Small Polycyclic Aromatic Hydrocarbons Research Articles

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 mass (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 frequently applied 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. Moreover, we compared the skin permeation kinetics of different PAH in pigskin. While small PAH (< 230 g/mol, logP < 6) permeate the skin quickly and are detected in the receptor fluid after 2 h, large PAH (> 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.

Proximity effects: Structural implications and quantum-chemical description. Review

Proximity effects occur between vicinal functional groups or between a group and a specific molecular fragment in close proximity, for example an endocyclic N atom in the ortho position relative to the group. They are a combination of through-bond inductive and resonance effects, and through-space interactions, which can be either attractive or repulsive. In this review, we discuss quantum-chemical tools and examples from the literature that explore the structural and physicochemical implications of proximity effects. We focus on derivatives of small molecules: benzene, small polycyclic aromatic hydrocarbons, simple heterocycles, and lastly, purine derivatives.

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.

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.

Smoke Points: A Crucial Factor in Cooking Oil Selection for Public Health

Abstract: Cooking oils and fats play a significant role in our daily diet and culinary practices by enhancing flavours, textures, and nutritional value. However, overheating these fats can compromise the quality and safety of cooked foods. When oils and fats exceed their smoke points, they undergo chemical breakdown, producing volatile compounds, off-flavours, and undesirable odors, including harmful substances like small chain fatty acids, trans fats, acrylamides, and polycyclic aromatic hydrocarbons. It is crucial to avoid overheating oils to mitigate the formation of these toxic substances and instead opt for those with higher smoke points for high-temperature cooking methods. The smoke point, indicating the temperature at which visible smoke is emitted, serves as a critical indicator of thermal stability and suitability for various cooking oils and fats. Therefore, understanding and considering the smoke points of different oils and fats are essential for maintaining food quality and safety in culinary practices. This review consolidates existing knowledge on the smoke points of various oils and fats and methods for determining smoke points, providing a list of fifty-one oils and fats with their respective smoke points and highlighting their applications in cooking. By considering the smoke point, chefs, cooks, and food manufacturers can select oils that optimize cooking, frying, taste, texture, flavour enhancement, salad dressings, marinades, baking, and overall safety in their culinary practices. Mindfulness of the smoke point helps prevent the degradation of nutritional value and the generation of harmful compounds during the cooking process.

Calibrating mid-infrared emission as a tracer of obscured star formation on H II-region scales in the era of JWST

Measurements of the star formation activity on cloud scales are fundamental to uncovering the physics of the molecular cloud, star formation, and stellar feedback cycle in galaxies. Infrared (IR) emission from small dust grains and polycyclic aromatic hydrocarbons (PAHs) is widely used to trace the obscured component of star formation. However, the relation between these emission features and dust attenuation is complicated by the combined effects of dust heating from old stellar populations and an uncertain dust geometry with respect to heating sources. We used images obtained with NIRCam and MIRI as part of the PHANGS–JWST survey to calibrate the IR emission at 21 μm, and the emission in the PAH-tracing bands at 3.3, 7.7, 10, and 11.3 μm as tracers of obscured star formation. We analysed ∼20 000 optically selected H II regions across 19 nearby star-forming galaxies, and benchmarked their IR emission against dust attenuation measured from the Balmer decrement. We modelled the extinction-corrected Hα flux as the sum of the observed Hα emission and a term proportional to the IR emission, with aIR as the proportionality coefficient. A constant aIR leads to an extinction-corrected Hα estimate that agrees with those obtained with the Balmer decrement with a scatter of ∼0.1 dex for all bands considered. Among these bands, 21 μm emission is demonstrated to be the best tracer of dust attenuation. The PAH-tracing bands underestimate the correction for bright H II regions, since in these environments the ratio of PAH-tracing bands to 21 μm decreases, signalling destruction of the PAH molecules. For fainter H II regions, all bands suffer from an increasing contamination from the diffuse IR background. We present calibrations that take this effect into account by adding an explicit dependence on 2 μm emission or stellar mass surface density.

