Planar Polycyclic Aromatic Hydrocarbons Research Articles

Electronic Spectroscopy of Ovalene: Reassignment of the S2(B3u)- S0(Ag) Transition.

As part of our experiments to characterize solid para-hydrogen (para-H2) as a matrix host for electronic spectroscopy with potential applications in the ongoing quest for the carriers of the diffuse interstellar bands (DIB), we studied dispersed fluorescence and fluorescence excitation spectra of ovalene (C32H14), a planar polycyclic aromatic hydrocarbon (PAH) of D2h symmetry. Although generally in good agreement with previously reported data for jet-cooled C32H14, our results, in conjunction with quantum-chemical calculations, indicate that the observed spectral progressions are associated with the S2(B3u)-S0(Ag) electronic transition instead of the originally assigned S1-S0 transition for C32H14 in a supersonic jet, and that the previously reported origin band was misassigned and should be located at ∼21050 cm-1. The reassignment is further supported by the comparably long fluorescence lifetime of ∼1.7 μs. From an analysis of spectral features located >1600 cm-1 in the fluorescence excitation spectrum, we estimate an S2(B3u)-S3(B1g) energy gap of ∼350 cm-1.

(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

Mutual induced fit transition structure stabilization of corannulene's bowl-to-bowl inversion in a perylene bisimide cyclophane.

Corannulene is known to undergo a fast bowl-to-bowl inversion at r.t. via a planar transition structure (TS). Herein we present the catalysis of this process within a perylene bisimide (PBI) cyclophane composed of chirally twisted, non-planar chromophores, linked by para-xylylene spacers. Variable temperature NMR studies reveal that the bowl-to-bowl inversion is significantly accelerated within the cyclophane template despite the structural non-complementarity between the binding site of the host and the TS of the guest. The observed acceleration corresponds to a decrease in the bowl-to-bowl inversion barrier of 11.6 kJ mol-1 compared to the uncatalyzed process. Comparative binding studies for corannulene (20 π-electrons) and other planar polycyclic aromatic hydrocarbons (PAHs) with 14 to 24 π-electrons were applied to rationalize this barrier reduction. They revealed high binding constants that reach, in tetrachloromethane as a solvent, the picomolar range for the largest guest coronene. Computational models corroborate these experimental results and suggest that both TS stabilization and ground state destabilization contribute to the observed catalytic effect. Hereby, we find a "mutual induced fit" between host and guest in the TS complex, such that mutual geometric adaptation of the energetically favored planar TS and curved π-systems of the host results in an unprecedented non-planar TS of corannulene. Concomitant partial planarization of the PBI units optimizes noncovalent TS stabilization by π-π stacking interactions. This observation of a "mutual induced fit" in the TS of a host-guest complex was further validated experimentally by single crystal X-ray analysis of a host-guest complex with coronene as a qualitative transition state analogue.

Tunable Crystallinity and Electron Conduction in Wavy 2D Conjugated Metal-Organic Frameworks via Halogen Substitution.

Currently, most reported 2D conjugated metal-organic frameworks (2D c-MOFs) are based on planar polycyclic aromatic hydrocarbons (PAHs) with symmetrical functional groups, limiting the possibility of introducing additional substituents to fine-tune the crystallinity and electrical properties. Herein, a novel class of wavy 2D c-MOFs with highly substituted, core-twisted hexahydroxy-hexa-cata-benzocoronenes (HH-cHBCs) as ligands is reported. By tailoring the substitution of the c-HBC ligands with electron-withdrawing groups (EWGs), such as fluorine, chlorine, and bromine, it is demonstrated that the crystallinity and electrical conductivity at the molecular level can be tuned. The theoretical calculations demonstrate that F-substitution leads to a more reversible coordination bonding between HH-cHBCs and copper metal center, due to smaller atomic size and stronger electron-withdrawing effect. As a result, the achieved F-substituted 2D c-MOF exhibits superior crystallinity, comprising ribbon-like single crystals up to tens of micrometers in length. Moreover, the F-substituted 2D c-MOF displays higher electrical conductivity (two orders of magnitude) and higher charge carrier mobility (almost three times) than the Cl-substituted one. This work provides a new molecular design strategy for the development of wavy 2D c-MOFs and opens a new route for tailoring the coordination reversibility by ligand substitution toward increased crystallinity and superior electric conductivity.

