Conformational control in the population of the triplet state and photoreactivity of nitronaphthalene derivatives

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 1-nitronaphthalene (1NN), 2-methyl-1-nitronaphthalene (2M1NN), and 2-nitronaphthalene (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.