A comparative laboratory study of soft X-ray-induced ionization and fragmentation of five small PAH cations

The interaction between polycyclic aromatic hydrocarbon (PAH) radical cations and X-rays predominantly leads to photofragmentation, a process that strongly depends on PAH size and geometry. In our experiments, five prototypical PAHs were exposed to monochromatic soft X-ray photons with energies in the C K-edge regime. As a function of soft X-ray photon energy, photoion yields were obtained by means of time-of-flight mass spectrometry. The resulting near-edge X-ray absorption mass spectra were interpreted using time-dependent density functional theory (TD-DFT) with a short-range corrected functional. We found that the carbon backbone of anthracene^+ (C_{14}H_{10}^+), pyrene^+ (C_{16}H_{10}^+) and coronene^+ (C_{24}H_{12}^+) can survive soft X-ray absorption, even though mostly intermediate size fragments are formed. In contrast, for hexahydropyrene^+ (C_{16}H_{16}^+) and triphenylene^+ (C_{18}H_{12}^+) molecular survival is not observed and the fragmentation pattern is dominated by small fragments. For a given excitation energy, molecular survival evidently does not simply correlate with PAH size but strongly depends on other PAH properties.Graphic abstract

Electronic spectroscopy of 1-cyanonaphthalene cation for astrochemical consideration

Context. Polycyclic aromatic hydrocarbons (PAHs) are believed to be the carriers of the aromatic infrared bands and have been proposed as candidates to explain other astronomical phenomena such as diffuse interstellar bands (DIBs). The first aromatic structures possessing more than one ring, 1- and 2-cyanonaphthalene (CNN), were recently detected by rotational spectroscopy in the dense molecular cloud TMC-1. Laboratory investigations have indicated that due to fast and efficient relaxation through recurrent fluorescence (RF), CNN+ may be photostable in the harsh conditions of the lower density, more diffuse regions of the interstellar medium (ISM) exposed to ultraviolet (UV) radiation. As a result, it has been suggested that the widely held belief that small PAHs present in these regions are dissociated may need to be revisited. If 1-CNN+ is able to survive in the diffuse ISM it may contribute to the population of 1-CNN observed in TMC-1. To investigate the abundance of 1-CNN+ in diffuse clouds, laboratory spectroscopy is required. The present work concerns the electronic spectroscopy of 1-CNN+ in absorption and the search for its spectroscopic fingerprints in diffuse clouds. Aims. The aim is to obtain laboratory data on the electronic transitions of gas-phase 1-CNN+ under conditions appropriate for comparison with DIBs and assess abundance in diffuse clouds. Methods. Spectroscopic experiments are carried out using a cryogenic ion trapping apparatus in which gas-phase 1-CNN+ is cooled to temperatures below 10 K through buffer gas cooling. Calculations are carried out using time-dependent density-functional theory. Results. Experimental and theoretical data on the D2 ← D0 and D3 ← D0 electronic transitions of 1-CNN+ are reported. The former transition has a calculated oscillator strength of f = 0.075 and possesses a pattern dominated by its origin band. The origin band is located at 7343 Å and has a full width at half maximum of 28 Å. In observational data, this falls in a region polluted by telluric water lines, hindering assessment of its abundance. Conclusions. Space-based observations are required to search for the spectroscopic signatures of 1-CNN+ and evaluate the hypothesis that this small aromatic system, stabilised by RF, may be able to survive in regions of the ISM exposed to UV photons.

(Invited) Optical Investigation of Single Nanographenes

Carbon based light emitters ranging from small organic molecules to carbon nanotubes have been subject of a great attention in the framework of diverse applications such as optoelectronics, bio-imaging, and quantum technologies. In this context, graphene quantum dots (GQD) whose size is between that of small polycyclic aromatic hydrocarbon molecules and of carbon nanotubes, have important assets. In particular the complete control of the structure allowed by their synthesis through bottom-up chemistry opens the way to a wide customization of their electronic, optical, and spin properties [1-3].The full benefit from these opportunities requires addressing GQD’s intrinsic photophysical properties. To do so, single molecule photoluminescence experiment is a powerful tool [4-6]. Nevertheless, until now, GQDs faced a problem of solubility that has complexified both the study of their intrinsic properties and their use for applications. In this presentation, we will report on the study of new structures that show a quasi-perfect solubility. We will show how this breakthrough allows us studying in depth their photophysics. Finally, we will see that the theoretical predictions of the evolution of the electronic properties of the GQDs with their structure are fully confirmed by the experiments.[1] M. G. Debije, J. Am. Chem. Soc. 2004, 126, 4641[2] X. Yan, X. Cui, and L.-s. Li, J. Am. Chem. Soc. 2010 132, 5944[3] A. Konishi et al, J. Am. Chem. Soc. 2010, 132, 11021[4] S. Zhao et al, Nature Communications, 2018, 9, 3470[5] T. Liu et al, Nanoscale, 14, 3826 – 3833 (2022)[6] T. Liu et al, Journal of Chemical Physics 156, 104302 (2022)[7] D. Medina-Lopez et al, in preparation

Autoionization from the plasmon resonance in isolated 1-cyanonaphthalene.

