On the Primary Reaction Pathways in the Photochemistry of Nitro-Polycyclic Aromatic Hydrocarbons

Abstract

The primary reaction pathways in the photochemistry of nitro-polycyclic aromatic hydrocarbons under specific laboratory conditions are briefly summarized. In addition, photochemical data is presented for 2-nitronaphthalene, 1-nitronaphthalene, and 2-methyl-1-nitronaphthalene in cyclohexane and acetonitrile solutions under aerobic and anaerobic conditions. It is shown that molecular oxygen significantly reduces the photodegradation quantum yield of 1-nitronaphthalene and 2-methyl-1-nitronaphthalene by 63% and 81%, respectively, whereas 2-nitronaphthalene is photoinert in both solvents under aerobic and anaerobic conditions. In addition, it is proposed that recombination of the aryl and nitrogen (IV) dioxide geminate radical pair within the solvent cage or internal conversion of an initially formed intramolecular charge transfer state may play an important role in the fraction of excited molecules that returns to the ground state in 1-nitronaphthalene and 2-methyl-1-nitronaphthalene. Scavenging of radical species by molecular oxygen and the generation of singlet oxygen in high yield are proposed to contribute to the photochemistry of these nitro-naphthalene derivatives in solution.

Nonadiabatic Dynamics Simulation Predict Intersystem Crossing in Nitroaromatic Molecules on a Picosecond Time Scale

“…Instead, we attributed relaxation within the singlet manifold to the ∼100 fs process, which is then followed by ISC on a defiantly longer ∼1 ps time scale. Based on our simulation, a new deactivation mechanism for 2NN was proposed which, while differing from the one suggested previously, could still explain the time constants for electronic decay processes measured in the same experimentally studies. Intrigued by these controversial results, in this work we investigate the excited‐state dynamics of another two nitronaphthalene (NN) derivatives, 1‐nitronaphthalene (1NN) and 2‐methyl‐1‐nitronaphthalene (2M1NN), for which ISC has also been suggested to occur on a femtosecond time scale.…”

“…Experimental excited‐state dynamics studies of the NN derivatives were conducted in solvent . The experimental observations suggest that the main influence of the solvent results in small changes of the measured time constants by a factor up to 2.…”

“…Despite this general rule of thumb, a number of organic molecules have been reported to display ultrafast ISC on a sub‐picosecond time scale after photoexcitation, despite possessing small SOCs. Of these molecules, a large part belong to the class of nitro polycyclic aromatic hydrocarbons (NPAHs) or the closely related nitrobenzene derivatives . The assumptions about the processes occurring after excitation and the existence of ultrafast ISC are derived mainly from the interpretation of time‐resolved spectroscopic experiments.…”

Solvent-dependent dual fluorescence of the push–pull system 2-diethylamino-7-nitrofluorene

“…The most likely candidate for this long-lived excited state species is a triplet state, 39 similar to what has been observed in nitro-aromatic compounds. [40][41][42][43][44][45] An alternative scenario would be the formation of a photoproduct; [46][47][48] however this would not result in a concomitant rise on the ms timescale of the ground state bleach, as is observed in the current data. The formation of a triplet state provides a non-radiative excited state deactivation mechanism for the initially excited singlet state and more importantly, illustrates how the solvent impacts the PES and completely changes the photophysics in cyclohexane compared to the polar solvents.…”

“…Furthermore, the energy gap between the interacting singlet and triplet states is essentially zero (DE = 0.001 eV). An ultrafast ISC process, happening on a sub 100 fs timescale, has been shown to occur in other nitro containing aromatic compounds [40][41][42][43][44][45][49][50][51][52][53][54][55][56] and could explain the appearance of the ESA band for de-ANF in cyclohexane, despite our previous assumption that the ISC channel was closed for de-ANF. 30 However, no ISC is apparent in the dynamics observed in the polar solvents.…”