Large Polycyclic Aromatic Hydrocarbons Research Articles

The Background Interstellar Medium as Observed from Off-order Low-resolution Spitzer-IRS Spectra

Spitzer “hidden” observations of the background are used to construct a catalog of 4090 spectra and examine the signature of polycyclic aromatic hydrocarbon (PAH) molecules and their connection to extinction by dust. A strong positive correlation is recovered between WISE12, E(B − V), and the 11.2 μm PAH band. For 0.06 ≤ E(B − V) ≤ 5.0, correlations of the 6.2, 11.2, and 12.7 μm PAH band are positive with E(B − V). Three dust temperature regimes are revealed. Correlations with WISE12 are well constrained and that with 12.7/11.2 is flat. Decomposition with the NASA Ames PAH IR Spectroscopic Database reveals a tentative positive correlation between the 6.2/11.2 and the PAH ionization fraction, while that with 12.7/11.2 is slightly negative, suggesting PAH structural changes. The relation with PAH size and 6.2/11.2 is negative, while that with 12.7/11.2 is positive. Averaging spectra into five E(B − V) and three T dust bins shows an evolution in PAH emission and variations in 12.7/11.2. Database-fits show an increase in f i and the PAH ionization parameter γ, but a more stable large PAH fraction. While the largest γs are associated with the highest T dust, there is no one-to-one correlation. The analysis is hampered by low-quality data at short wavelengths. There are indications that PAHs in the more-diffuse backgrounds behave differently from those in the general interstellar medium. However, they are often still associated with larger scale filamentary cloud-like structures. The spectra and auxiliary data have been made available through the Ames Background Interstellar Medium Spectral Catalog and may guide JWST programs.

Oxidation of Polycyclic Aromatic Hydrocarbons (PAHs) Triggered by a Photochemical Synergistic Effect between High- and Low-Molecular-Weight PAHs.

Photooxidation of polycyclic aromatic hydrocarbons (PAHs), which are widely observed in atmospheric particulate matter (PM), largely determines their atmospheric fate. In the environment, PAHs are highly complex in chemical composition, and a great variety of PAHs tend to co-occur. Despite extensive investigation on the photochemical behavior of individual PAH molecules, the photochemical interaction among these coexisting PAHs is still not well understood. Here, we show that during photooxidation, there is a strong photochemical synergistic effect among PAHs extracted from soot particles. We find that neither small PAHs with low molecular weights of 200-350 Da and 4-8 aromatic rings (named PAHsmall) nor large PAHs with high molecular weights of 350-600 Da and 8-14 aromatic rings (named PAHlarge) undergo photooxidation under red-light irradiation (λ = 648 nm), even though PAHlarge can absorb light with this wavelength. Interestingly, when PAHlarge is mixed with PAHsmall, substantial photooxidation is observed for both PAHlarge and PAHsmall. Comparisons of in situ infrared (IR), high-resolution mass spectrometry, and electron paramagnetic resonance analysis indicate that the presence of PAHsmall inhibits the light quenching effect arising from the π-π stacking of PAHlarge. This leads to the formation of singlet oxygen (1O2), which initiates the photooxidation. Our findings reveal a new mechanism for the photooxidation of PAHs and suggest that complex atmospheric PAHs exhibit distinct photoreactivity from simple systems.

First direct detection of large polycyclic aromatic hydrocarbons on asteroid (162173) Ryugu samples: An interstellar heritage

