Stable Polycyclic Aromatic Hydrocarbons Research Articles

Late Pleistocene emergence of an anthropogenic fire regime in Australia’s tropical savannahs

At the time of European arrival on the Australian continent, sophisticated Indigenous societies practiced land management across Australia’s extensive tropical savannahs. Fire was one of the main tools people used to manipulate fuel loads and connectivity to reduce uncontrolled wildfire, maintain vegetation structure and enhance biodiversity. When this alteration of a ‘natural’ fire regime to a human-dominated fire regime occurred is not known. Here we assessed fire incidence and intensity over the past 150,000 years through a continuous lacustrine record by comparing the accumulation rates of micro-charcoal and stable polycyclic aromatic hydrocarbon that form during the combustion of vegetation. We also compared grass (mainly C4) pollen as a percentage of total dryland pollen with the carbon isotope composition of the stable polycyclic aromatic hydrocarbon. We established with high statistical certainty that a change in fire regime occurred at least 11,000 years ago from less-frequent, more-intense fires to more-frequent, less-intense fires. This change marked the overprinting of a largely natural fire regime by one at least modulated by Indigenous management. Our findings demonstrate that human fire use has modified fire regimes throughout the Holocene and also show how people have managed the potential for the type of high-intensity fires that are likely to increase in the future.

Organic carbon, elemental carbon and particulate semivolatile organic compound emissions from a common-rail diesel engine: Insight into effect of fuel injection pressure at different loads

Effect of fuel injection pressure on organic carbon (OC), elemental carbon (EC) and particulate semivolatile organic compounds (SVOCs), i.e., n-alkanes and polycyclic aromatic hydrocarbons (PAHs), emissions from a common-rail diesel engine was analyzed comprehensively. EC emission rate evidently decreased with increasing injection pressure at low fuel injection pressure ranges (80–120 MPa), while engine load effect on the EC emission was insignificant at high injection pressure ranges (140–160 MPa). The higher fraction of EC2 in the total EC emission appeared at the highest injection pressure ranges (140–160 MPa) under middle and high loads, suggesting the spontaneous carbonization process from soot precursor to ordered soot during the high temperature process. Low injection pressure provided poor combustion condition and caused unburned diesel fuel to volatilize more 2–3 ring PAHs. The percentage of 4-ring PAHs exhibited a rise-then-fall trend with increasing injection pressure, while the maximum percentage of 5–7 ring PAHs appeared at the highest injection pressure ranges (140–160 MPa) under high load condition, suggesting that higher combustion temperature and larger pyrolysis zone under the high injection pressure promoted the formation of lager and more stable PAHs. The fractions of fuel-derived short chain (C16-C21) and oil-derived long chain (C22-C33) in the total n-alkanes exhibited obvious load and injection pressure dependence.

Polycyclic aromatic hydrocarbons (PAHs) in air, foliage, and litter in a subtropical forest: Spatioseasonal variations, partitioning, and litter-PAH degradation

Forest canopies play a vital role in scavenging airborne semi-volatile organic compounds. The present study measured polycyclic aromatic hydrocarbons (PAHs) in the understory air (at two heights), foliage, and litterfall in a subtropical rainforest (the Dinghushan mountain) in southern China. ∑17PAH concentrations in the air ranged from 2.75 to 44.0 ng/m3 (mean = 8.91 ng/m3), showing a spatial variation depending on the forest canopy coverage. Vertical distributions of the understory air concentrations also indicated PAH inputs from the above-canopy air. The concentrations of PAHs in fresh litter (with a mean of 261 ± 163 ng/g dry weight (dw)) were slightly lower than those in the foliage (362 ± 291 ng/g dw). Unlike the stable air PAH concentrations for most of the time of the year, the temporal variations of foliage and litter concentrations were remarkable but generally similar. Higher or comparable leaf/litter-air partition coefficients (KLA) in fresh litter compared with living KLA in leaves suggest that the forest litter layer is an efficient storage media for PAHs. Degradation of three-ring PAHs in litter under the field conditions follows first-order kinetics (R2 = 0.81), while the degradation is moderate for four-ring PAHs and insignificant for five- and six-ring PAHs. The yearly net cumulative deposition of PAHs through forest litterfall in the whole Dinghushan forest area over the sampling year was about 1.1 kg, 46% of the initial deposition (2.4 kg). This spatial variations study provides the results of in-field degradation of litter PAHs and makes a quantitative assessment of the litter deposition of PAHs, deducing their residence dynamics in the litter layer in a subtropical rainforest.