Experimental and Theoretical Structure Elucidation of the [2 : 1] Complex Ion of Carbo[n]helicene with n=6, 7 and 8 and Ag.

Gas-phase complexes of [n]helicenes with n=6, 7 and 8 and the silver(I) cation are generated utilizing electrospray ionization mass spectrometry (ESI-MS). Besides the well-established [1 : 1] helicene/Ag+ -complex in which the helicene provides a tweezer-like surrounding for the Ag+ , there is also a [2 : 1] complex formed. Density functional theory (DFT) calculations in conjunction with energy-resolved collision-induced dissociation (ER-CID) experiments reveal that the second helicene attaches via π-π stacking to the first helicene, which is part of the pre-formed [1 : 1] tweezer complex with Ag+ . For polycyclic aromatic hydrocarbons (PAHs) of planar structure, the [2 : 1] complex with silver(I) is typically structured as an Ag+ -bound dimer in which the Ag+ would bind to both PAHs as the central metal ion (PAH-Ag+ -PAH). For helicenes, the Ag+ -bound dimer is of similar thermochemical stability as the π-π stacked dimer, however, it is kinetically inaccessible. Coronene (Cor) is investigated in comparison to the helicenes as an essentially planar PAH. In analogy to the π-π stacked dimer of the helicenes, the Cor-Ag+ -Cor-Cor complex is also observed. Competition experiments using [n]helicene mixtures reveal that the tweezer complexes of Ag+ are preferably formed with the larger helicenes, with n=6 being entirely ignored as the host for Ag+ in the presence of n=7 or 8.

Electronic excited states of planar vs bowl-shaped polycyclic aromatic hydrocarbons in interaction with water clusters: a TD-DFT study

Electronic excited states of planar vs bowl-shaped polycyclic aromatic hydrocarbons in interaction with water clusters: a TD-DFT study

Polarizability of Kekulene, Septulene, and Nearest Non-Planar Polycyclic Aromatic Hydrocarbons

The polarizability of polycyclic aromatic hydrocarbons (PAHs) is an important property that relates to their abundance in natural environments. To assess the differences in the mean polarizability of planar and non-planar polycyclic aromatic hydrocarbons (PAHs), we computationally studied the PAH series of circulenes (kekulene C48H24, [13]circulene C52H26, and septulene C56H28), a number of the nearest helicenes, and their “expanded” isomers. We mean under “nearest” a close number of aromatic rings: 11 (C46H26), 12 (C50H28), 13 (C54H30), and 14 (C58H32). For these PAHs, we performed the quantum chemical calculations of thermodynamic and polarizability parameters with the PBE/3ζ density functional theory method, which is widely used in the theoretical chemistry of fullerenes and PAHs. The calculated mean polarizabilities (in Å3) ranged from 80.1 for [11]helicene to 135.5 for septulene, and while the circulenes and expanded helicenes had similar values, the mean polarizability of the normal helicenes was markedly lower. In all four pairs of helical PAHs, the expanded helicene was energetically considerably more favorable than its standard helicene isomer. Herewith, the ratio of their polarizabilities was equal to 1.3.

Solvation of Large Polycyclic Aromatic Hydrocarbons in Helium: Cationic and Anionic Hexabenzocoronene.