AbstractPolycyclic aromatic hydrocarbons (PAHs) are considered major players in the physics and chemistry of star‐ and planet‐forming regions. The interstellar PAH hypothesis is based on our understanding of the origin of the aromatic infrared bands (AIBs), a set of bright emission features that are now the focus of observations by the James Webb telescope. While AIB carriers are expected to be large free PAHs (50 carbon atoms or more), laboratory analysis of primitive carbonaceous chondrites (CCs) has mainly revealed relatively small PAHs, up to 24 carbon atoms. In this study, we present a comprehensive analysis of aromatic species in bulk samples from the carbonaceous asteroid Ryugu using a surface mass spectrometry technique provided by two‐step laser desorption ionization. The resulting molecular distribution differs significantly from that obtained for a sample from the CC Orgueil, revealing aromatic species extending up to 61 carbon atoms. The species identified are composed of both peri‐condensed PAHs and non‐condensed aromatics. These results directly support the interstellar PAH hypothesis and open up new perspectives on the formation and evolution of organic matter in star‐forming regions and in the solar nebula.Key Points First direct detection of free aromatic species of large sizes with up to 61 carbon atoms in primitive extraterrestrial matter by applying a highly sensitive two‐step laser mass spectrometry analysis to grain samples from the carbonaceous asteroid Ryugu (Hayabusa2 mission). First direct support for the interstellar polycyclic aromatic hydrocarbon (PAH) hypothesis, according to which large free PAHs are responsible for the aromatic emission bands that are major infrared features currently observed by the James Webb Space Telescope. The large aromatic species detected are present in trace amounts and future research is needed to develop sensitive techniques for studying these compounds in sample return missions and meteorites.

(Invited) Synthesis and Functionalization of Nanographenes with Red to Near-Infrared Emission for Photonic Applications

Nanostructures of graphene demonstrate a wide range of optical, electronic, and magnetic properties depending on their size and chemical structures, which renders them promising as next-generation carbon-based nanomaterials, e.g., for nanoelectronics, spintronics, and photonics. However, it is challenging to accurately control the chemical structures while “cutting” graphene. To this end, large polycyclic aromatic hydrocarbons (PAHs) with nanoscale graphene structures are attracting a renewed attention as atomically precise nanographenes [1]. We have developed the synthesis of dibenzo[hi,st]ovalene (DBOV) as an unprecedented nanographene with high stability, strong red fluorescence, and stimulated emission, demonstrating the potential for organic lasers [2]. The functionalization of DBOV was achieved through regioselective bromination and Suzuki coupling, enabling the introduction of various substituents, for example fluoranthene imide (FAI) groups, inducing the red-shift of the emission to the near-infrared (NIR) region [3]. Different effects on the photophysical properties of the DBOV core have been observed by changing the peripheral functional groups. More recently, we have synthesized hexabenzoperihexacene (HBPH) with zigzag and fjord edges, which displayed high stability and NIR emission with maximum at ~800 nm [4]. Notably, HBPH could be made chiral by introducing bulky tert-butyl groups on the helical fjord edges, allowing the separation of the enantiomers and showing the circular dichroism responses up to 830 nm. These results provide a new insight into the structure-property relationship of such nanographenes and pave the way toward their photonic applications.[1] Paternò, G. M.; Goudappagouda,; Chen, Q.; Lanzani, G.; Scotognella, F; Narita A., Adv. Optical Mater. 2021, 9, 2100508.[2] Paternò, G. M.; Chen, Q.; Wang, X.-Y.; Liu, J.; Motti, S. G.; Petrozza, A.; Feng, X.; Lanzani, G.; Müllen, K.; Narita, A.; Scotognella, F., Angew. Chem. Int. Ed. 2017, 56, 6753–6757.[3] Paternò, G. M.; Chen, Q.; Muñoz-Mármol, R.; Guizzardi, M.; Bonal, V.; Kabe, R.; Barker, A. J.; Boj, P. G.; Chatterjee, S.; Ie, Y.; Villalvilla, J. M.; Quintana, J. A.; Scotognella, F.; Müllen, K.; Díaz-García, M. A.; Narita, A.; Lanzani, G., Mater. Horiz. 2022, 9, 393–402.[4] Xu, X.; Muñoz-Mármol, R.; Vasylevskyi, S.; Villa, A.; Folpini, G.; Scotognella, F.; Paternò, G. M.; Narita, A., Angew. Chem. Int. Ed. 2023, 62, e202218350.

