Primary Photochemistry of Nitrated Aromatic Compounds: Excited-State Dynamics and NO^·Dissociation from 9-Nitroanthracene

Abstract

We report results of femtosecond-resolved ex-periments which elucidate the time scale for the primary photoinduced events in the model nitroaromatic compound 9-nitroanthracene. Through time-resolved fluorescence measurements, we observed the ultrafast decay of the initially excited singlet state, and through transient absorption experiments, we observed the spectral evolution associated with the formation of the relaxed phosphorescent T(1) state. Additionally, we have detected for the first time the accumulation of the anthryloxy radical which results from the nitro-group rearrangement and NO(•) dissociation from photoexcited 9-nitroanthracene, a photochemical channel which occurs in parallel with the formation of the phosphorescent state. The spectral evolution in this molecule is highly complex since both channels take place in similar time ranges of up to a few picoseconds. Despite this complexity, our experiments provide the general time scales in which the primary products are formed. In addition, we include calculations at the time-dependent density functional level of theory which distinguish the molecular orbitals responsible for the n-π* character of the "receiver" vibronic triplet states that couple with the first singlet state and promote the ultrafast transfer of population between the two manifolds. Comparisons with the isoelectronic compounds anthracene-9-carboxylic acid and its conjugated base, which are highly fluorescent, show that in these two compounds the near-isoenergeticity of the S(1) with an appropriate "receiver" triplet state is disrupted, providing support to the idea that a specific energy coincidence is important for the ultrafast population of the triplet manifold, prevalent in polycyclic nitrated aromatic compounds.

Stilbene photoisomerization driving force as revealed by the topology of the electron density and QTAIM properties

“…The favouring of one mechanism over the others depends strongly on the nature of the substituents of stilbene [7][8][9][10]. This notion is consistent with the observation that different functional groups are capable of promoting or impairing several photophysical relaxation pathways for deactivation of excited states to the ground state [11][12][13].…”

Investigation of Photophysical Properties of Ceric Ammonium Nitrate Mediated Oxidative Dimerization of 1-Methoxynaphthalene and 4,4ʹ-Diamino-1,1ʹ-dimethoxy-2,2ʹ-binaphthyl

“…This study explains the technological importance especially for constructing light emitting diodes (LEDs), photovoltaic cells, organic solar cells and artificial or model photosynthetic devices, etc [13][14][15][16][17] . Generally, p-extended compounds in which the nitro group is introduced directly into the fluorophore show very weak fluorescent as compared to unsubstituted compounds, since the nitro group elongates the path for internal conversion immediately after intersystem crossing [18][19][20][21][22] . The fluorescence of nitro compound was investigated by Ajro Hachia et al, 23 .…”

Comparison of photoreactions of flutamide in acetonitrile and 2-propanol solvents in the absence of cage-forming compounds

“…Aromatic nitro compounds are known to undergo various photoreactions, such as photosubstitution, photoredox reactions, photodissociation and photo-induced nitro-nitrite rearrangement [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17]. Photosubstitution and photoredox reactions have been well investigated [1][2][3][4][5][6][7][8][9][10][11], but photodissociation and nitro-nitrite rearrangement have not yet been fully elucidated [10][11][12][13][14][15][16][17].…”

“…Photosubstitution and photoredox reactions have been well investigated [1][2][3][4][5][6][7][8][9][10][11], but photodissociation and nitro-nitrite rearrangement have not yet been fully elucidated [10][11][12][13][14][15][16][17]. Nitro-nitrite rearrangement and photodissociation have low reaction efficiencies, and the short-lived intermediates of these reactions are difficult to observe.…”