Air Stable Chalcogen-Doped Rubicenes with Diradical Character

Air Stable Chalcogen-Doped Rubicenes with Diradical Character

Photodissociation of aliphatic PAH derivatives under relevant astrophysical conditions

Context. The interaction of polycyclic aromatic hydrocarbons (PAHs) with vacuum ultraviolet (VUV) photons triggers the emission of the well-known aromatic infrared bands (AIBs), but other mechanisms, such as fragmentation, can be involved in this interaction. Fragmentation leads to selection effects that favor specific sizes and structures. Aims. Our aim is to investigate the impact of aliphatic bonds on the VUV photostability of PAH cations in a cryogenic and collisionless environment with conditions applicable for photodissociation regions (PDRs). Methods. The studied species are derived from pyrene (C16H10) and coronene (C24H12) and contain aliphatic bonds either in the form of methyl or ethyl sidegroups or of superhydrogenation. Their cations are produced by laser desorption ionization and isolated in the cryogenic ion cell of the PIRENEA setup, where they are submitted to VUV photons of 10.5 eV energy over long timescales (~1000 s). The parent and fragment ions are mass-analyzed and their relative intensities are recorded as a function of the irradiation time. The fragmentation cascades are analyzed with a simple kinetics model from which we identify fragmentation pathways and derive fragmentation rates and branching ratios for both the parents and their main fragments. Results. Aliphatic PAH derivatives are found to have a higher fragmentation rate and a higher carbon to hydrogen loss compared to regular PAHs. On the other hand, the fragmentation of PAHs with alkylated sidegroups forms species with peripheral pentagonal cycles, which can be as stable as, or even more stable than, the bare PAH cations. This stability is quantified for the main ions involved in the fragmentation cascades by the comparison of the fragmentation rates with the photoabsorption rates derived from theoretical photoabsorption cross sections. The most stable species for which there is an effective competition of fragmentation with isomerization and radiative cooling are identified, providing clues on the structures favored in PDRs. Conclusions. This work supports a scenario in which the evaporation of nanograins with a mixed aliphatic and aromatic composition followed by VUV photoprocessing results in both the production of the carriers of the 3.4 μm AIB by methyl sidegroups and in an abundant source of small hydrocarbons at the border of PDRs. An additional side effect is the efficient formation of stable PAHs that contain some peripheral pentagonal rings. Our experiments also support the role of isomerization processes in PAH photofragmentation, including the H-migration process, which could lead to an additional contribution to the 3.4 μm AIB.

Identification of wintertime carbonaceous fine particulate matter (PM2.5) sources in Kaunas, Lithuania using polycyclic aromatic hydrocarbons and stable carbon isotope analysis

Abstract The study aimed at demonstrating the application of PAHs and δ13C indoor and outdoor measurements towards resolving airborne carbonaceous particle sources in wintertime. Concurrent outdoor and indoor measurements of PM2.5 and size-resolved (PM0.056-2.5) particulate matter were conducted at 6 one-family houses in Kaunas, Lithuania during January–March 2013. PM2.5 and size-resolved filter samples were analyzed for total carbon (TC) and stable carbon isotopic composition by elemental analyzer connected to isotope ratio mass spectrometer. Moreover, the indoor-outdoor concentrations of 10 selected polycyclic aromatic hydrocarbons (PAHs) in PM2.5 were quantified. The simultaneous stable carbon isotope ratio and PAHs diagnostic ratio analysis enabled to identify main anthropogenic sources (traffic, biomass burning, and coal combustion) of PM2.5. Our study indicated that during winter period outdoor PM2.5 in Kaunas were mainly derived from biomass burning (solid fuel stoves) and vehicular emissions. Whereas indoor particulate matter had different stable carbon isotope composition of TC indicating the existence of pollution sources which was likely cooking emissions. The differences of indoor/outdoor stable carbon isotope composition in PM2.5 and size-resolved aerosol particles supported the existence of distinct TC sources and suggested that cooking emissions inside the house could strongly affect δ13C values.