(Invited) Electrochemistry of the 1,1,N,N-Tetramethyl[N](2,11)Teropyrenophanes

Polycyclic aromatic hydrocarbons (PAH) have been of broad interest for many decades. This is due largely to their extended π systems, which imbue them with optical and electronic properties that can be exploited in a variety of organic electronic devices. A current trend in PAH chemistry is the construction of ever-larger and more elaborate aromatic systems. A major challenge associated with this work is to maintain solubility. For innately planar PAH, the standard tactic is to append multiple solubilizing groups to the periphery of the aromatic system. The drawbacks to this are that 1) the mass of solubilizing groups can outweigh that of the PAH, 2) the presence of the solubilizing groups severely limits or rules out doing chemistry on the periphery and 3) the tactic becomes less and less effective as the size of the PAH becomes very large. An alternative and very effective way to boost solubility is to bring about nonplanarity in a PAH. This can be achieved through the presence of nonbonded interactions (as in helicenes), the presence of non-6-membered rings (as in corannulene) and by bridging the aromatic system (i.e. incorporating it into a cyclophane).Cyclophanes constitute a very large and diverse class of compounds. The simplest cyclophanes (the [n]cyclophanes) consist of just one aromatic system and one aliphatic bridge. These cyclophanes are especially interesting because the structure of the aromatic system can be changed by varying the length of the bridge. This enables the study of how the chemical and physical properties of a particular PAH are affected by incremental changes in structure without any complications arising from intramolecular π-π interactions or changes in the character or position of the substituents (the bridge). Understanding the relationships between structure and properties can underpin efforts to tailor properties of organic compounds for use as materials in devices. Very few [n]cyclophanes that have a large PAH are known, so there is plenty of room for exploration in this area.The PAH of interest in this work is teropyrene. It is a member of the ropyrene series of aromatic compounds, which evolve into armchair-edged graphene nanoribbons (GNR) as they propagate. Teropyrene has 10 6-membered rings, 36 carbon atoms and vanishingly low solubility. Only a handful of teropyrene derivatives are known. We have developed a synthetic route to a series of 1,1,n,n-tetramethyl[n](2,11)teropyrenophanes (n = 7-10), which feature a teropyrene system with an end-to-end bend ranging from 145° (n = 10) to 178° (n = 7) and have very good solubility in a variety of organic solvents. Details of the redox behaviour of these teropyrenophanes will be presented. Figure 1

Supramolecular Exchange Reaction in the Cavity of Naphthalene Diimides Extended-Pillar[6]arene.

A new naphthalene diimides extended-pillar[6]arene 1 with a large cavity and rich host-guest complexation properties was synthesized in high yield. It can not only form 1 : 2 complexes with large size polycyclic aromatic hydrocarbons but also form 1 : 1 : 1 ternary complex with perylene and 2,7-diazapyrenium. Moreover, the supramolecular exchange reaction from a 1 : 2 host-guest complex 1 ⋅ (G3)2 formed by 1 and perylene to a 1 : 1 : 1 ternary complex 1 ⋅ G3 ⋅ G5 formed by 1 with perylene and 2,7-diazapyrenium salt was also investigated by 1H NMR experiments as well as theoretically calculations.

Intermolecular interaction energies with AROFRAG-A systematic approach for fragmentation of aromatic molecules.

Intermolecular interactions with polycyclic aromatic hydrocarbons (PAHs) represent an important area of physisorption studies. These investigations are often hampered by a size of interacting PAHs, which makes the calculation prohibitively expensive. Therefore, methods designed to deal with large molecules could be helpful to reduce the computational costs of such studies. Recently we have introduced a new systematic approach for the molecular fragmentation of PAHs, denoted as AROFRAG, which decomposes a large PAH molecule into a set of predefined small PAHs with a benzene ring being the smallest unbreakable unit, and which in conjunction with the Molecules-in-Molecules (MIM) approach provides an accurate description of total molecular energies. In this contribution we propose an extension of the AROFRAG, which provides a description of intermolecular interactions for complexes composed of PAH molecules. The examination of interaction energy partitioning for various test cases shows that the AROFRAG3 model connected with the MIM approach accurately reproduces all important components of the interaction energy. An additional important finding in our study is that the computationally expensive long-range electron-correlation part of the interaction energy, that is,the dispersion component, is well described at lower AROFRAG levels even without MIM, which makes the latter models interesting alternatives to existing methods for an accurate description of the electron-correlated part of the interaction energy.

