Planar Polycyclic Aromatic Hydrocarbons Research Articles

Janusarene: A Homoditopic Molecular Host.

A homoditopic molecular host, janusarene, is presented that has two back-to-back compactly arranged nanocavities for guest complexation. The unique two-face structural feature of janusarene allows it to bind and align various guest compounds concurrently, which include spherical pristine fullerene C60 and planar polycyclic aromatic hydrocarbons (PAHs), such as pyrene, perylene, and 9,10-dimethylanthracene. The host-guest interactions were characterized by single-crystal X-ray diffraction. A pairwise encapsulation of the PAH guests by janusarene enables PAH dimers to be obtained that deliver spectroscopic properties distinct from those of PAHs dissolved in solution, or in the bulk state. A monotopic control host was also synthesized and used to characterize the host-guest complexing behavior in solution.

Quantum Design of π-Electron Ring Currents in Polycyclic Aromatic Hydrocarbons: Parallel and Antiparallel Ring Currents in Naphthalene.

Control of π-electrons in polycyclic aromatic hydrocarbons (PAHs) is one of the fundamental issues in optoelectronics for ultrafast optical switching devices. We have proposed an effective scenario for design of the generation of coherent ring currents in naphthalene (D2h), which is the smallest unit of planar PAHs. It has been demonstrated by using quantum chemical calculations and quantum optimal control (QOC) simulations that two types of ring currents, parallel and antiparallel, can be generated by resonance excitations by two linearly polarized lasers. A parallel (antiparallel) ring current means that the currents of two benzene rings run in the same (opposite) directions. The two types of ring currents may be experimentally identified by magnetic force microscopy. The QOC simulations indicate that a parallel ring current can be generated by using continuous wave and Gaussian pulse lasers with their time delay without relying on a sophisticated experimental apparatus. The present results provide a guiding principle of coherent π-electronics in PAHs for next-generation organic optical switching devices.

Perylene Bisimide Cyclophanes with High Binding Affinity for Large Planar Polycyclic Aromatic Hydrocarbons: Host-Guest Complexation versus Self-Encapsulation of Side Arms.

Binding of guest molecules with high affinity and selectivity requires well-designed hosts to provide optimized interactions in the host-guest complexes. Herein we report the design and synthesis of new cyclophanes 2PBI(2,6-iPr)2 and 2PBI(2,6-Ph)2 based on core-disubstituted perylene bisimide (PBI) chromophores bearing two phenoxy bay-substituents that evoke almost planar PBI scaffolds. This strategy afforded the new cyclophanes with conformationally rigid cavities to ensure strong binding to stiff planar polycyclic aromatic hydrocarbons (PAHs). Our detailed host-guest binding studies with different PAHs by UV/Vis and fluorescence titration experiments revealed record binding strengths for PAHs with binding constants of up to 1.6×106 m-1 in chloroform at room temperature. One- and two-dimensional NMR experiments and solvent-dependent titration studies revealed self-encapsulation of the side arms of PBI bay-substituents into the cavities that attained high fluorescence quantum yields of these cyclophanes close to unity by preventing the interaction of PBI subunits in the excited states. The binding constants and Gibbs free energies of host-guest complexations disclosed significant effects of PBI bay-substituents and core twist on the binding affinity of the cyclophanes.

Iodine-doped sumanene and its application for the synthesis of chalcogenasumanenes and silasumanenes.

The synthesis of an iodine-doped sumanene, which represents the first example of halogen-doped buckybowls, is described. Pristine trithiasumanene, trisilasumanene and the hitherto unknown parent triselenasumanene, three interesting heterobuckybowls with significant synthetic challenges, were efficiently prepared from the iodine-doped sumanene through non-pyrolytic conditions. This work demonstrates the high potential of hypervalent iodines embedded in extended π-conjugated systems for the synthesis of novel curved and planar polycyclic aromatic hydrocarbons.

Redox-Active M8 L6 Cubic Hosts with Tetraphenylethylene Faces Encapsulate Organic Dyes for Light-Driven H2 Production.