Polycyclic aromatic hydrocarbons have widely been conjectured to be ubiquitous in space, as supported by the recent discovery of two isomers of cyanonaphthalene, indene, and 2-cyanoindene in the Taurus molecular cloud-1 using radioastronomy. Here, the photoionization dynamics of 1-cyanonaphthalene (1-CNN) are investigated using synchrotron radiation over the hν = 9.0-19.5eV range, revealing that prompt autoionization from the plasmon resonance dominates the photophysics for hν = 11.5-16.0eV. Minimal photo-induced dissociation, whether originating from an excited state impulsive bond rupture or through internal conversion followed by a statistical bond cleavage process, occurs over the microsecond timescale (as limited by the experimental setup). The direct photoionization cross section and photoelectron angular distributions are simulated using an ezDyson model combining Dyson orbitals with Coulomb wave photoejection. When considering these data in conjunction with recent radiative cooling measurements on 1-CNN+, which showed that cations formed with up to 5eV of internal energy efficiently stabilize through recurrent fluorescence, we conclude that the organic backbone of 1-CNN is resilient to photodestruction by VUV and soft XUV radiation. These dynamics may prove to be a common feature for the survival of small polycyclic aromatic hydrocarbons in space, provided that the cations have a suitable electronic structure to support recurrent fluorescence.

Spectral variations among different scenarios of PAH processing or formation

ABSTRACT We examine the variations in the spectral characteristics and intensities of polycyclic aromatic hydrocarbons (PAHs) in two different scenarios of PAH processing (or formation): (1) small PAHs are being destroyed (or equivalently large PAHs are being formed, referred to as SPR, i.e. small PAHs removed), and (2) large PAHs are being destroyed (or equivalently small PAHs are being formed, referred to as LPR, i.e. large PAHs removed). PAH emission was measured considering both the presence and absence of plateau components. The variation in the PAH band intensities as a function of the average number of carbon atoms 〈NC〉 has the highest dynamic range in the SPR case, suggesting that smaller PAHs have higher impact on the PAH band strengths. The plateaus show overall declining emission with 〈NC〉, and their higher dynamic range in the SPR case also suggests that smaller PAHs are mainly contributing to the plateau emission. The 7.7/(11.0 + 11.2) $\mu$m PAH band ratio presents the least amount of variance with the lowest dynamic range, rendering this ratio as the better choice for tracing PAH charge. The 3.3/(11.2 + 11.0) $\mu$m PAH band ratio is the only ratio that has both a monotonic variance and fully separated values among the SPR and LPR scenarios, highlighting its efficiency as PAH size tracer but also allowing the characterization of the dominant scenario of processing or formation in a given region or source. We present new PAH charge–size diagnostic diagrams, which can provide insights into the average, maximum, or minimum NC within astrophysical sources.

Efficient stabilization of cyanonaphthalene by fast radiative cooling and implications for the resilience of small PAHs in interstellar clouds

After decades of searching, astronomers have recently identified specific Polycyclic Aromatic Hydrocarbons (PAHs) in space. Remarkably, the observed abundance of cyanonaphthalene (CNN, C10H7CN) in the Taurus Molecular Cloud (TMC-1) is six orders of magnitude higher than expected from astrophysical modeling. Here, we report unimolecular dissociation and radiative cooling rate coefficients of the 1-CNN isomer in its cationic form. These results are based on measurements of the time-dependent neutral product emission rate and kinetic energy release distributions produced from an ensemble of internally excited 1-CNN+ studied in an environment similar to that in interstellar clouds. We find that Recurrent Fluorescence – radiative relaxation via thermally populated electronic excited states – efficiently stabilizes 1-CNN+, owing to a large enhancement of the electronic transition probability by vibronic coupling. Our results help explain the anomalous abundance of CNN in TMC-1 and challenge the widely accepted picture of rapid destruction of small PAHs in space.

Experimental radiative cooling rates of a polycyclic aromatic hydrocarbon cation.