Efficient Reaction‐Based Approaches for Gas‐Phase Enthalpy of Formation Prediction and their Application to Large (C32) Polycyclic Aromatic Hydrocarbons

AbstractIn this work, an efficient and generally applicable scheme for the automatic generation of the minimal set of independent model reactions to be used for the calculation of enthalpies of formation is presented. A post‐processing procedure targeting the selection of the most suitable model reactions by assigning them larger weights is suggested. The developed computational protocol exploiting high‐level ab initio calculations and accurate reference enthalpies of formation retrieved from the Active Thermochemical Tables reproduces with chemical accuracy well‐established enthalpies of formation of 15 relatively small (C10–C24) polycyclic aromatic hydrocarbons (PAHs). A promising alternative single‐reaction strategy encompassing all reference species is outlined. Both methods are then applied to predict for the set of 43 larger (C32) PAHs, and revealed significant deviation (Mean Unsigned Error, MUE = 23.2 kJ mol−1) compared to the earlier theoretical results obtained from B3LYP calculations with subsequent group‐based empirical corrections. In the absence of the experimental data, these evaluations of are expected to be more realistic as the approach employed in this work demonstrated better performance on the benchmarking set of 15 smaller PAHs with the MUE of 1.3–1.5 versus 6.2 kJ mol−1.

Fluorescence Excitation and Dispersed Fluorescence Spectra of the First Electronic Excited (S1) State of peri-Hexabenzocoronene (C42H18) Isolated in Solid para-Hydrogen.

Large polycyclic aromatic hydrocarbons (PAH) and their cationic, hydrogenated, and protonated derivatives have long been considered as promising candidates for the carriers of the diffuse interstellar bands. peri-Hexabenzocoronene (peri-HBC, C42H18) is a large, compact PAH, and, to the best of our knowledge, the largest centrosymmetric all-benzenoid PAH for which electronic spectroscopy data has been published. In this work, we present the dispersed fluorescence and fluorescence excitation spectra of the first electronic excited (S1) state of peri-HBC isolated in solid para-H2 and provide the first detailed vibronic analysis of observed features. The observed spectra agree with the emission and absorption spectra simulated according to optimized geometries and scaled harmonic vibrational frequencies calculated at the density functional theory (DFT) level using a Franck-Condon Herzberg-Teller approach; the spectral bands are associated solely with vibrational normal modes of approximate e2g symmetry and their combinations with vibrational modes of approximately a1g symmetry. We clearly observed the position of the S1-S0 electronic transition origin of peri-HBC at 22,088 cm-1 (452.7 nm), which was unreported previously. The matrix shift of ∼110 cm-1 to the red relative to the gas-phase value was estimated by comparison of two reported gas-phase bands with our work. Because of the significant deviation from the reported wavelengths of DIB, the weakness of the S1-S0 electronic transitions, and the lack of reported DIB at <400 nm where the intense S4 ← S0 band of peri-HBC is located, peri-HBC is unlikely to contribute to DIB.

Polycyclic aromatic hydrocarbon skin permeation efficiency in vitro is lower through human than pigskin and decreases with lipophilicity

Polycyclic aromatic hydrocarbons (PAH) are persistent environmental pollutants, which occasionally appear as contaminants in consumer products. Upon dermal contact, transfer of PAH into the stratum corneum (s.c.) and migration through the skin may occur, resulting in this class of highly toxic compounds to become bioavailable. In this study, dermal penetration through human and porcine skin of 24 PAH, comprising broad molar mass (M: 152–302 g/mol) and octanol-water partition coefficient (logP: 3.9–7.3) ranges, was evaluated via Franz diffusion cell in vitro assays. More lipophilic and potentially more toxic PAH had decreased permeation rates through the rather lipophilic s.c. into the more hydrophilic viable (epi-)dermis. Furthermore, human skin was less permeable than pigskin, a commonly used surrogate in skin penetration studies. In particular, the s.c. of human skin retains a greater share of PAH, an effect that is more pronounced for smaller PAH. Additionally, we compared the skin permeation kinetics of different PAH in pigskin. While small PAH (M < 230 g/mol, logP < 6) permeate the skin quickly and are detected in the receptor fluid after 2 h, large PAH (M > 252 g/mol, logP ≥ 6) do not fully permeate the skin up to 48 h. This indicates that highly lipophilic PAH do not become bioavailable as readily as their smaller congeners when transferred to the skin surface. Our data suggest that pigskin could be used as a surrogate for worst case scenario estimates of dermal PAH permeation through human skin.

