compounds increases from 0.1° in 2NN to 56.8° in 2M1NN; with 1NN showing an intermediate torsion angle of 33.1° at the density functional level of theory (DFT). The calculations show that this torsion angle varies slightly with solvent. The torsion angle is modulated by steric forces acting between the oxygen atoms on the nitro-group and their neighboring atoms on the naphthalene moiety. In the case of 1NN, a peri hydrogen atom forces the torsion angle away from 0°. In 2M1NN, a methyl-group 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 why an almost planar conformation is obtained for this molecule. The ground-state calculations have also shown that a distribution of torsion angles with energies lower than the thermal energy (k_BT) is accessible at room temperature in these three compounds. The distribution of torsion angles increases in going from 2NN to 2M1NN indicating that resonance stabilization between the nitro-group and the naphthalene moiety also plays a role in this distribution. Strikingly, a direct correlation is seen between the torsion angle distribution and the experimentally measured photodegradation quantum yield, with 2M1NN showing the highest photodegradation yield and 2NN being photo-inert. In line with these results, 2NN shows a small but measurable fluorescence while 1NN and 2M1NN do not.

We have used femtosecond transient absorption spectroscopy to investigate the excited state dynamics of 2NN, 1NN and 2M1NN in polar and non-polar solvents. The time-resolved experiments have been complemented with excited-state calculations at the DFT level of theory that include solvent effects. We have recently shown that UVA excitation of 1NN and 2M1NN results in ultrafast branching of the initial excited singlet state population (S₁, ππ*) to two nonradiative decay channels. The main decay channel connects the S₁ state with a receiver triplet state (T_n) that has considerable nπ* character. The receiver T_n state undergoes internal conversion to the lowest-energy triplet state (T₁, ππ*) in 0.5 to 4 picoseconds depending on the solvent used. The vibrationally excited T₁ state relaxes by energy transfer to the solvent with average lifetimes in the range from 6 to 12 ps depending on the solvent used. The relaxed T₁ state undergoes intersystem crossing back to the ground state within a few microseconds in N₂-saturated solutions.

The second minor channel involves conformational relaxation of the S₁ state in less than 200 femtoseconds (primarily rotation of the NO₂-group to a torsion angle of 90°) to a dissociative singlet state with significant charge-transfer character and negligible oscillator strength. This dissociative channel is proposed to be responsible for the formation of nitrogen(II) oxide (NO) and aryloxy (ArO) radicals and for the observed photochemistry in 1NN and 2M1NN. For 2NN, a sequential relaxation mechanism is proposed. UVA excitation of 2NN results in the population of the S₁ state that relaxes to the S₁ energy minimum (primarily rotation of the NO₂-group to a torsion angle of 0°) in ultrafast time scales. A small fraction of the relaxed S₁ state decays by fluorescence emission to the ground state in 3 picoseconds. The largest fraction of the S₁ state population decays by intersystem crossing to the T_n state, which populates the T₁ state with excess vibrational energy as in 1NN and 2M1NN. The T₁ state returns to the ground state in a microsecond time scale after transferring its excess vibrational energy to the solvent.

Our results satisfactorily explain the observed steady-state photochemistry in these nitronaphthalene compounds. The ultrafast nature of the dissociative channel in 1NN and 2M1NN suggests that the distribution of torsion angles in the ground-state controls whether or not the NO and ArO radicals are formed; revealing a strong link between conformation before light absorption and photochemistry. Work is in progress to validate this hypothesis in other nitro-PAHs.