The adsorption of helium on charged hexabenzocoronene (Hbc, C42H18), a planar polycyclic aromatic hydrocarbon (PAH) molecule of D6h symmetry, was investigated by a combination of high-resolution mass spectrometry and classical and quantum computational methods. The ion abundance of HenHbc+ complexes versus size n features prominent local anomalies at n = 14, 38, 68, 82, and a weak one at 26, indicating that for these “magic” sizes, the helium evaporation energies are relatively large. Surprisingly, the mass spectra of anionic HenHbc− complexes feature a different set of anomalies, namely at n = 14, 26, 60, and 62, suggesting that the preferred arrangement of the adsorbate atoms depends on the charge of the substrate. The results of our quantum calculations show that the adsorbate layer grows by successive filling of concentric rings that surround the central benzene ring, which is occupied by one helium atom each on either side of the substrate. The helium atoms are fairly localized in filled rings and they approximately preserve the D6h symmetry of the substrate, but helium atoms in partially filled rings are rather delocalized. The first three rings contain six atoms each; they account for magic numbers at n = 14, 26, and 38. The size of the first ring shrinks as atoms are filled into the second ring, and the position of atoms in the second ring changes from hollow sites to bridge sites as atoms are filled into the third ring. Beyond n = 38, however, the arrangement of helium atoms in the first three rings remains essentially frozen. Presumably, another ring is filled at n = 68 for cations and n = 62 for anions. The calculated structures and energies do not account for the difference between charge states, although they agree with the measurements for the cations and show that the first solvation shell of Hbc± is complete at n = 68. Beyond that size, the adsorbate layer becomes three-dimensional, and the circular arrangement of helium changes to hexagonal.

Adsorption of Helium and Hydrogen on Triphenylene and 1,3,5-Triphenylbenzene.

The adsorption of helium or hydrogen on cationic triphenylene (TPL, C18H12), a planar polycyclic aromatic hydrocarbon (PAH) molecule, and of helium on cationic 1,3,5-triphenylbenzene (TPB, C24H18), a propeller-shaped PAH, is studied by a combination of high-resolution mass spectrometry and classical and quantum computational methods. Mass spectra indicate that HenTPL+ complexes are particularly stable if n = 2 or 6, in good agreement with the quantum calculations that show that for these sizes, the helium atoms are strongly localized on either side of the central carbon ring for n = 2 and on either side of the three outer rings for n = 6. Theory suggests that He14TPL+ is also particularly stable, with the helium atoms strongly localized on either side of the central and outer rings plus the vacancies between the outer rings. For HenTPB+, the mass spectra hint at enhanced stability for n = 2, 4 and, possibly, 11. Here, the agreement with theory is less satisfactory, probably because TPB+ is a highly fluxional molecule. In the global energy minimum, the phenyl groups are rotated in the same direction, but when the zero-point harmonic correction is included, a structure with one phenyl group being rotated opposite to the other two becomes lower in energy. The energy barrier between the two isomers is very small, and TPB+ could be in a mixture of symmetric and antisymmetric states, or possibly even vibrationally delocalized.

Water Clusters in Interaction with Corannulene in a Rare Gas Matrix: Structures, Stability and IR Spectra

The interaction of polycyclic aromatic hydrocarbons (PAHs) with water is of paramount importance in atmospheric and astrophysical contexts. We report here a combined theoretical and experimental study of corannulene-water interactions in low temperature matrices and of the matrix’s influence on the photoreactivity of corannulene with water. The theoretical study was performed using a mixed density functional based tight binding/force field approach to describe the corannulene-water clusters trapped in an argon matrix, together with Born-Oppenheimer molecular dynamics to determine finite-temperature IR spectra. The results are discussed in the light of experimental matrix isolation FTIR spectroscopic data. We show that in the solid phase, π isomers of (C20H10)(H2O)n, with n = 2 or 3, are energetically favored. These π complexes are characterized by small shifts in corannulene vibrational modes and large shifts in water bands. These π structures, particularly stable in the case of the water trimer where the water cluster is trapped “inside” the corannulene bowl, may account for the difference in photoreactivity of non-planar–compared to planar–PAHs with water. Indeed, planar PAHs such as pyrene and coronene embedded in H2O:Ar matrices form σ isomers and react with water to form alcohols and quinones under low energy UV irradiation, whereas no photoreactivity was observed for corannulene under the same experimental conditions.