Direct Observation of Covalently Bound Clusters in Resonantly Stabilized Radical Reactions and Implications for Carbonaceous Particle Growth.

Based on quantum mechanically guided experiments that observed elusive intermediates in the domain of inception that lies between large molecules and soot particles, we provide a new mechanism for the formation of carbonaceous particles from gas-phase molecular precursors. We investigated the clustering behavior of resonantly stabilized radicals (RSRs) and their interactions with unsaturated hydrocarbons through a combination of gas-phase reaction experiments and theoretical calculations. Our research directly observed a sequence of covalently bound clusters (CBCs) as key intermediates in the evolution from small RSRs, such as benzyl (C7H7), indenyl (C9H7), 1-methylnaphthyl (1-C11H9), and 2-methylnaphthyl (2-C11H9), to large polycyclic aromatic hydrocarbons (PAHs) consisting of 28 to 55 carbons. We found that hydrogen abstraction and RSR addition drive the formation and growth of CBCs, leading to progressive H-losses, the generation of large PAHs and PAH radicals, and the formation of white smoke (incipient carbonaceous particles). This mechanism of progressive H-losses from CBCs (PHLCBC) elucidates the crucial relationship among RSRs, CBCs, and PAHs, and this study provides an unprecedentedly seamless path of observed assembly from small RSRs to large nanoparticles. Understanding the PHLCBC mechanism over a wide temperature range may enhance the accuracy of multiscale models of soot formation, guide the synthesis of carbonaceous nanomaterials, and deepen our understanding of the origin and evolution of carbon within our galaxy.

Energy gap and aromatic molecular rings.

The manuscript combines rational density functional theory simulations and experimental data to investigate the electrical properties of eight polycyclic aromatic hydrocarbons (PAHs). The optimized geometries reveal a preference for one-row, two-row and three-row ring distributions. Band structure plots demonstrate an inverse correlation between the number of aromatic rings and band gap size, with a specific order observed across the PAHs. Gas phase simulations support these findings, though differences in values are noted compared to the literature. Introducing a two-row ring distribution concept resolves discrepancies, particularly in azulene. The B3LYP function successfully bridges theoretical and experimental gaps, particularly in large PAHs. The manuscript highlights the potential for designing electronic devices based on different-sized PAHs, emphasizing a multi-ring distribution approach and opening new avenues for practical applications.

A Terrylene Bisimide based Universal Host for Aromatic Guests to Derive Contact Surface‐Dependent Dispersion Energies

Abstractπ–π interactions are among the most important intermolecular interactions in supramolecular systems. Here we determine experimentally a universal parameter for their strength that is simply based on the size of the interacting contact surfaces. Toward this goal we designed a new cyclophane based on terrylene bisimide (TBI) π‐walls connected by para‐xylylene spacer units. With its extended π‐surface this cyclophane proved to be an excellent and universal host for the complexation of π‐conjugated guests, including small and large polycyclic aromatic hydrocarbons (PAHs) as well as dye molecules. The observed binding constants range up to 108 M−1 and show a linear dependence on the 2D area size of the guest molecules. This correlation can be used for the prediction of binding constants and for the design of new host–guest systems based on the herewith derived universal Gibbs interaction energy parameter of 0.31 kJ/molÅ2 in chloroform.

A Terrylene Bisimide based Universal Host for Aromatic Guests to Derive Contact Surface-Dependent Dispersion Energies.

π-π interactions are among the most important intermolecular interactions in supramolecular systems. Here we determine experimentally a universal parameter for their strength that is simply based on the size of the interacting contact surfaces. Toward this goal we designed a new cyclophane based on terrylene bisimide (TBI) π-walls connected by para-xylylene spacer units. With its extended π-surface this cyclophane proved to be an excellent and universal host for the complexation of π-conjugated guests, including small and large polycyclic aromatic hydrocarbons (PAHs) as well as dye molecules. The observed binding constants range up to 108 M-1 and show a linear dependence on the 2D area size of the guest molecules. This correlation can be used for the prediction of binding constants and for the design of new host-guest systems based on the herewith derived universal Gibbs interaction energy parameter of 0.31 kJ/molÅ2 in chloroform.