Low-temperature Formation of Carbonaceous Dust Grains from PAHs

Abstract Carbon is ubiquitous in space and plays a key role in prebiotic chemistry. Astronomical observations have found interstellar carbon in the form of polycyclic aromatic hydrocarbons (PAHs) as well as carbonaceous dust, confirming its presence in both gaseous and solid phases. The goal of this study is to experimentally investigate low-temperature chemical pathways between these two phases in order to better understand the evolution of cosmic carbon. Cosmic dust analogs are produced in the supersonic expansion of an argon jet seeded with aromatic molecules: benzene, naphthalene, anthracene, phenanthrene, and pyrene. These are prototype aromatic molecules of compact and noncompact structure, and are used to evaluate the effect of precursor structure on the molecular complexity of the resulting grains. The seeded jet is exposed to an electrical discharge and the carbonaceous grains are collected and probed ex situ via laser desorption mass spectrometry. Mass spectra reveal a rich molecular diversity within the grains, including fragments of the parent molecule but also growth into larger complex organic molecules (COMs). In all experiments, the largest number of products is found in the m/z range 200–250, and C16H10 (attributed to pyrene and/or its isomers) is found to be a dominant product, pointing at the formation of this stable PAH as a preferential route in the growth to larger PAHs. Comparison to mass spectra from the Murchison meteorite reveals a similar dominance of compounds related to C16H10 at m/z = 202. Evidence of the methyl-addition-cyclization mechanism in the anthracene experiment is reported. PAH structure is found to impact the dust production yield, as seen by the greater yield for the anthracene compared to the phenanthrene experiment. PAH growth at low temperatures via barrierless routes involving the addition of alkyl- and phenyl-type radicals is suggested as a viable pathway to COMs. These results suggest that PAH growth and dust formation from PAHs are feasible at low temperatures in photon-dominated regions and circumstellar envelopes.

X-Shaped Polycyclic Aromatic Hydrocarbons: Optical Properties and Tunable Assembly Ability.

Although a number of synthetic methodologies have been developed to prepare stable polycyclic aromatic hydrocarbons (PAHs), much less research has been devoted to functionalizing the peripheries of molecules to tune the self-assembly ability or introduce functional groups without altering their photophysical properties. Herein, we report twisted "X"-shaped molecules prepared through annulation of hexacene with benzoanthracene on the zigzag edge, and an investigation of their photophysical properties and self-assembly properties. The shape-complementary "X"-shaped molecules prefer to dimerize, while the π-extension would lead to one-dimensional π-stacking. Our findings give some insights into the design of stable PAHs without disturbing the electronic structures.

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.

A method for controlling the synthesis of stable twisted two-dimensional conjugated molecules.

Thermodynamic stabilization (π-electron delocalization through effective conjugation) and kinetic stabilization (blocking the most-reactive sites) are important considerations when designing stable polycyclic aromatic hydrocarbons displaying tunable optoelectronic properties. Here, we demonstrate an efficient method for preparing a series of stable two-dimensional (2D) twisted dibenzoterrylene-acenes. We investigated their electronic structures and geometries in the ground state through various experiments assisted by calculations using density functional theory. We find that the length of the acene has a clear effect on the photophysical, electrochemical, and magnetic properties. These molecules exhibit tunable ground-state structures, in which a stable open-shell quintet tetraradical can be transferred to triplet diradicals. Such compounds are promising candidates for use in nonlinear optics, field effect transistors and organic spintronics; furthermore, they may enable broader applications of 2D small organic molecules in high-performance electronic and optical devices.

Biomonitoring method for the determination of polycyclic aromatic hydrocarbons in hair by online in-tube solid-phase microextraction coupled with high performance liquid chromatography and fluorescence detection.

Polycyclic aromatic hydrocarbons (PAHs) are formed from the incomplete combustion or pyrolysis of organic matter during industrial processing and various human activities, but human exposure to PAHs has not yet been elucidated in detail. To assess long-term exposure to PAHs, we developed a simple and sensitive method for measuring PAHs in hair by online in-tube solid-phase microextraction using a CP-Sil 19CB capillary column as an extraction device, followed by high-performance liquid chromatography using a Zorbax Eclipse PAH column and fluorescence detection. Seventeen PAHs could be analyzed simultaneously, with good linearity from 20 to 1000pg/mL each as determined using stable isotope-labeled PAH internal standards. The detection limits of PAHs were 0.5-20.4pg/mL. PAHs in human hair samples were extracted by ultrasonication in 50mM NaOH in methanol, and successfully analyzed without any interference peaks, with good recovery rates above 70% in spiked hair samples. Using this method, we evaluated the suitability of using hair PAHs as biomarkers for long-term exposure.