The design of artificial systems that mimic highly evolved and finely tuned natural enzymes is a promising subject of intensive research. The assembly of O-symmetric cubic structures with an Fe8 L6 formula was reported through the direct combination of a C4 -symmetric tetraphenylethylene-based ligand with a C3 -symmetric tris(bipyridine)iron node. The robust metal-organic cubes are rich in π-electron density and provide favorable interactions with planar polycyclic aromatic hydrocarbons. Within the confined space of the host, the aromatic hydrocarbons molecules are forced closer to the redox active host, and the photoinduced electron transfer (PET) is modified into a pseudo-intramolecular pathway. These iron vertices within the cubes exhibit suitable redox potential for electrochemical reduction of protons and the well-modified PET is further tailored to create artificial systems for light-driven hydrogen evolution from water through the encapsulation of fluorescein dyes. Control experiments based on a mononuclear compound resembling the iron corner of the octahedron suggest an enzymatic dynamic behavior. The new, well-elucidated reaction pathways and the increased molarity of the reaction within the confined space render these supramolecular systems superior to other relevant systems.

Polyiodide chains resolve starch test mystery

In the course of developing new semiconducting materials, a research team including Ram Seshadri and Fred Wudl of the University of California, Santa Barbara, has solved a historical puzzle regarding the structure of the starch-iodine complex (Angew. Chem. Int. Ed. 2016, DOI: 10.1002/anie.201601585). Iodine dissolved in potassium iodide solution has long been used as a test for the presence of starch. The helical iodide chain that forms wraps around starch molecules (amylose) to create a complex that produces a deep purple color. Although scientists have speculated that polymeric iodide chains are involved, the exact structure has remained elusive. Chemists have reported other organic polyiodide complexes, but when characterized they have been found to have chain lengths shorter than 10 iodide units and contain molecular iodine (I2) within the chain. The perylene-iodine complex, for example, made up of stacks of the planar polycyclic aromatic hydrocarbon perylene interspersed with iodide strands, was

Understanding the Reactivity of Planar Polycyclic Aromatic Hydrocarbons: Towards the Graphene Limit.

The Diels-Alder reactivity of maleic anhydride towards the bay regions of planar polycyclic aromatic hydrocarbons was explored computationally in the DFT framework. The process becomes more and more exothermic and the associated activation barriers become lower and lower when the size of the system increases. This enhanced reactivity follows an exponential behavior that reaches its maximum for systems having 18-20 benzenoid rings in their structures. This peculiar behavior was analyzed in detail by using the activation strain model of reactivity in combination with energy decomposition analysis. The influence of the change in the aromaticity of the polycyclic compound during the process on the respective activation barriers was also studied.

Solvation of carbonaceous molecules by para-H2 and ortho-D2 clusters. I. Polycyclic aromatic hydrocarbons.

This work theoretically examines the progressive coating of planar polycyclic aromatic hydrocarbon (PAH) molecules ranging from benzene to circumcoronene (C54H18) by para-hydrogen and ortho-deuterium. The coarse-grained Silvera-Goldman potential has been extended to model the interactions between hydrogen molecules and individual atoms of the PAH and parametrized against quantum chemical calculations for benzene-H2. Path-integral molecular dynamics simulations at 2 K were performed for increasingly large amounts of hydrogen coating the PAH up to the first solvation shell and beyond. From the simulations, various properties were determined such as the size of the first shell and its thickness as well as the solvation energy. The degree of delocalization was notably quantified from an energy landscape perspective, by monitoring the fluctuations among inherent structures sampled by the trajectories. Our results generally demonstrate a high degree of localization owing to relatively strong interactions between hydrogen and the PAH, and qualitatively minor isotopic effects. In the limit of large hydrogen amounts, the shell size and solvation energy both follow approximate linear relations with the numbers of carbon and hydrogen in the PAH.

Sorption of polycyclic aromatic hydrocarbons to polystyrene nanoplastic.

Microplastic has become an emerging contaminant of global concern. Bulk plastic can degrade to form smaller particles down to the nanoscale (<100 nm), which are referred to as nanoplastics. Because of their high surface area, nanoplastic may bind hydrophobic chemicals very effectively, increasing their hazard when such nanoplastics are taken up by biota. The present study reports distribution coefficients for sorption of polycyclic aromatic hydrocarbons (PAHs) to 70 nm polystyrene in freshwater, and PAH adsorption isotherms spanning environmentally realistic aqueous concentrations of 10(-5) μg/L to 1 μg/L. Nanopolystyrene aggregate state was assessed using dynamic light scattering. The adsorption isotherms were nonlinear, and the distribution coefficients at the lower ends of the isotherms were very high, with values up to 10(9) L/kg. The high and nonlinear sorption was explained from π-π interactions between the planar PAHs and the surface of the aromatic polymer polystyrene and was higher than for micrometer-sized polystyrene. Reduction of nanopolystyrene aggregate sizes had no significant effect on sorption, which suggests that the PAHs could reach the sorption sites on the pristine nanoparticles regardless of the aggregation state. Pre-extraction of the nanopolystyrene with C18 polydimethylsiloxane decreased sorption of PAHs, which could be explained by removal of the most hydrophobic fraction of the nanopolystyrene. Environ Toxicol Chem 2016;35:1650-1655. © 2015 SETAC.