Characterisation of the transition type in optical band gap analysis of in-flame soot

Semiconductor materials are characterised by a band gap between conduction and valence band, which can either be of direct or indirect nature depending on the crystal momentum. In case of applying the so-called Tauc method to infer the band gap value, both transition types are described by different values for the transition type exponent. In the case of soot particles, the optical band gap is an important quantity during the evolution process to describe the optical characteristics mainly linked to the underlying structural properties. In this work, an approach to infer the optical band gap value together with the transition type exponent is used to shed light on the electronic nature and optical properties of soot during its formation process. In situ UV–Vis-NIR extinction measurements as well as ex situ Raman spectroscopy are performed in a non-premixed ethylene/air flame at two different positions in the flame reflecting different stages of the soot formation process. The results reveal two important tendencies: first, an increase in the band gap energy of young soot is observed, possibly caused by the curvature of initially planar polycyclic aromatic hydrocarbon monomers due to the presence of odd-numbered rings in the hexagonal aromatic lattice. Second, a decrease of the transition type exponent value can be observed, indicating a shift from an indirect transition to a more direct band structure. Further, the indirect band gap assumption for mature soot and its band gap value close to 0.2 eV can be confirmed experimentally. The Raman results provide additional insight in the atomic fine structure of the soot particles regarding the defect density and the loss of amorphous character with increasing maturity.

In Situ Formation of Polycyclic Aromatic Hydrocarbons as an Artificial Hybrid Layer for Lithium Metal Anodes.

Nonuniform Li deposition causes dendrites and low Coulombic efficiency (CE), seriously hindering the practical applications of Li metal. Herein, we developed an artificial solid-state interphase (SEI) with planar polycyclic aromatic hydrocarbons (PAHs) on the surface of Li metal anodes by a facile in situ formation technology. The resultant dihydroxyviolanthron (DHV) layers serve as the protective layer to stabilize the SEI. In addition, the oxygen-containing functional groups in the soft and conformal SEI film can regulate the diffusion and transport of Li ions to homogenize the deposition of Li metal. The artificial SEI significantly improves the CEs and shows superior cyclability of over 1000 h at 4 mAh cm-2. The LiFePO4/Li cell (2.8 mAh cm-2) enables a long cyclability for 300 cycles and high CEs of 99.8%. This work offers a new strategy to inhibit Li dendrite growth and enlightens the design on stable SEI for metal anodes.

Design, synthesis, and controllable transformation of bowl-shaped heterosumanenes

<p indent="0mm">Buckybowls are the key intermediate structures for transforming planar polycyclic aromatic hydrocarbons (PAHs) into fullerenes, and they show unique physical and chemical properties as compared with PAHs and fullerenes. In this review, we summarize our recent progress on the bowl-shaped heterosumanenes, including molecular design rationale, highly efficient synthetic strategies, structure-property relationship, chemical reactions, and controllable transformation. It is proved that trichalcogenasumanenes (TCSs) can serve as a promising scaffold to create unconventional polycylces with structural and property diversity such as red fluorescence hetero-PAHs, molecular spoon with multi-functionality, electron-acceptors, and chiral π-system.

Synthesis of Helical and Planar Extended‐Phenanthridinium Salts

AbstractThe investigation of new synthetic routes towards positively charged N‐heterocycles is of high importance for the further development of medicinal chemistry, functional materials, catalysis and other areas of application. For accessing acridinium dyes, we previously reported an approach based on an aryne‐imine‐aryne coupling followed by the subsequent oxidation of the acridane intermediates. Herein, we now present an unusual reaction outcome when phenanthryne is used as aryne component. Under optimized conditions, this two‐step synthetic methodology led to the formation of a helical tetrabenzophenanthridinium derivative. Furthermore, the susceptibility of this product to photoinduced cyclodehydrogenation was observed, providing a highly fluorescent planar polycyclic aromatic hydrocarbon with a positively charged nitrogen. The photophysical and electrochemical properties of the mesityl‐phenyltetrabenzophenanthridinium tetrafluoroborate were also determined.