PAH EMISSION AT THE BRIGHT LOCATIONS OF PDRs: THE grandPAH HYPOTHESIS

The polycyclic aromatic hydrocarbon (PAH) emission observed in the Spitzer Infrared Spectrograph spectra of bright mid-IR locations of NGC 7023, NGC 2023, and NGC 1333 was analyzed. These objects show large variations in PAH band ratios when studied through spectral mapping. Nevertheless, the mid-IR spectra at these bright spots show a remarkably similar PAH emission. We used the NASA Ames PAH IR Spectroscopic Database to fit the observations and analyze the derived PAH populations. Our results show that PAH emission in the 5–15 μm range appears to be rather insensitive to variations of the radiation field. Similar PAH populations of neutral small to medium-sized PAHs (∼50%), with ionized species contributing in slightly less than 50%, provide very good fits. Analyzing the degeneracy of the results shows that subtle (but intrinsic) variations in the emission properties of individual PAHs lead to observable differences in the resulting spectra. On top of this, we found that variations of <30% in the PAH abundances would lead to noticeable spectral differences between the three photodissociation regions (PDRs). Therefore, PAH populations must be remarkably similar at these different lines of sight. To account for this, we suggest the concept of grandPAHs as a unique mixture of the most stable PAHs emitting at these spots. Using NGC 7023 as an example, the grandPAHs refer to the robust PAH population that results from the intense processing of PAHs at the border limit between the PDR and the molecular cloud, where, due to the UV radiation that destroys the PAH population, the abundance of PAHs starts decreasing as we move toward the star.

Unconventional, chemically stable, and soluble two-dimensional angular polycyclic aromatic hydrocarbons: from molecular design to device applications.

Polycyclic aromatic hydrocarbons (PAHs), consisting of laterally fused benzene rings, are among the most widely studied small-molecule organic semiconductors, with potential applications in organic field-effect transistors (OFETs) and organic photovoltaics (OPVs). Linear acenes, including tetracene, pentacene, and their derivatives, have received particular attention due to the synthetic flexibility in tuning their chemical structure and properties and to their high device performance. Unfortunately, longer acenes, which could exhibit even better performance, are susceptible to oxidation, photodegradation, and, in solar cells which contain fullerenes, Diels-Alder reactions. This Account highlights recent advances in the molecular design of two-dimensional (2-D) PAHs that combine device performance with environmental stability. New synthetic techniques have been developed to create stable PAHs that extend conjugation in two dimensions. The stability of these novel compounds is consistent with Clar's sextet rule as the 2-D PAHs have greater numbers of sextets in their ground-state configuration than their linear analogues. The ionization potentials (IPs) of nonlinear acenes decrease more slowly with annellation in comparison to their linear counterparts. As a result, 2-D bistetracene derivatives that are composed of eight fused benzene rings are measured to be about 200 times more stable in chlorinated organic solvents than pentacene derivatives with only five fused rings. Single crystals of the bistetracene derivatives have hole mobilities, measured in OFET configuration, up to 6.1 cm(2) V(-1) s(-1), with remarkable Ion/Ioff ratios of 10(7). The density functional theory (DFT) calculations can provide insight into the electronic structures at both molecular and material levels and to evaluate the main charge-transport parameters. The 2-D acenes with large aspect ratios and appropriate substituents have the potential to provide favorable interstack electronic interactions, and correspondingly high carrier mobilities. In stark contrast to the 1-D acenes that form mono- and bis-adducts with fullerenes, 2-D PAHs show less reactivity with fullerenes. The geometry of 2-D PAHs plays a crucial role in determining both the barrier and the adduct stability. The reactivity and stability of the 2-D PAHs with regard to Diels-Alder reactions at different reactive sites were explained via DFT calculations of the reaction kinetics and of thermodynamics of reactions and simple Hückel molecular orbital considerations. Also, because of their increased stability in the presence of fullerenes, these compounds have been successfully used in OPVs. The small-molecule semiconductors highlighted in this Account exhibit good charge-transport properties, comparable to those of traditional linear acenes, while being much more environmentally stable. These features have made these 2-D PAHs excellent molecules for fundamental research and device applications.