Contorted Arene‐fused Metallophthalocyanines

AbstractThree phthalonitriles 9–11 fused with planar and nonplanar polycyclic aromatic hydrocarbons were synthesized through a combination of palladium‐catalyzed coupling, electrophilic cyclization, and Scholl oxidation reactions. The equilibrium geometries and electronic structures of the frontier orbitals of 9–11 were simulated by computation calculations. The optimized structure of 11 is found to have a larger torsional angle than that of 10 because of the non‐planarity arising from some cove and fjord regions. Zinc complexes 1–3 fused with polycyclic aromatic hydrocarbons were synthesized by the cyclotetramerization of 9–11. The attachment of nonplanar polycyclic aromatic hydrocarbons with phthalocyanine rings prevented the formation of aggregates in the solid state.

Dibenzocarbazolediimides: synthesis, solid structure, self-assembly behavior, and optoelectronic properties.

Both planar and nonplanar polycyclic aromatic hydrocarbons (PAHs) have attracted attention owing to their potential applications in optoelectronic materials. Four twist benzopicenediimides with good optoelectronic properties have been reported previously. Following on from this work, four functionalized dibenzocarbazoles have been synthesized and reported herein. The fluorescence quantum yields of these compounds were high in dichloromethane and moderate in the solid state. They have interesting self-assembling behavior and tunable packing motifs in single crystals obtained by introducing different functional groups. Their good optoelectronic properties make them potential candidates for organic devices, bioimaging, and biolabeling.

The infrared spectra of nonplanar polycyclic aromatic hydrocarbons with five- or seven-membered rings

The infrared (IR) spectra are computed for compact polycyclic aromatic hydrocarbons (PAHs) with one five- or one seven-membered central ring, which have a bowl and saddle shape, respectively. In spite of the large geometric distortion compared with the planar PAHs with only six-membered rings, the IR spectra are surprisingly similar. Species with more than one five-membered ring show larger difference compared with typical PAHs. The C–C modes in the “ball” shaped C60 and C70 are shifted somewhat from those in typical PAHs. The ions of C60 and C70 show the typical enhancement of these modes compared with neutrals. Cases where the B3LYP method fail are discussed as is the choice of functional.

Search for polycyclic aromatic hydrocarbons in the Perseus molecular cloud with the Green Bank Telescope

Polycyclic Aromatic Hydrocarbons (PAHs) are believed to be the small-size tail of the interstellar carbonaceous dust grain population. Their vibrational emission is the most widely accepted source of the aromatic near-infrared features, and their rotational radiation is a likely explanation for the dust-correlated anomalous microwave emission (AME). Yet, no individual interstellar PAH molecule has been identified to date. It was recently recognized that quasi-symmetric planar PAHs ought to have a well identifiable comb-like rotational spectrum, and suggested to search for them in spectroscopic data with matched-filtering techniques. We report the results of the first such search, carried out with the Green Bank Telescope, and targeting the star-forming region IC348 in the Perseus molecular cloud, a known source of AME. Our observations amounted to 16.75 hours and spanned a 3 GHz-wide band extending from 23.3 to 26.3 GHz. Using frequency switching, we achieved a sensitivity of 0.4 mJy per 0.4 MHz channel (1-sigma). The non-detection of comb-like spectra allowed us to set upper bounds on the abundance of specific quasi-symmetric PAH molecules (specified uniquely by their moments of inertia) of approximately 0.1% of the total PAH abundance. This bound generically applies to PAHs with approximately 15 to 100 carbon atoms.

