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.

Excited-State Dynamics of the Thiopurine Prodrug 6-Thioguanine: Can N9-Glycosylation Affect Its Phototoxic Activity?

“…The excited-state character was estimated from visual inspection of the Kohn-Sham orbitals and oscillator strengths. The percentage of single-electron contribution (y%) to the vertical excitation energies was calculated using the following expression as done previously for TD-DFT calculations: [ 65 , 66 ]: where single-electron transition.…”

Ab Initio Study of Decay Dynamics of 1-Nitronaphthalene Initiated from the S2(ππ* + nNOπ*) State

“…Observation of pyrenoxy radical produced simultaneously with T 1 within laser pulse indicates that the generation of aryloxy radical takes place from higher excited states. Recently, Crespo-Hernández and co-workers proposed aryloxy radical could be formed via an intramolecular charge-transfer state with dissociative character S diss (CT), competition with intersystem crossing from the lowest S 2 (ππ*) state via a high-energy receiver triplet state T n (nπ*) that internally converts to the lowest-energy triplet state. , The predicted wavelength dependence of yields of T 1 and aryloxy radical for 1,6-dintiropyrene may mostly because of the energy suggest the presence of an energy barrier leading to aryloxy radical formation in the singlet manifold . On the other hand, the triplet manifold was supported by Ushida and co-workers on the basis of phosphorescence and laser flash photolysis experiments results on 9-nitroanthracence.…”

Effect of Noncovalent Interactions on the Intersystem Crossing Behavior in Charge-Transfer Cocrystals of 3,5-Dinitrobromobenzene

“…It was known that nitro-aromatics undergo an efficient ultrafast ISC between lowest excited singlet and triplet state manifold, results phosphorescence emission. − Our aim in this work is to study the optical behavior, in particular, ISC followed by phosphorescence of DNBB and its binary cocrystals. Therefore, the microcrystalline CT complexes and DNBB were dispersed in a MeOH and EtOH (1:4) solvent mixture .…”

“…Nitro-polycyclic aromatic hydrocarbons (PAHs) are considered as environmental pollutants. In order to remove these pollutants through photochemical degradation, the excited state relaxation pathways in solution were studied extensively. − It is important to note that the orientation of the nitro group with respect to the aromatic ring dictates the photoreactivity of nitro-PAHs which is called as Chapman’s orientation-photoreactivity relationship. The relaxation studies indicate that, upon photoexcitation of nitro-aromatics, the molecule may undergo intramolecular nitro–nitrite rearrangement and forms a nitrite intermediate if the nitro group is perpendicular to the aromatic ring .…”

“…The relaxation studies indicate that, upon photoexcitation of nitro-aromatics, the molecule may undergo intramolecular nitro–nitrite rearrangement and forms a nitrite intermediate if the nitro group is perpendicular to the aromatic ring . However, if the nitro group is in plane with the aromatic ring, the molecule which is in the S 1 state may undergo ultrafast nonradiative intersystem crossing (ISC) to the isoenergetic triplet state. − These results motivated us to study the nitro-aromatics in the solid state to understand the impact of the “nitro group” on photophysical properties. Our group and others have reported recently the significance and ability of luminescent coordination polymers , and metal organic frameworks in the detection of nitro-aromatics.…”