Influence of the CH/B replacement on the Reactivity of Boranthrene and Related Compounds.

The influence of the replacement of CH groups by boron atoms on the reactivity of planar polycyclic aromatic hydrocarbons has been explored by means of computational tools. To this end, [4 + 2]-cycloaddition reactions involving anthracene and neutral boranthrene with different dienophiles such as ethylene, acetylene, and CO2 have been compared. In addition, the influence of additional fused aromatic rings (pentacene or borapentacene) on the reactivity of these species has been also explored. It was found that the B-doped systems are systematically much more reactive than their all-carbon counterparts from both kinetic and thermodynamic points of view. The observed trends in reactivity are quantitatively analyzed in detail using state-of-the-art methods, namely, the activation strain model of reactivity and the energy decomposition analysis method. Our calculations reveal the importance of molecular orbital interactions as the key factor responsible for the enhanced reactivity of the B-doped systems.

Imide-Functionalized Helical PAHs: A Step towards New Chiral Functional Materials

AbstractAttachment of cyclic imide groups to polycyclic aromatic hydrocarbons (PAHs) leads to fascinating electronic and luminescence properties, with rylene diimides being a representative example. The close to unity fluorescence quantum yields and electron-acceptor properties render them suitable for application in organic electronics and photovoltaics. Recent reports show that, in line with planar PAHs, the imide functionalization has also endowed helical three-dimensional PAHs with similar beneficial photophysical properties. In this article, we have summarized the state-of-the-art research developments in the field of helicene–imide hybrid functional molecules, with a particular focus on synthesis, (chir)optical and redox properties, and applications in electronics. Additionally, we have highlighted our recent work, introducing a novel family of functional chiral molecules, namely, [n]helicene diimides, as three-dimensional relatives of rylene diimides.

Adsorption of Helium on Small Cationic PAHs: Influence of Hydrocarbon Structure on the Microsolvation Pattern.

The adsorption of up to ∼100 helium atoms on cations of the planar polycyclic aromatic hydrocarbons (PAHs) anthracene, phenanthrene, fluoranthene, and pyrene was studied by combining helium nanodroplet mass spectrometry with classical and quantum computational methods. Recorded time-of-flight mass spectra reveal a unique set of structural features in the ion abundance as a function of the number of attached helium atoms for each of the investigated PAHs. Path-integral molecular dynamics simulations were used with a polarizable potential to determine the underlying adsorption patterns of helium around the studied PAH cations and in good general agreement with the experimental data. The calculated structures of the helium–PAH complexes indicate that the arrangement of adsorbed helium atoms is highly sensitive toward the structure of the solvated PAH cation. Closures of the first solvation shell around the studied PAH cations are suggested to lie between 29 and 37 adsorbed helium atoms depending on the specific PAH cation. Helium atoms are found to preferentially adsorb on these PAHs following the commensurate pattern common for graphitic surfaces, in contrast to larger carbonaceous molecules like corannulene, coronene, and fullerenes that exhibit a 1 × 1 commensurate phase.

Scholl Reaction of Perylene-Based Polyphenylene Precursors under Different Conditions: Formation of Hexagon or Octagon?

A planar dibenzo-peri-hexacene derivative (2) was synthesized via FeCl3 -mediated Scholl reaction from a cyclopenta-fused perylene (CP) based polyphenylene precursor (1). However, an unexpected octagon-containing, negatively curved molecule (3) was obtained in nearly quantitative yield when 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) and methanesulfonic acid (MeSO3 H) were used. Similar results were observed when two smaller-sized precursors containing one (4) or two CP units (5) were tested. X-ray crystallographic analysis also revealed that there is no close π-π stacking between neighboring π-conjugated skeletons. DFT calculations suggest a radical cation mechanism in the presence of FeCl3 while an arenium ion pathway for the DDQ/MeSO3 H mediated Scholl reaction, which can well explain the selective formation of hexagons and octagons under different conditions. The obtained compounds showed tunable optical and electrochemical properties.