Inversion and rotation processes involving non-planar aromatic compounds catalyzed by extended polycyclic aromatic hydrocarbons

Using accurate quantum chemical calculations, we show that extended planar polycyclic aromatic hydrocarbons (PAHs) can efficiently catalyze a range of chemical processes involving non-planar aromatic systems. These include (i) bowl-to-bowl inversion of curved PAHs (e.g. corannulene and sumanene), (ii) ‘flip-flop’ inversion of helicenes (e.g. benzo[c]phenanthrene), and (iii) rotation about the PhPh bond in biphenyls. Non-covalent π–π interactions between the planar catalyst and the substrate stabilize the planar transition structures to a greater extent than they stabilize the non-planar reactants. These result in surprisingly large catalytic enhancements (namely, the reaction barrier heights are reduced by 21–63% of the uncatalyzed reaction barriers).

Site-selective covalent functionalization at interior carbon atoms and on the rim of circumtrindene, a C36H12 open geodesic polyarene.

Circumtrindene (6, C36H12), one of the largest open geodesic polyarenes ever reported, exhibits fullerene-like reactivity at its interior carbon atoms, whereas its edge carbons react like those of planar polycyclic aromatic hydrocarbons (PAHs). The Bingel–Hirsch and Prato reactions – two traditional methods for fullerene functionalization – afford derivatives of circumtrindene with one of the interior 6:6 C=C bonds modified. On the other hand, functionalization on the rim of circumtrindene can be achieved by normal electrophilic aromatic substitution, the most common reaction of planar PAHs. This peripheral functionalization has been used to extend the π-system of the polyarene by subsequent coupling reactions and to probe the magnetic environment of the concave/convex space around the hydrocarbon bowl. For both classes of functionalization, computational results are reported to complement the experimental observations.

Photophysical and Electrochemical Properties of Functionalized Benzo[ghi]perylene and Coronene Derivatives

Planar polycyclic aromatic hydrocarbons (PAHs) such as benzo[ghi]perylene (Bp) and coronene (Cor) are expected for electronic device applications and solar energy conversion because of their π-conjugated structures and ordered aggregate properties. In this work, a series of Bp and Cor derivatives substituted by methoxycarbonyl (COOMe: electron-withdrawing unit) and methoxy (MeO: electron-donating unit) groups were systematically synthesized considering the differences in types and numbers of substituent groups (Figure 1).The electrochemical, spectroscopic and photophysical properties of these molecules were investigated in full detail by cyclic voltammetry, steady-state and time-resolved spectroscopy and quantum yield measurements of fluorescence and intersystem crossing. The synthetic introduction of proper substituents on the aromatic ring therefore enables us to successfully control the electrochemical behavior. For example, with increasing the number of COOMe groups, the reduction and oxidation potentials are significantly shifted to positive direction. On the other hand, the steady-state absorption and fluorescence spectra also become red-shifted and broadened as compared to those of unsubstituted Bp and Cor.Additionally, a systematic comparison of quantum yields and rate constants of the excited-state dynamics also reveals that the fluorescence quantum yields dramatically increase with an increase of the number of COOMe groups in both Bp and Cor derivatives. The fabrication and measurement of filed effect transistors (FET) utilizing these derivatives are also discussed in this presentation.

Hydrogen-Driven Cage Unzipping of C60 into Nano-Graphenes

Annealing of C60 in hydrogen at temperatures above the stability limit of C–H bonds in C60Hx (500–550 °C) is found to result in direct collapse of the cage structure, evaporation of light hydrocarbons, and formation of solid mixture composed of larger hydrocarbons and few-layered graphene sheets. Only a minor part of this mixture is soluble; this was analyzed using matrix-assisted laser desorption/ionization MS, Fourier transform infrared (FTIR), and nuclear magnetic resonance spectroscopy and found to be a rather complex mixture of hydrocarbon molecules composed of at least tens of different compounds. The sequence of most abundant peaks observed in MS, which corresponds to C2H2 mass difference, suggests a stepwise breakup of the fullerene cage into progressively smaller molecular fragments edge-terminated by hydrogen. A simple model of hydrogen-driven C60 unzipping is proposed to explain the observed sequence of fragmentation products. The insoluble part of the product mixture consists of large planar polycyclic aromatic hydrocarbons, as evidenced by FTIR and Raman spectroscopy, and some larger sheets composed of few-layered graphene, as observed by transmission electron microscopy. Hydrogen annealing of C60 thin films showed a thickness-dependent results with reaction products significantly different for the thinnest films compared to bulk powders. Hydrogen annealing of C60 films with the thickness below 10 nm was found to result in formation of nanosized islands with Raman spectra very similar to the spectra of coronene oligomers and conductivity typical for graphene.