Experimental and modeling study of fuel-rich oxidation of gasoline surrogate components/dimethyl carbonate blends in an atmospheric flow reactor

Fuel-rich oxidation of blending fuels consisting of three typical gasoline surrogate components (n-heptane, iso-octane, and toluene) and dimethyl carbonate (DMC) was investigated in an atmospheric-pressure flow reactor at mean gas temperatures from 1000 to 1350 K, equivalence ratio of 9.0, and residence time of 1.2 s. Blending ratios of DMC were 35 and 70 % on a molar basis. The mole fractions of not only polycyclic aromatic hydrocarbons (PAHs) up to 3 ring structure but also small intermediate species from C1 to C5 contained in sampled gas were experimentally quantified. A kinetic model that includes not only oxidation mechanism of gasoline surrogate components and DMC but also PAH growth reactions was developed to reproduce the experimental data. The experimental and simulated results showed that the blending effect of DMC on PAH production was different according to gasoline surrogate components. By blending DMC with n-heptane and iso-octane, the mole fractions of PAHs were generally reduced with an increase of the DMC blending ratio in the temperature range studied here. On the other hand, when DMC was blended with toluene, PAH production in the blending fuels was larger than that in pure toluene at a temperature below 1150 K, while the mole fractions of PAHs in pure toluene were identical to or larger than those in the blending fuels at a temperature above 1200 K. Because a reactivity of DMC is higher than toluene, toluene conversion was facilitated at a low temperature, leading to the promoted PAH production in the blending fuels compared to pure fuel. However, such a promoting effect of DMC was diminished at an elevated temperature, explaining the identical or large PAH production in neat toluene compared to the blending fuels. On the other hand, n-heptane and isooctane can be easily reacted even at a low temperature, suggesting that the promoting effect of DMC cannot be observed. This study reasonably succeeded in explaining the different DMC blending effects on PAH formation depending on the gasoline surrogate components. Novelty and significance statementExperiments and kinetic modeling for the mixtures consisting of dimethyl carbonate (DMC) and gasoline surrogate components, such as n-heptane, iso-octane, and toluene, were conducted to reveal the blending effect of DMC on polycyclic aromatic hydrocarbon (PAH) formation under the fuel-rich environment. It was found that DMC had a dilution effect in n-heptane/DMC and iso-octane/DMC blends, resulting in a reduction in PAH production with an increase of a DMC blending ratio. On the other hand, while DMC had the dilution effect in the toluene/DMC blend at an elevated temperature similar to other blends, toluene conversion was facilitated in the presence of DMC at a low temperature, resulting in the enhanced PAH production in the toluene/DMC blends compared to pure toluene.

AROFRAG─A Systematic Approach for Fragmentation of Aromatic Molecules.

We present a new systematic fragmentation scheme of polycyclic aromatic hydrocarbons (PAHs), including fullerenes and nanotubes, based on an idea to treat a sextet ring as a single unbreakable unit so that the basic unit of aromaticity remains preserved upon fragmentation. In the approach, denoted as AROFRAG (from aromatic fragmentation), a set of predefined elementary subsystems, such as naphthalene and biphenyl in the first model and larger PAHs in the second and third models, is generated with appropriate weights with the aim of reproducing the structure of the original molecule. The energies of the molecules are approximated as weighted sums of the energies of these subsystems. For symmetric cases, e.g., fullerenes, the point-group symmetry is preserved during the decomposition. The AROFRAG is used in conjunction with the molecule-in-molecule (MIM) technique to obtain an accurate description of the electronic energies. The new approach has been applied for selected graphene structures and fullerene doped with boron and nitrogen atoms, for which isomerization energies were calculated, as well as for several nanotubes and regular fullerene molecules. The combination of the third AROFRAG model and the MIM approach leads to the reproduction of electronic energies with a few milli-hartree accuracy at a fraction of the computational cost of the original method.