Several small Polycyclic Aromatic Hydrocarbons (PAHs) have been identified recently in the Taurus Molecular Cloud (TMC-1) using radio telescope observations. Reproducing the observed abundances of these molecules has been a challenge for astrochemical models. Rapid radiative cooling of PAHs by Recurrent Fluorescence (RF), the emission of optical photons from thermally populated electronically excited states, has been shown to efficiently stabilize small PAHs following ionization, augmenting their resilience in astronomical environments and helping to rationalize their observed high abundances. Here, we use a novel method to experimentally determine the radiative cooling rate of the cation of 1-cyanonaphthalene (C10H7CN, 1-CNN), the neutral species of which has been identified in TMC-1. Laser-induced dissociation rates and kinetic energy release distributions of 1-CNN cations isolated in a cryogenic electrostatic ion-beam storage ring are analysed to track the time evolution of the vibrational energy distribution of the initially hot ion ensemble as it cools. The measured cooling rate is in good agreement with the previously calculated RF rate coefficient. Improved measurements and models of the RF mechanism are needed to interpret astronomical observations and refine predictions of the stabilities of interstellar PAHs.

Connections between the accuracy of rotational constants and equilibrium molecular structures.

Rotational spectroscopy is the technique of choice for investigating molecular structures in the gas phase. Indeed, rotational constants are strongly connected to the geometry of the molecular system under consideration. Therefore, they are powerful tools for assessing the accuracy that quantum chemical approaches can reach in structural determinations. In this review article, it is shown how it is possible to measure the accuracy of a computed equilibrium geometry based on the comparison of rotational constants. But, it is also addressed what accuracy is required by computations for providing molecular structures and thus rotational constants that are useful to experiment. Quantum chemical methodologies for obtaining the "0.1% accuracy" for rotational constants are reviewed for systems ranging in size from small molecules to small polycyclic aromatic hydrocarbons. This accuracy for systems containing two dozen or so atoms opens the way towards future applications such as the accurate characterization of non-covalent interactions, which play a key role in several biological and technological processes.

Evidence That Shocks Destroy Small PAH Molecules in Low-luminosity Active Galactic Nuclei

We combined mapping-mode mid-infrared Spitzer spectra with complementary infrared imaging to perform a spatially resolved study of polycyclic aromatic hydrocarbon (PAH) emission from the central regions of 66 nearby galaxies, roughly evenly divided into star-forming systems and low-luminosity active galactic nuclei (AGNs). In conjunction with similar measurements available for quasars, we aim to understand the physical properties of PAHs across a broad range of black hole accretion power, with the goal of identifying observational diagnostics that can be used to probe the effect of AGNs on the host galaxy. Whereas the PAH emission correlates tightly with far-ultraviolet luminosity in star-forming regions, the spatially resolved regions of AGNs tend to be PAH deficient. Moreover, AGN regions exhibit on average smaller PAH 6.2 μm/7.7 μm and larger PAH 11.3 μm/7.7 μm band ratios. Although the current data are highly restrictive, they suggest that these anomalous PAH band ratios cannot be explained by the effects of the AGN radiation field alone. Instead, they hint that small grains may be destroyed by the combined effects of radiative processes and shocks, which are plausibly linked to jets and outflows preferentially associated with highly sub-Eddington, radiatively inefficient AGNs. While quasars also present a PAH deficit and unusual PAH band ratios, their characteristics differ in detail compared to those observed in more weakly accreting AGNs, a possible indicator of fundamental differences in their modes of energy feedback.

HOMO-LUMO gaps of large polycyclic aromatic hydrocarbons and their implication on the quantum confinement behavior of flame-formed carbon nanoparticles

Flame-formed carbon nanoparticles exhibit apparent size-dependent optical and electronic band gaps, consistent with the quantum confinement effect found in semiconductor materials. Understanding the effect of chemical composition on the band gap variations in carbon nanoparticles requires the investigation of the HOMO-LUMO gap of large aromatics. Geometry optimization with empirical force fields, followed by a density functional theory calculation, is employed here to enable the calculation of HOMO-LUMO gaps of aromatic species of sizes substantially larger than those reported in the past. Large hexagonal polycyclic aromatic hydrocarbons (PAHs) are shown to have diminishing HOMO-LUMO gaps, consistent with the behavior of two-dimensional quantum dots and approaching the zero band gap of graphene, while the HOMO-LUMO gap of polyacenes levels off at a finite value. The HOMO-LUMO gap of an aromatic cluster is found to be determined mostly by the large PAH molecules with smaller HOMO-LUMO gaps; small PAH molecules in the cluster make small or negligible contributions to the gap size of the overall cluster. Thus, if the quantum confinement of flame-formed carbon nanoparticles must be understood within the context of particle composition, the dependency of the band gap on particle size may be explained simply by the fact that larger flame-formed carbon nanoparticles contain larger PAH molecules or perhaps more precisely, have higher probabilities of containing more light absorbing species, e.g., large PAH molecules.

