Large Polycyclic Aromatic Hydrocarbons Research Articles

Functionalized Contorted Polycyclic Aromatic Hydrocarbons by a One-Step Cyclopentannulation and Regioselective Triflyloxylation.

The oxidative cyclodehydrogenation (often named the Scholl reaction) is still a powerful synthetic tool to construct even larger polycyclic aromatic hydrocarbons (PAHs) by multiple biaryl bond formations without the necessity of prior installation of reacting functional groups. Scholl-type reactions are usually very selective although the resulting products bear sometimes some surprises, such as the formation of five-membered instead of six-membered rings or the unexpected migration of aryl moieties. There are a few examples, where chlorinated byproducts were found when FeCl3 was used as reagent. To our knowledge, the direct functionalization of PAHs during Scholl-type cyclization by triflyloxylation has not been observed. Herein we describe the synthesis of functionalized PAHs by the formation of five-membered rings and a regioselective triflyloxylation in one step. The triflyloxylated PAHs can be used as reactants for further transformation to even larger contorted PAHs.

Using Density Functional Theory Based Methods to Investigate the Photophysics of Polycyclic Aromatic Hydrocarbon Radical Cations: A Benchmark Study on Naphthalene, Pyrene and Perylene Cations

AbstractUnrestricted DFT (UDFT), time‐dependent DFT (TDDFT) and spin‐flip TDDFT (SF‐TDDFT) were used to investigate the potential energy surfaces of the ground and first two electronic excited states of the naphthalene, pyrene and perylene radical cations. In particular, conical intersections (which play a central role in the photophysics of these cations) were located with these DFT‐based methods. The results are consistent with accurate multiconfigurational wavefunction‐based ab initio methods. These show that naphthalene and pyrene cations can quickly relax nonradiatively from their excited states back down to the original ground state species through easily accessible conical intersections, but the perylene cation cannot do so, due to the absence of any accessible funnels between the lowest excited state and the ground state, leaving radiative decay as the most probable photophysical pathway. This study paves the way for using computationally efficient density functional theory (DFT)‐based methods in future investigations of the photophysics of much larger polycyclic aromatic hydrocarbons, for which multiconfigurational wavefunction‐based methods become prohibitively expensive.

Photomodulation of Two-Dimensional Self-Assembly of Azobenzene–Hexa-peri-hexabenzocoronene–Azobenzene Triads

Achieving exquisite control over self-assembly of functional polycyclic aromatic hydrocarbons (PAH) and nanographene (NG) is essential for their exploitation as active elements in (nano)technological applications. In the framework of our effort to leverage their functional complexity, we designed and synthesized two hexa-peri-hexabenzocoronene (HBC) triads, pAHA and oAHA, decorated with two light-responsive azobenzene moieties at the pseudo-para and ortho positions, respectively. Their photoisomerization in solution is demonstrated by UV–vis absorption. 1H NMR measurements of oAHA suggested 23% of Z-form can be obtained at a photostationary state with UV irradiation (366 nm). Scanning tunneling microscopy imaging revealed that the self-assembly of pAHA and oAHA at the solid–liquid interface between highly oriented pyrolytic graphite (HOPG) and their solution in 1,2,4-trichlorobenzene can be modulated upon light irradiation. This is in contrast to our previous work using HBC bearing a single azobenzene moiety, which did not show such photomodulation of the self-assembled structure. Upon E-Z isomerization both pAHA and oAHA displayed an increased packing density on the surface of graphite. Moreover, pAHA revealed a change of self-assembled pattern from an oblique unit cell to a dimer row rectangular crystal lattice whereas the assembly of oAHA retained a dimer row structure before and after light irradiation, yet with a modification of the inter-row molecular orientation. Molecular mechanics/molecular dynamics simulations validated the self-assembly patterns of pAHA and oAHA, comprising azobenzenes in their Z-forms. These results pave the way toward use of suitably functionalized large PAHs, as well as NGs, to develop photoswitchable devices.

