Understanding the dynamics of the electronically excited states of nitrated polycyclic aromatic hydrocarbons (NPAHs) is of great importance since photochemical reactions determine the atmospheric stability of these toxic pollutants. From previous studies, it is known that electronically excited NPAHs evolve through two parallel pathways: The formation of the first triplet state and the dissociation of nitrogen (II) oxide. In this contribution, we present the first time-resolved emission measurements of the singlet excited states which are the precursors in the aforementioned photoprocesses. We analyzed 1-nitronaphthalene, 9-nitroanthracene, 1-nitropyrene, 6-nitrochrysene, and 3-nitrofluoranthene in solution samples. Although these compounds are considered nonfluorescent, with the frequency up-conversion method it was possible to detect the emission from the S1 states despite their femtosecond and picosecond lifetimes. Except for 1-nitronapthalene, where a single exponential is observed, for the rest of the compounds, the emission shows double-exponential decays indicating ultrafast structural changes in the excited states. From anisotropy measurements, we conclude that no significant internal conversion occurs in the singlet manifold after excitation in the first absorption band. In accord with El-Sayed rules and with previous calculations, the highly efficient intersystem crossing implied by the large triplet yields and the ultrafast S1 decays is accounted by the pi-pi* nature of the S1 and T1 states together with the existence of higher triplet configurations which act as receiver states. Our measurements show that NPAHs have the largest intersystem crossing rates observed to date in an organic molecule.

The rotational dynamics of 6,7- and 5,7-dihydroxy-4-methylcoumarin in a series of linear alcohols have been studied by time-resolving their fluorescence anisotropy decay with the frequency up-conversion method. Through estimations of their rotational diffusion coefficients in a series of linear alcohols, it was verified that these two coumarins keep nearly the same hydrodynamic contributions to friction, which accounts for only about 35% of the observed reorientational times. Whereas the former compound has the two -OH groups bonded to adjacent carbon atoms in the aromatic frame, in the latter compound, the two hydroxyl groups are separated by enough space to develop more stable interactions involving a network of several solvent molecules. These findings show that this structural difference results in significantly slower rotational relaxation for the 5,7-dihydroxylated coumarin as a result of specific hydrogen-bonding networks as determined at B3LYP/6-311G(d,p) level of theory.