Adsorption of Polycyclic Aromatic Hydrocarbons and C60 onto Forsterite: C-H Bond Activation by the Schottky Vacancy.

Understanding how to catalytically break the C–H bond of aromatic molecules, such as polycyclic aromatic hydrocarbons (PAHs), is currently a big challenge and a subject of study in catalysis, astrochemistry, and planetary science. In the latter, the study of the breakdown reaction of PAHs on mineral surfaces is important to understand if PAHs are linked to prebiotic molecules in regions of star and planet formation. In this work, we employed a periodic density functional theory along with Grimme’s D4 (DFT-D4) approach for studying the adsorption of a sample of PAHs (naphthalene, anthracene, fluoranthene, pyrene, coronene, and benzocoronene) and fullerene on the [010] forsterite surface and its defective surfaces (Fe-doped and Ni-doped surfaces and a MgO-Schottky vacancy) for their implications in catalysis and astrochemistry. On the basis of structural and binding energy analysis, large PAHs and fullerene present stronger adsorption on the pristine, Fe-doped, and Ni-doped forsterite surfaces than small PAHs. On a MgO-Schottky vacancy, parallel adsorption of the PAH leads to the chemisorption process (C–Si and/or C–O bonds), whereas perpendicular orientation of the PAH leads to the catalytic breaking of the aromatic C–H bond via a barrierless reaction. Spin density and charge analysis show that C–H dissociation is promoted by electron donation from the vacancy to the PAH. As a result of the undercoordinated Si and O atoms, the vacancy acts as a Frustrated Lewis Pair (FLP) catalyst. Therefore, a MgO-Schottky vacancy [010] forsterite surface proved to have potential catalytic activity for the activation of C–H bond in aromatic molecules.

Synthesis of N/P/S Co‐Doped 3D Cross‐Linked Carbon Nanosheets by Double Activation Method for High‐Performance Supercapacitors

AbstractIt is still a challenge to prepare heteroatoms‐doped carbon materials for supercapacitors by an acid‐free and alkali‐free synthesis method. Herein, the N/P/S co‐doped carbon nanosheets (NPS‐CNs) were synthesized by an in‐situ K2CO3 and Na2S2O3 ⋅ 5H2O double activation method from pyrene as carbon source, melamine‐phytic acid supramolecular aggregate as N and P source without the presence of acid and alkali. The as‐prepared NPS‐CN featured three‐dimensional cross‐linked nanosheets with abundant pores, boosting the stability of carbon skeleton and promoting the transfer of ion and electron. The abundant micropores together with high heteroatom content (15.09 %) provided abundant active sites and defects for ion adsorption. Benefitting from these merits, the NPS‐CN electrode exhibits a high specific capacitance of 485.1 F g−1 at 0.05 A g−1 as well as good rate performance of 350.3 F g−1 at 20 A g−1 in three electrode system. Simultaneously, the specific capacitance of the NPS‐CN capacitor reached 305.0 F g−1 at 0.05 A g−1 and retained 201.2 F g−1 at 100 A g−1 in two electrode system. Importantly, NPS‐CN capacitor presented an excellent cycle stability (93.4 % of initial capacitance after 30,000 charge/discharge cycles). This work opened a less harmful route to prepare NPS‐CNs from small polycyclic aromatic hydrocarbons for energy storage devices.

Fragmentation Dynamics of Fluorene Explored Using Ultrafast XUV-Vis Pump-Probe Spectroscopy

We report on the use of extreme ultraviolet (XUV, 30.3 nm) radiation from the Free-electron LASer in Hamburg (FLASH) and visible (Vis, 405 nm) photons from an optical laser to investigate the relaxation and fragmentation dynamics of fluorene ions. The ultrashort laser pulses allow to resolve the molecular processes occurring on the femtosecond timescales. Fluorene is a prototypical small polycyclic aromatic hydrocarbon (PAH). Through their infrared emission signature, PAHs have been shown to be ubiquitous in the universe, and they are assumed to play an important role in the chemistry of the interstellar medium. Our experiments track the ionization and dissociative ionization products of fluorene through time-of-flight mass spectrometry and velocity-map imaging. Multiple processes involved in the formation of each of the fragment ions are disentangled through analysis of the ion images. The relaxation lifetimes of the excited fluorene monocation and dication obtained through the fragment formation channels are reported to be in the range of a few tens of femtoseconds to a few picoseconds.