Sequential Extraction of Coal Tar Pitch and Structural Characterization of Enriched Large Polycyclic Aromatic Hydrocarbons

AbstractFour fractions separated sequentially from a coal tar pitch by two different methods are examined by matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry (MALDI TOF‐MS) and 50/24 matrix analysis, to evaluate both separation effectiveness and characteristics of large polycyclic aromatic hydrocarbons (PAHs) in variable fractions. Both Soxhlet extraction and anti‐solvent precipitation can sequentially produce four fractions that have narrower molecular mass distribution with specific structural features of PAHs. The results show that the overall molecular mass (MM) and ring size of large PAHs abundant in the fractions gradually shift to higher values as solvent strength increases, with some overlap between two adjacent fractions. A broad range of peaks in MS spectra has been resolved as 4‐ to about 25‐ ring PAHs by 50/24 matrix analysis, with some detailed molecular structure elucidated. For example, the PAHs in the light HS fraction include 4 to 11‐ring aromatics with more ortho‐condensed structure. On the other hand, heavier fractions contain more aromatic rings fused by bay‐condensations that result in quaternary aromatic carbons. Besides structural insights, SEM morphologically reveals differences in the extraction effectiveness between Soxhlet and anti‐solvent, due to mass transfer and diffusion limitations in bituminous material.

A chemical kinetic modeling study of indene pyrolysis

An improved polycyclic aromatic hydrocarbon (PAH) model is developed to predict the decomposition of indene and the formation of large PAHs under pyrolytic conditions. This model is developed based on experimental study of pyrolytic kinetics of indene in a flow reactor at low and atmospheric pressures (30 and 760 Torr) by using synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). A general map of PAH growth is presented according to the observations in this study and those in literature. Indene dissociates via indanyl forming mono-cyclic aromatics and small intermediates, while its dominant decomposition product is indenyl. As a resonantly stabilized radical, indenyl serves as a platform molecule in PAH growth process which links small unsaturated hydrocarbons and mono-aromatic species to multi-cyclic ones. Reactions of indenyl radical are proposed to form commonly studied and recently observed PAHs. Rate constants of these reactions are evaluated by analyzing literature data of rate constant measurements, quantum chemical calculations and analogy to cyclopentadienyl radical. The main PAH formation pathways are the bi-molecular addition reactions of indenyl radical with indene and a series of intermediates, forming C10C18 and larger PAHs. Meanwhile, radical chain reactions provide huge passage for PAH growth form one resonantly stabilized radical (RSR) to larger ones. Particular contribution has been found from the reactions of RSRs that have five-member ring in their molecular structures, such as fluorenyl, benz-indenyl, cyclopenta-phenanthrenyl and benzo-fluorenyl.

Dilution and Injection Pressure Effects on Ignition and Onset of Soot at Threshold-Sooting Conditions by Simultaneous PAH-PLIF and Soot-PLII Imaging in a Heavy Duty Optical Diesel Engine

<div class="section abstract"><div class="htmlview paragraph">Although accumulated in-cylinder soot can be measured by various optical techniques, discerning soot formation rates from oxidation rates is more difficult. Various optical measurements have pointed toward ways to affect in-cylinder soot oxidation, but evidence of effects of operational variables on soot formation is less plentiful. The formation of soot and its precursors, including polycyclic aromatic hydrocarbons (PAHs), are strongly dependent on temperature, so factors affecting soot formation may be more evident at low-temperature combustion conditions. Here, in-cylinder PAHs are imaged by planar laser-induced fluorescence (PAH-PLIF) using three different excitation wavelengths of 355, 532, and 633 nm, to probe three different size-classes of PAH from 2-3 to 10+ rings. Simultaneous planar laser-induced incandescence of soot (soot-PLII) using 1064-nm excitation provides complementary imaging of soot formation near inception. To achieve low combustion temperatures at the threshold of PAH and soot formation, the engine operating conditions are highly diluted, with intake-O<sub>2</sub> mole-fractions as low as 7.5%. The optical diagnostics show that increasing dilution delays the inception of PAH by over 2.5 ms as the intake-O<sub>2</sub> mole-fraction decreases from 15.0% to 9.0%. At 7.5% intake-O<sub>2</sub>, no large PAH or soot are formed, while the 9.0% intake-O<sub>2</sub> condition forms PAH but virtually no detectable soot. Conditions with 10.0% or more intake-O<sub>2</sub> form both PAH and soot. For the threshold-sooting condition with 10.0% intake-O<sub>2</sub>, large PAH typically forms broadly throughout the cross-section of the downstream jets and along the bowl-wall. Soot appears after PAH, and in narrower ribbons in the jet-jet interaction region. These soot ribbons are on the periphery of the PAH, near the diffusion flame, where the highest temperatures are expected. With increasing intake-O<sub>2</sub>, the delay time between soot and PAH shortens, and soot tends to shift upstream to the jet region prior to wall impingement, though still on the periphery of the PAH. The spatial distributions of PAH and soot overlap slightly under these threshold-sooting conditions, with soot typically surrounding the PAH. This may indicate that temperatures are only high enough for soot formation on the jet periphery, near the diffusion flame. The minimal overlap also suggests that PAHs are rapidly consumed and/or adsorbed when soot is formed. Additionally, increasing the fuel-injection pressure from 533 to 800 and then to 1200 bar increases soot and large PAH formation, which is opposite to the trend for conventional diesel combustion.</div></div>