Molecular composition of fossil charcoal and relationship with incomplete combustion of wood

Upper Triassic charcoal extracts were studied using gas chromatography–mass spectrometry (GC–MS) to recognize their molecular composition. Extractable compounds were divided into: (i) biomarkers, i.e. diagenetically changed primary wood components and (ii) products of combustion. Major compounds in the first group were: 1,2,5-trimethylnaphthalene and 1,2,5,6-tetramethylnaphthalene, cadalene, dehydroabietane, simonellite and retene. All of these are derived from resins. Moreover, propyl phenols, butyl acetophenones and pentyl acetophenones, as products of lignin breakdown, as well as fatty acids with a predominance of palmitic acid, typical constituents of wood, were also detected. Polycyclic aromatic hydrocarbons (PAHs), as well as ketones and aryl phenols, considered as high temperature combustion products, occurred at relatively low concentration in the samples due to their enhanced solubility in gelified, non-charred wood fragments, and vaporization of the major part of the burn products. Despite the low PAH concentrations, their distribution, with a significant contribution from typical pyrolytic compounds such as anthracene, 4H-cyclopenta[def]phenanthrene, benz[a]anthracene and benzo[a]pyrene was typical for rapid combustion. We propose to estimate paleo-wildfire temperature based on the PAH concentrations in the paleo-charcoal samples. The presence of thermally less stable organic compounds and low PAH abundances indicates a temperature < 400°C. High PAH amounts seem to be characteristic for charring between 400 and 500°C. Above these temperatures PAH concentrations again decrease, but less stable compounds are absent.

Revealing the Dual Role of Hydrogen for Growth Inhibition and Defect Healing in Polycyclic Aromatic Hydrocarbon Formation: QM/MD Simulations

Quantum mechanical molecular dynamics simulations are employed to reveal the influence of hydrogen on polycyclic aromatic hydrocarbon (PAH) formation in oxygen-lean combustion. While higher hydrogen concentration leads to the inhibition of PAH growth, it simultaneously facilitates pentagon and heptagon defect healing, leading to thermodynamically more stable PAH fragments with more hexagons. We therefore propose the existence of an optimal H/C ratio that facilitates the growth of all-hexagon-containing PAH species. Analysis of the PAH edge reconstruction in our simulations shows a near-equal ratio of armchair and zigzag edge structures. As armchair edge structures are thermodynamically considerably more stable than zigzag edge structures, the present simulations show that both kinetic and thermodynamic factors are needed to understand PAH/graphene edge reconstruction.

Reactivity of polycyclic aromatic compounds (PAHs, NPAHs and OPAHs) adsorbed on natural aerosol particles exposed to atmospheric oxidants

Reactivity of polycyclic aromatic compounds (PACs) adsorbed on natural aerosol particles exposed to different atmospheric oxidants (O3, OH and NO2/O3 mixture) was studied. Decay of polycyclic aromatic hydrocarbons (PAHs) and formation/decay of oxygenated PAHs (OPAHs) and nitrated PAHs (NPAHs) were monitored. Overall, benzo[a]pyrene appeared to be the most reactive PAH (degradation of 50%). Only its nitrated derivative, 6-nitrobenzo[a]pyrene, was significantly formed explaining just 0.4% of reacted benzo[a]pyrene. No other nitrated or oxygenated benzo[a]pyrene derivatives were detected. Interestingly, B[e]P and In[1,2,3,c,d]P, which are usually considered as quite stable PAHs, also underwent decay in all experiments. In presence of O3, ketones were significantly formed but their amount was not totally explained by decay of parent PAH. These results suggest that PAH derivatives could be formed from the reaction of other compounds than their direct parent PAHs and raise the question to know if the oxidation of methyl-PAHs, identified in vehicle-exhausts, could constitute this missing source of OPAHs. NPAHs were significantly formed in presence of O3/NO2 and OH. Surprisingly, NPAH formation was clearly observed during O3 experiments. Nitrated species, already associated with aerosol particles (NO3−, NO2−) or formed by ozonation of particulate nitrogen organic matter, could react with PAHs to form NPAHs. Heterogeneous formation of 2-nitropyrene from pyrene oxidation was for the first time observed, questioning its use as an indicator of NPAH formation in gaseous phase. Equally, formation of 2-nitrofluoranthene by heterogeneous reaction of fluoranthene with O3/NO2 was clearly shown, while only its formation by homogeneous processes (gaseous phase) is reported in the literature. Finally, results obtained highlighted the dependence of heterogeneous PAH reactivity with the substrate nature and the importance to focus reactivity studies on natural particles, whatever the quality of the models previously studied.

ChemInform Abstract: Open‐Shell Polycyclic Aromatic Hydrocarbons

AbstractReview: 137 refs.

