HR: 17:50h
AN: B44A-07    [Abstracts]
TI: Decay of the Singlet Excited States of Nitro-Polycyclic Aromatic Hydrocarbons and its Relevance in the Photochemical Degradation
AU: * Peon, J
EM: jpeon@servidor.unam.mx
AF: Instituto de Quimica, Universidad Nacional Autónoma de México, Instituto de Química, Ciudad Universitaria, 04510, México, D.F., México., Mexico City, 04510, Mexico
AB: Nitro-polycyclic aromatic hydrocarbons (NPAHs) are important toxic pollutants formed directly during fuel combustion and indirectly by atmospheric nitration of polyaromatic hydrocarbons. Since photochemical reactions determine the atmospheric ambient stability of these compounds, understanding the dynamics of their singlet excited states is of great importance. Electronically excited NPAHs evolve through two parallel pathways: Intersystem crossing leading to the formation of the first triplet state and the photoinduced 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 these primary photo-processes. We analyzed 1-nitronaphthalene, 9- nitroanthracene, 1-nitropyrene, 6-nitrochrysene and 3-nitrofluoranthene in solution samples. 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. Our measurements show that NPAHs have the largest intersystem crossing rates observed to date in an organic molecule. The n* nature of the S1 and T1 states together with the presence of upper n* triplet states nearly isoenergetic to S1 accounts for the ultrafast triplet formation rates. This appears to be a common feature in all NPAHs.
DE: 0300 ATMOSPHERIC COMPOSITION AND STRUCTURE
DE: 0320 Cloud physics and chemistry
DE: 0345 Pollution: urban and regional (0305, 0478, 4251)
DE: 0360 Radiation: transmission and scattering
DE: 0394 Instruments and techniques
SC: Biogeosciences [B]
MN: 2007 Joint Assembly