Photoprocessing of large PAH cations.

In cosmic environments, polycyclic aromatic hydrocarbons (PAHs) strongly interact with vacuum ultraviolet (VUV) photons emitted by young stars. Trapped PAH cations ranging in size from 30 to 48 carbon atoms were irradiated by tunable synchrotron light (DESIRS beamline at SOLEIL). Their ionization and dissociation cross sections were determined and compared with TD-DFT computed photoabsorption cross sections. Evidence for radiative cooling is reported.

Structural effects on the growth of large polycyclic aromatic hydrocarbons by C2H2

Soot particles are composed of planar and curved polycyclic aromatic hydrocarbons with different types of reactive sites, where their growth and oxidation reactions occur. This study presents the effect of curvatures in PAHs present in soot on their growth in the flame environment. For this, six planar and curved model PAH molecules having five to nine aromatic rings with armchair and zigzag sites are selected. Density functional theory (B3LYP functional and 6-311G (d,p) basis set) is used to study the reaction energetics for their growth through hydrogen-abstraction-C2H2-addition mechanism that is primarily used in all soot models for their chemical growth. The rate constants evaluated using transition state theory for the reactions involved in the mechanism are provided. Through energetics and kinetics comparison, the differences in the reactivity of planar and curved PAHs at different site types are observed. The growth at armchair site was found to be sensitive to the PAH structure, with the curved one having higher growth rate at all temperatures between 1000 and 2500 K studied in this work. The PAH curvature had less impact on the growth at zigzag sites of the model PAHs. The fast conversion of planar PAH to a curved one through ring addition and the high growth rate of curved PAHs at low temperatures suggests that tortuous PAHs can also be present in nascent soot.

Release of large polycyclic aromatic hydrocarbons and fullerenes by cosmic rays from interstellar dust

Context. Top-down chemistry is believed to be responsible for the formation of the large molecular compounds such as the polycyclic aromatic hydrocarbon-like molecules and the fullerenes observed in the interstellar medium. The release of these large molecules from the parent grains remains an important issue to be investigated. Aims. Cosmic rays irradiate the dust grains during their journey in the interstellar medium. In this study we probe to what extent electronic sputtering and/or desorption processes induced by high-energy ion projectiles contribute to the creation of the large molecular component in space. Methods. Carbonaceous dust analogues were produced in an ethylene flame. The resulting soot nanoparticles generated under well-defined conditions were irradiated by swift heavy ions, and mass spectra of the ionic and neutral molecular fragments emitted shortly after the impact were monitored. Results. Large molecular fragments were detected, including neutral and ionic polycyclic aromatic hydrocarbons containing up to several tens of carbon atoms, as well as ionic fullerenes. Although the absolute efficiencies were not obtained, these experiments provide a proof of principle of a top-down scenario involving interaction processes of interstellar dust with high-energy projectiles yielding large molecular compounds observed in space.