Open-shell polycyclic aromatic hydrocarbons

Polycyclic aromatic hydrocarbons (PAHs) with unpaired or partially unpaired electrons will display open-shell radical character (monoradical, biradical, polyradical). This will lead to special features in electronic structures, magnetic properties and crystalline packing, which endow them with great potential as a new generation of materials. Stability issues are always the focus on the way towards functionalized open-shell PAHs, and chemists have made huge endeavours in synthesizing stable PAHs with open-shell characters. In this article, we summarize recent developments on open-shell PAHs covering both theoretical advances and experimental progresses, and touch on a few examples of them for practical applications.

Effects of weathering on organic matter: I. Changes in molecular composition of extractable organic compounds caused by paleoweathering of a Lower Carboniferous (Tournaisian) marine black shale

A detailed bulk and molecular study on paleoweathering of a Lower Carboniferous (Tournaisian) black shale from the Kowala quarry in the Holy Cross Mountains of Poland, revealed significant changes in total organic carbon (TOC), total sulfur (TS) and extract compositions. Paleoweathering resulted in a 97% decrease in TOC and total loss of sulfur, as well as changes in carbonate contents, extract yields and percentage yields of the organic fractions. Pyrite framboids, which are used extensively in paleoecological studies, decreased considerably in the partially weathered zone and totally vanished in the weathered zone. The decrease in TOC is accompanied by a pronounced reduction of organic compound concentrations, but the degradation range differs in the individual weathering zones. Here we show that less stable compounds such as low molecular weight aromatics (e.g. methylnaphthalenes, dibenzofuran, and dibenzothiophene), isorenieratane and its diagenetic products, or maleimides decrease significantly or disappear already in the partially weathered zone, while the more stable polycyclic aromatic hydrocarbons (PAHs) decrease (~ 90%) only in the weathered and highly weathered zones. Besides the organic matter (OM) content, the influence of paleoweathering on the distributions of organic compounds is important in the context of paleoenvironment, source and maturity interpretations. Almost all sterane and triterpane biomarker parameters change their values in the highly weathered zone, but some ratios, e.g. the 2-MeH index, are almost totally resistant to change. The aryl isoprenoid ratio (AIR) values decrease gradually with weathering. This modifies completely the potential interpretation of the nature of the photic zone anoxia. In addition to degradation of OM, some PAHs like benzo[ b]fluoranthene increase in concentration in the partially weathered zone due to their formation from phenyl-derivatives. The correct recognition of paleoweathering in outcrop and drill core samples aids in the proper interpretation of biomarker parameters and contributes to a better understanding of the processes which took place during weathering.

Soluble and Columnar Liquid Crystalline Peropyrenequinones by Coupling of Phenalenones in Caesium Hydroxide

Perylene tetracarboxylic diimides (PTCDIs) 1 are very versatile, stable and synthetically very accessible red dyes with high extinction coefficients, that have, amongst a variety of other applications, found widespread use in molecular optoelectronics, including electrophotography, solar cells and light-emitting diodes. When substituted with appropriate alkyl or trialkylaryl substituents, they exhibit lamellar or columnar liquid crystalline self-assembly, which gives rise to high intermolecular charge carrier mobilities. The columnar liquid-crystalline state is also prominent in the corresponding orange perylene tetracarboxylic tetraalkyl esters (PTCTEs) 2 in which it can be stabilised at room temperature with racemically branched alkyl groups. A key feature of tetracarboxylic perylene derivatives is their intense absorption at relatively long wavelengths (about 525 nm for dialkyl imides and about 475 nm for tetraalkyl esters in solution) with respect to their small chromophore size (which favours solubility), when compared with other relatively stable polycyclic aromatic hydrocarbon (PAH) ACHTUNGTRENNUNGderivatives. Considerable efforts have been undertaken to obtain perylene derivatives that absorb at yet longer wavelengths, whilst maintaining solubility. These approaches include the introduction of arylimidazole moieties, donor substitution in the bay regions, extension of the core in the rylene series and lateral oligomerisation. To shift the absorption maximum to longer wavelengths by a particularly simple approach that maintains the small chromophore size of 1, and to ensure at the same time columnar liquid-crystalline self-assembly at room temperature, we have investigated the known base-induced dehydro-dimerisation of 3-hydroxy-1-phenalenone (3a). The resulting violet dihydroxyperopyrenequinone 4 can be regarded as a homologue of 1 in which both nitrogen atoms have been exchanged for methine moieties (Scheme 1). Since 4 is enolised (no H NMR coupling in alkyl-substituted 4 between the a protons of R with any ring proton), which yields to two fully sp-configured terminal rings, its colour is