Methyl-Cyclohexane Methanol (MCHM) Isomer-Dependent Binding on Amorphous Carbon Surfaces.

In January 2014, over 10,000 gallons of methyl-cyclohexane methanol (MCHM) leaked into the Elk River in West Virginia, in a chemical spill incident that contaminated a large portion of the state’s water supply and left over 300,000 residents without clean water for many days and weeks. Initial efforts to remove MCHM at the treatment plant centered on the use of granulated activated carbon (GAC), which removed some of the chemical from the water, but MCHM levels were not lowered to a “non-detect” status until well after the chemical plume had moved downstream of the intake. Months later, MCHM was again detected at the outflow (but not the inflow) at the water treatment facility, necessitating the full and costly replacement of all GAC in the facility. The purpose of this study is to investigate the hypothesis that preferential absorbance of one of the two MCHM isomers, coupled with seasonal variations in water temperature, explain this contrary observation. Calculated intermolecular potentials between ovalene (a large planar polycyclic aromatic hydrocarbon) and the MCHM isomers were compared to physisorption potentials of MCHM onto an amorphous carbon model. While a molecular mechanics (MM) force field predicts no difference in the average interaction potentials between the cis- and trans-MCHM with the planar ovalene structure, MM predicts that the trans isomer binds stronger than the cis isomer to the amorphous carbon surface. Semi-empirical and density functional theory also predict stronger binding of trans-MCHM on both the planar and amorphous surfaces. The differences in the isomer binding strengths on amorphous carbon imply preferential absorbance of the trans isomer onto activated charcoal filter media. Considering seasonal water temperatures, simple Arrhenius kinetics arguments based on these predicted binding energies help explain the environmental observations of MCHM leeching from the GAC filters months after the spill. Overall, this work shows the important implications that can arise from detailed interfacial chemistry investigations.

Growth network of PAH with 5-membered ring: Case study with acenaphthylene molecule

Numerous experimental and theoretical evidence indicates that polycyclic aromatic hydrocarbon (PAH) is the precursor of soot. However, the formation mechanism of PAH at flame conditions is not well understood. In this study, the growth of PAHs with a 5-membered ring via the hydrogen-abstraction-acetylene-addition route was systematically investigated using acenaphthylene (A2R5) as a model molecule. Potential energy surfaces were searched using DFT/B3LYP/6-311+G(d,p) and ROCCSD(T)/cc-pVDZ methods. The reaction rate coefficients of elementary reactions were evaluated using the transition state theory in the temperature range of 500–2500 K. The results showed that H abstractions by H, OH, and CH3 radicals, and the subsequent C2H2 addition reactions, were highly sensitive to the 5-membered ring. H abstraction reaction from the C atom on a 5-membered ring is not favorable due to the relatively higher energy barrier among all competing reactions. In contrast, the reaction rate coefficient of subsequent C2H2 addition reaction on a 5-membered ring is the highest. The planar PAH with no substitution, planar PAH with C2H substitution, curved PAH with two adjacent 5-membered rings, and PAH with a CCH2C functional group, can be formed in the A2R5H-C2H2 system investigated. The reactions investigated were merged into a detailed PAH mechanism to check the product distribution in a premixed C2H4 sooting flame. Owing to the existence of the 5-membered ring, the formation of planar PAHs with a C2H substitution was much more favorable than the formation of new 5-membered ring and 6-membered rings. The predicted mole fraction of all C14H8 species with one C2H substitution was as high as 2.8 × 10−5, higher than that of phenanthrene with same C atom number (1.2 × 10−5). A similar phenomenon was observed for C16H8 species with two C2H substitutions. In the future, more attention should be given to PAHs with C2H substitution, due to their high concentrations and enhanced reactivity, as compared to the benzenoid PAH.