Laboratory Photochemistry of Covalently Bonded Fluorene Clusters: Observation of an Interesting PAH Bowl-forming Mechanism

Abstract The fullerene C60, one of the largest molecules identified in the interstellar medium (ISM), has been proposed to form top-down through the photochemical processing of large (more than 60 C atoms) polycyclic aromatic hydrocarbon (PAH) molecules. In this article, we focus on the opposite process, investigating the possibility that fullerenes form from small PAHs, in which bowl-forming plays a central role. We combine laboratory experiments and quantum chemical calculations to study the formation of larger PAHs from charged fluorene clusters. The experiments show that with visible laser irradiation, the fluorene dimer cation—[C13H9−C13H9]+—and the fluorene trimer cation—[C13H9−C13H8−C13H9]+—undergo photodehydrogenation and photoisomerization, resulting in bowl-structured aromatic cluster ions, C26H12 + and C39H20 +, respectively. To study the details of this chemical process, we employ quantum chemistry that allows us to determine the structures of the newly formed cluster ions, to calculate the dissociation energies for hydrogen loss, and to derive the underlying reaction pathways. These results demonstrate that smaller PAH clusters (with less than 60 C atoms) can convert to larger bowled geometries that might act as building blocks for fullerenes, because the bowl-forming mechanism greatly facilitates the conversion from dehydrogenated PAHs to cages. Moreover, the bowl-forming induces a permanent dipole moment that—in principle—allows one to search for such species using radio astronomy.

Temperature effects on anharmonic infrared spectra of large compact polycyclic aromatic hydrocarbons

Aims. Large compact polycyclic aromatic hydrocarbon molecules (PAHs) present special interest in the astrochemical community. A key issue in analyses of large PAHs is understanding the effect that temperature and anharmonicity have on different vibrational bands, and thus interpreting the infrared (IR) spectra for molecules under various conditions. Methods. Because of the huge amount of interactions/resonances in large PAHs, no anharmonic IR spectrum can be produced with static/time-independent ab initio method, especially for the molecules with D6h symmetry, e.g., coronene and circumcoronene. In this work, we performed molecular dynamics simulations to generate anharmonic IR spectra of coronene and circumcoronene. Results. The method is validated for small PAHs, i.e., naphthalene and pyrene. We find that the semiempirical method PM3 produces accurate band positions with an error <5 cm−1. Furthermore, we calculate the spectra at multiple temperatures and find a clear trend toward band shifting and broadening.

Examining the Class B to A Shift of the 7.7 μm PAH Band with the NASA Ames PAH IR Spectroscopic Database

Abstract We present insights into the behavior of the astronomical 7.7 μm polycyclic aromatic hydrocarbon (PAH) emission complex as gleaned from analyzing synthesized spectra, utilizing the data and tools from the NASA Ames PAH IR Spectroscopic Database. We specifically study the influence of PAH size, charge, aliphatic content, and nitrogen substitution on the profile and peak position of the 7.7 μm feature (λ 7.7). The 7.7 μm band is known to vary significantly from object to object in astronomical observations, but the origin of these variations remains highly speculative. Our results indicate that PAH size can accommodate the largest shift in λ 7.7 (≃0.4 μm), where relatively small PAHs are consistent with class A spectra ( ≤ 60), while large PAHs are consistent with red/very red class B spectra. Aliphatic PAHs, of which our sample only contains a few, can produce redshifts typically around 0.15 μm; changes in ionization fraction, depending on the species, produce shifts of up to 0.1 μm; and nitrogen substitution has no effect on λ 7.7. Within the limits of our study, the class B → A transition is best explained with a changing PAH size distribution, with a relatively minor role assigned to aliphatic content and varying charge states. The resulting astronomical picture is that the photochemical evolution of PAHs moving from shielded class C/B environments into exposed ISM-like class A environments may be intrinsically different from the reverse class A → B transition of interstellar PAHs being incorporated into newly forming star systems.

PAH under XUV excitation: an ultrafast XUV-photochemistry experiment for astrophysics

Understanding processes induced by XUV excitation of Polycyclic Aromatic Hydrocarbons (PAHs) is at the heart of molecular astrophysics, which aims at understanding molecular evolution in interstellar media. We used ultrashort XUV pulses to produce highly excited PAHs cations. The photo-induced dynamics is probed using a pump-probe XUV-IR spectroscopy. By studying PAH from small (naphthalene) to large (hexabenzocoronene) PAHs, we show that the dynamic is governed by the large density of states, in which many-body quantum effects are dominant.

