title: The Photochemical Branching Ratio in 1,6-Dinitropyrene Depends on the Excitation Energy.
abstract: Nitropolycyclic aromatic hydrocarbons constitute one of the most disconcerting classes of pollutants. Photochemical degradation is thought to be a primary mode of their natural removal from the environment, but the microscopic mechanism leading to product formation as a function of excitation wavelength is poorly understood. In this Letter, it is revealed that excitation of 1,6-dinitropyrene with 425, 415, or 340 nm radiation leads to an increasing amount of radical production through photodissociation at the expense of triplet-state population-the two primary reaction pathways in this class of pollutants. Radical formation requires overcoming an energy barrier in the excited singlet manifold. This activation energy explains the large fraction of the initial singlet-state population that intersystem crosses to a doorway triplet state, instead of leading overwhelmingly to photodissociation. The unforeseen excitation wavelength dependence of this branching process is expected to regulate the photochemistry of 1,6-dinitropyrene and possibly of other nitroaromatic pollutants in the environment.
authors.affiliation: Department of Chemistry, Case Western Reserve University , 10900 Euclid Avenue, Cleveland, Ohio 44106, United States.
authors.forename: Matthew M
authors.initials: MM
authors.lastname: Brister
authors.affiliation: Department of Chemistry, University of Puerto Rico at Humacao , Humacao Campus, Humacao, Puerto Rico 00792.
authors.forename: Luis E
authors.initials: LE
authors.lastname: Piñero-Santiago
authors.affiliation: Department of Chemistry, University of Puerto Rico , Río Piedras Campus, San Juan, Puerto Rico 00931.
authors.forename: María
authors.initials: M
authors.lastname: Morel
authors.affiliation: Department of Chemistry, University of Puerto Rico , Río Piedras Campus, San Juan, Puerto Rico 00931.
authors.forename: Rafael
authors.initials: R
authors.lastname: Arce
authors.affiliation: Department of Chemistry, Case Western Reserve University , 10900 Euclid Avenue, Cleveland, Ohio 44106, United States.
authors.forename: Carlos E
authors.identifier: http://orcid.org/0000-0002-3594-0890
authors.initials: CE
authors.lastname: Crespo-Hernández