Compact and accurate chemical mechanism for methane pyrolysis with PAH growth

A reliable and compact chemical mechanism of gas-phase methane pyrolysis leading to formation of large polycyclic aromatic hydrocarbon (PAH) molecules has been developed. This model is designed for studies of carbon nanostructure synthesis such as carbon black and graphene flakes, including soot growth kinetics. Methane pyrolysis with carbon nanostructure synthesis is a two-stage process where conversion of CH4 to C2H2 precedes the growth of PAH molecules from acetylene. We present a single chemical mechanism that accurately describes both stages. We have constructed a compact and accurate chemical mechanism capable of modeling both stages of methane pyrolysis based on the ABF [1] mechanism which was expanded with the most prominent reaction pathways from the mechanism by Tao [2] for small PAH molecules and HACA pathways for larger PAH molecules, up to 37 aromatic rings. The resulting mechanism was validated through comparison to multiple available sets of experimental data. Good agreement with the experimental data for both processes was obtained. Performance of the mechanism was tested for pyrolysis of methane-rich mixtures under long residence times leading to abundant formation of PAH molecules. It is shown that the inclusion of larger PAH species (up to A37) in the chemical mechanism is important for accurate prediction of the fraction of carbon converted to PAH molecules and, correspondingly, the residual fraction of acetylene in the mixture. The mechanism file is available upon request.

Precise synthesis of BN embedded perylene diimide oligomers for fast-charging and long-life potassium-organic batteries.

Replacing the C[double bond, length as m-dash]C bond with an isoelectronic BN unit is an effective strategy to tune the optoelectronic properties of polycyclic aromatic hydrocarbons (PAHs). However, precise control of the BN orientations in large PAH systems is still a synthetic challenge. Herein, we demonstrate a facile approach for the synthesis of BN embedded perylene diimide (PDI) nanoribbons, and the polarization orientations of the BN unit were precisely regulated in the two PDI trimers. These BN doped PDI oligomers show great potential as organic cathodes for potassium-ion batteries (PIBs). In particular, trans-PTCDI3BN exhibits great improvement in voltage potential, reversible capacities (ca. 130 mA h g-1), superior rate performance (19 s to 69% of the maximum capacity) and ultralong cyclic stability (nearly no capacity decay over 30 000 cycles), which are among those of state-of-the-art organic-based cathodes. Our synthetic approach stands as an effective way to access large PAHs with precisely controlled BN orientations, and the BN doping strategy provides useful insight into the development of organic electrode materials for secondary batteries.

Clar covering polynomials of polycyclic aromatic hydrocarbons

&lt;abstract&gt;&lt;p&gt;Polycyclic aromatic hydrocarbon (PAH) is a compound composed of carbon and hydrogen atoms. Chemically, large PAHs contain at least two benzene rings and exist in a linear, cluster, or angular arrangement. Hexagonal systems are a typical class of PAHs. The Clar covering polynomial of hexagonal systems contains many important topological properties of condensed aromatic hydrocarbons, such as Kekulé number, Clar number, first Herndon number, which is an important theoretical quantity for predicting the aromatic stability of PAH conjugation systems, and so on. In this paper, we first obtained some recursive formulae for the Clar covering polynomials of double hexagonal chains and proposed a Matlab algorithm to compute the Clar covering polynomial of any double hexagonal chain. Moreover, we presented the characterization of extremal double hexagonal chains with maximum and minimum Clar covering polynomials in all double hexagonal chains with fixed $ s $ naphthalenes.&lt;/p&gt;&lt;/abstract&gt;

New light on the imbroglio surrounding the C8H+6 isomers formed from ionized azulene and naphthalene using ion-molecule reactions.

Most polycyclic aromatic hydrocarbons (PAHs) can isomerize with internal energies near to or below the dissociation threshold. The C10H+8 group of ions, made up of the naphthalene (Naph+) and the azulene (Azu+) radical cations, is a prototypical example. C8H+6 isomers are important species in the growth kinetics and formation of complex organic molecules, and more generally fragments from larger PAHs, yet information about C8H+6 structures is scarce and contradictory. Here, ion-molecule reactions were carried out and the tunable photoionization chemical monitoring technique was used to probe the C8H+6 isomers formed upon C2H2-loss from Naph+ and Azu+. The experimental findings were interpreted with the support of ab initio and kinetics calculations. To facilitate the interpretation of these data, chemical reactivity starting from phenylacetylene (PA) was studied. It was found that most of the C8H+6 ions formed from C10H8, in a timescale of 40 μs, are PA+ in the vicinity of the dissociation threshold. No evidence of the pentalene radical cation (PE+) was observed and explanations to reconcile previous results are presented.