Effect of molecular structure on the infrared signatures of astronomically relevant PAHs

Emission bands from polycyclic aromatic hydrocarbons (PAHs) dominate the mid-infrared spectra of a wide variety of astronomical sources, encompassing nearly all stages of stellar evolution. Despite their similarities, details in band positions and shapes have allowed a classification of PAH emission to be developed. It has been suggested that this classification is in turn associated with the degree of photoprocessing of PAHs. Over the past decade, a more complete picture of the PAH interstellar life-cycle has emerged, in which a wide range of PAH species are formed during the later stages of stellar evolution. After this they are photoprocessed, increasing the relative abundance of the more stable (typically larger and compact) PAHs. For this work we have tested the effect of the symmetry, size, and structure of PAHs on their fragmentation pattern and infrared spectra by combining experiments at the free electron laser for infrared experiments (FELIX) and quantum chemical computations. Applying this approach to the cations of four molecular species, perylene (C20H12), peropyrene (C26H14), ovalene (C32H14) and isoviolanthrene (C34H18), we find that a reduction of molecular symmetry causes the activation of vibrational modes in the 7–9 μm range. We show that the IR characteristics of less symmetric PAHs can help explain the broad band observed in the class D spectra, which are typically associated with a low degree of photoprocessing. Such large, nonsymmetrical irregular PAHs are currently largely missing from the NASA Ames PAH database. The band positions and shapes of the largest more symmetric PAH measured here, show the best resemblance with class A and B sources, representative of regions with high radiation fields and thus heavier photoprocessing. Furthermore, the dissociation patterns observed in the mass spectra hint to an enhanced stability of the carbon skeleton in more symmetric PAHs with respect to the irregular and less symmetric species, which tend to loose carbon containing units. Although not a direct proof, these findings are fully in line with the grandPAH hypothesis, which claims that symmetric large PAHs can survive as the radiation field increases, while their less symmetric counterparts are destroyed or converted to symmetric PAHs.

Density functional theory study of CO formation through reactions of polycyclic aromatic hydrocarbons with atomic oxygen (O(3P))

To provide a qualitative analysis on how the reaction energies and mechanisms of initial steps of char combustion depend on a) the size of polycyclic aromatic hydrocarbons (PAHs) and b) the reaction sites, we investigate the initial steps of the individual reactions of thirteen PAHs with atomic oxygen with density functional theory. Specifically, we have studied three elementary steps: i) O(3P) addition, ii) subsequent oxyradical formation through H abstraction, and iii) CO elimination from the oxyradicals. We find that the CO elimination from the oxyradicals follows either a three, two, or single step mechanism, depending on the geometrical strain at the reaction site. In accordance, we observe that as the geometrical strain at the reaction site increases, the activation energy of the first step of the CO elimination from the oxyradicals increases. Important findings that are relevant for a multiscale modeling of the oxidation of large PAHs, e.g. during char burnout, are that the activation energies for these reactions can be related to the reaction energies through Bell-Evans-Polanyi (BEP) fits, and that for the O addition and the last step of the three or two step CO elimination mechanisms, i.e. the CO elimination from carbonyl radicals, the reaction energies are correlated with the stabilization energies calculated from the Hückel molecular orbital theory.

Impact of urban pressure on the spatial and temporal dynamics of PAH fluxes in an urban tributary of the Seine River (France)

Polycyclic aromatic hydrocarbons (PAHs) produced by numerous anthropogenic activities are ubiquitous in the environment and have become a priority concern due to their potential severe biological impacts. A better understanding of PAH transfer at the catchment scale is therefore necessary to improve the management of PAH contaminants and protect rivers. Furthermore, the impact of changes in hydrological regimes and land uses on PAH fluxes should be specifically investigated. Accordingly, the current research monitors the contamination in atmospheric fallout, soils and rivers in a 950-km2 catchment (Orge River) characterized by an increasing urban gradient in downstream direction. During an entire hydrological year, river water contamination was quantified through regular sampling of both particulate and dissolved material at four river-monitoring stations, reflecting the increasing urbanization gradient. The significant input of PAHs from urban areas in downstream river sections corresponded to a specific PAH flux that reached 23 g km−2 y−1 despite the low sediment yield. Moreover, the comparison with runoff-specific fluxes reported in the literature underlined the major impact of urban runoff on the Orge River water and sediment quality. Nevertheless, the annual PAH load exported by the river (21 kg y−1) remained lower than the PAH inputs from atmospheric fallout (173 kg y−1), demonstrating the continuous accumulation of PAH from atmospheric fallout in the catchment soils. Consequently, the notably large PAH stock (close to 1000 tons) resulting from historical contamination of this early-industrialized region continues to increase due to ongoing atmospheric inputs.