Permeation of low-Z atoms through carbon sheets: Density functional theory study on energy barriers and deformation effects

Energetic and geometric aspects of the permeation of the atoms hydrogen to neon neutral atoms through graphene sheets are investigated by investigating the associated energy barriers and sheet deformations. Density functional theory calculations on cluster models, where graphene is modeled by planar polycyclic aromatic hydrocarbons (PAHs), provide the energies and geometries. Particularities of our systems, such as convergence of both energy barriers and deformation curves with increasing size of the PAHs, are discussed. Three different interaction regimes, adiabatic, planar and vertical, are investigated by enforcing different geometrical constraints. The adiabatic energy barriers range from 5 eV for hydrogen to 20 eV for neon. We find that the permeation of oxygen and carbon into graphene is facilitated by temporary chemical bonding while for other, in principle reactive atoms, it is not. We discuss implications of our results for modeling chemical sputtering of graphite.

Structural effects on the oxidation of soot particles by O2: Experimental and theoretical study

Soot particles are composed of polycyclic aromatic hydrocarbons (PAHs), which have either planar or curved structures. The oxidation behaviors of soot particles differ depending on their structures, arrangement of PAHs, and the type of surface functional groups. The oxidation rate of curved PAHs in soot is thought to be higher than that of planar ones. To understand the role that PAH structure plays in soot reactivity towards O2, experimental studies are conducted on two types of commercially produced soot, Printex-U and Fullerene soot, using high resolution transmission electron microscopy, electron energy loss spectroscopy, thermo-gravimetric analysis and elemental analysis. The relative concentrations of active sites, oxygenated functional groups, aliphatics and aromatics present in soots are evaluated. The activation energies for soot oxidation at different conversion levels are determined. The average activation energies of the two soots are found to differ by 26kJ/mol. To understand the reason for this difference, quantum calculations using density functional (B3LYP) and Hartree–Fock theories are conducted to study the reaction pathways of the oxidation by O2 of planar and curved PAHs using 4-pyrenyl and 1-corannulenyl as their model molecules, respectively. The energetically preferred channels for curved PAH oxidation differ from the planar one. The addition of O2 on a radical site of a six-membered ring to form a peroxyl radical is found to be barrierless for both the model PAHs. For peroxyl decomposition, three pathways are suggested, each of which involve the activation energies of 108, 170 and 121kJ/mol to form stable molecules in the case of planar PAH, and 94, 155 and 125kJ/mol in the case of curved PAH. During the oxidation of a five-membered ring, to form stable molecules, the activation energies of 90kJ/mol for the curved PAH and 169kJ/mol for the planar PAH relative to the energy of the peroxyl radical are required. The low activation barriers of preferred pathways for the oxidation of six and five-membered rings present on curved PAH (as compared to the planar one) explain and support its experimentally observed high reactivity.

Ca2+ Promoted the Low Transformation Efficiency of Plasmid DNA Exposed to PAH Contaminants

The effects of interactions between genetic materials and polycyclic aromatic hydrocarbons (PAHs) on gene expression in the extracellular environment remain to be elucidated and little information is currently available on the effect of ionic strength on the transformation of plasmid DNA exposed to PAHs. Phenanthrene and pyrene were used as representative PAHs to evaluate the transformation of plasmid DNA after PAH exposure and to determine the role of Ca2+ during the transformation. Plasmid DNA exposed to the test PAHs demonstrated low transformation efficiency. In the absence of PAHs, the transformation efficiency was 4.7 log units; however, the efficiency decreased to 3.72–3.14 log units with phenanthrene/pyrene exposures of 50 µg·L–1. The addition of Ca2+ enhanced the low transformation efficiency of DNA exposed to PAHs. Based on the co-sorption of Ca2+ and phenanthrene/pyrene by DNA, we employed Fourier-transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and mass spectrometry (MS) to determine the mechanisms involved in PAH-induced DNA transformation. The observed low transformation efficiency of DNA exposed to either phenanthrene or pyrene can be attributed to a broken hydrogen bond in the double helix caused by planar PAHs. Added Ca2+ formed strong electrovalent bonds with “–POO––” groups in the DNA, weakening the interaction between PAHs and DNA based on weak molecular forces. This decreased the damage of PAHs to hydrogen bonds in double-stranded DNA by isolating DNA molecules from PAHs and consequently enhanced the transformation efficiency of DNA exposed to PAH contaminants. The findings provide insight into the effects of anthropogenic trace PAHs on DNA transfer in natural environments.