Giant Star‐Shaped Nitrogen‐Doped Nanographenes

AbstractStar‐shaped nanographenes (SNGs) are large monodisperse polycyclic aromatic hydrocarbons that are larger than a nanometer and have shown a lot of promise in a wide range of applications including electronics, energy conversion, and sensing. Herein, we report a new family of giant star‐shaped N‐doped nanographenes with diameters up to 6.5 nm. Furthermore, the high solubility of this SNG family in neutral organic solvents at room temperature allowed a complete structural, optoelectronic, and electrochemical characterisation, which together with charge transport studies illustrate their n‐type semiconducting character.

Giant Star-Shaped Nitrogen-Doped Nanographenes.

Star-shaped nanographenes (SNGs) are large monodisperse polycyclic aromatic hydrocarbons that are larger than a nanometer and have shown a lot of promise in a wide range of applications including electronics, energy conversion, and sensing. Herein, we report a new family of giant star-shaped N-doped nanographenes with diameters up to 6.5 nm. Furthermore, the high solubility of this SNG family in neutral organic solvents at room temperature allowed a complete structural, optoelectronic, and electrochemical characterisation, which together with charge transport studies illustrate their n-type semiconducting character.

Effects of large aromatic precursors on soot formation in turbulent non-premixed sooting jet flames

In this paper, we computationally study the effects of large Polycyclic Aromatic Hydrocarbons (PAH) (more than two aromatic rings) on soot yield and distribution in turbulent flames, when they are considered as nucleating species for soot formation. This is examined in two turbulent non-premixed sooting jet flames using ethylene and a jet fuel (JP-8) surrogate as fuels. For each flame, two Large-Eddy Simulations (LES) are performed with two different soot nucleation strategies. In the first strategy, a range of PAH from naphthalene to cyclopenda[cd]pyrene are considered as nucleating species, while in the second strategy, naphthalene is considered as the only soot nucleating species and the effects of larger PAH are represented entirely by naphthalene. Flamelet-based chemistry-tabulation is used for the major thermochemical quantities, such as density, temperature, and major species mass fractions. Turbulence-chemistry interactions for PAH are accounted for by transporting their mass fractions and using a recently developed PAH relaxation model for their source term closure. The effects of large PAH on soot are highlighted by comparing the PAH profiles, soot nucleation rate, and soot volume fraction distributions obtained from both simulations for each test flame. These results are also compared against experimental measurements, when available. From these comparisons, it is first shown that naphthalene is predominantly formed along the flame centerline, and larger PAH species with more than two aromatic rings are primarily formed away from the centerline. Further, it is found that these larger PAH species only contribute to around of the overall soot nucleation rate along the flame centerline but their contributions are more substantial (up to ) away from the centerline. Finally, it is shown that representing the effects of these large PAH species by naphthalene leads to an under-prediction of around for the soot volume fraction magnitude along the flame centerline. Away from the centerline, this under-prediction can be as much as 100

Partitioning of polycyclic aromatic hydrocarbons in heterogeneous clusters

The morphologies of heterogeneous clusters of polycyclic aromatic hydrocarbons (PAHs) are investigated using molecular modelling. Clusters of up to 100 molecules containing combinations of the different sized PAHs circumcoronene, coronene, ovalene, or pyrene are evaluated. Replica exchange molecular dynamics simulations using an all-atom force field parameterised for PAHs sample many configurations at high and low temperatures to determine stable low energy structures. The resulting cluster structures are evaluated using molecular radial distances and coordination numbers, and are found to be independent of initial configuration and the cluster sizes studied. Stable clusters consist of stacked PAHs in a core-shell structure, where the larger PAHs are found closer to the cluster core and the smaller PAHs are located on the cluster surface. This work provides novel insight into the molecular partitioning of heterogeneous aromatic clusters, with particular relevance to the structure of nascent soot particles.