Abstract

Olympicene, systematically named 6''H''-benzo[''cd'']pyrene (CAS Registry Number: 191-33-3), represents a structurally distinctive polycyclic aromatic hydrocarbon composed of five fused rings arranged in a configuration reminiscent of the Olympic symbol. This C₁₉H₁₂ compound exhibits characteristic aromatic properties with 18 π-electrons distributed across its ring system. The molecule displays a planar geometry with the exception of the central ring, which adopts a non-aromatic character. Olympicene manifests as a white crystalline solid with a density of 1.28 g/cm³ and a boiling point of 511.75 °C at standard atmospheric pressure. Its synthesis involves a multi-step approach utilizing Wittig and Friedel-Crafts reactions, followed by reduction and dehydration steps. The compound's unique topology and electronic structure make it particularly suitable for scanning probe microscopy studies and fundamental investigations of aromatic systems.

Introduction

Olympicene belongs to the class of polycyclic aromatic hydrocarbons (PAHs), characterized by fused benzene rings in various topological arrangements. The compound derives its common name from its structural resemblance to the Olympic rings symbol, consisting of five annulated rings with four being aromatic benzene rings. This organic molecule was conceptually designed in 2010 by Graham Richards of the University of Oxford and Antony Williams as a chemical tribute to the 2012 London Olympics. The first successful synthesis was achieved by Anish Mistry and David Fox at the University of Warwick, while computational studies of its electronic structure and relative stability compared to isomers were conducted by Andrew Valentine and David Mazziotti at the University of Chicago.

Molecular Structure and Bonding

Molecular Geometry and Electronic Structure

The molecular geometry of olympicene exhibits approximate planarity with minor deviations from perfect flatness due to subtle steric interactions. X-ray crystallographic analysis of the closely related compound benzo[''c'']phenanthrene suggests that olympicene maintains greater planarity owing to the presence of a methylene bridge that alleviates steric clash between hydrogen atoms at the ring junctions. The central ring, consisting of six carbon atoms, does not participate in the aromatic system and displays bond length alternation characteristic of cyclohexadiene-like structure.

Molecular orbital calculations indicate that olympicene contains 18 π-electrons distributed across the four peripheral benzene rings, satisfying Hückel's rule for aromaticity (4n+2 π-electrons where n=4). The highest occupied molecular orbital (HOMO) demonstrates significant electron density localization on the outer rings, while the lowest unoccupied molecular orbital (LUMO) shows more uniform distribution across the entire π-system. The HOMO-LUMO gap measures approximately 3.2 eV, as determined by computational methods, indicating moderate electronic stability.

Chemical Bonding and Intermolecular Forces

Covalent bonding in olympicene follows typical patterns for fused aromatic systems, with carbon-carbon bond lengths ranging from 1.38 Å to 1.42 Å for aromatic bonds and 1.46 Å to 1.54 Å for single bonds in the aliphatic region. The methylene bridge carbon exhibits sp³ hybridization with bond angles of approximately 109.5°, while the aromatic carbons maintain sp² hybridization with bond angles near 120°.

Intermolecular forces in crystalline olympicene are dominated by van der Waals interactions and π-π stacking between adjacent molecules. The compound demonstrates limited hydrogen bonding capability due to the absence of strong hydrogen bond donors or acceptors. The molecular dipole moment measures approximately 0.8 Debye, primarily resulting from the asymmetry introduced by the methylene bridge and the uneven distribution of electron density across the ring system.

Physical Properties

Phase Behavior and Thermodynamic Properties

Olympicene presents as a white crystalline powder at standard temperature and pressure. The compound exhibits a density of 1.28 g/cm³ in the solid state. Thermal analysis indicates a boiling point of 511.75 °C at 760 mmHg and a flash point of 254.20 °C. The melting point has not been precisely determined experimentally but is estimated to fall within the range of 280-300 °C based on analogous compounds.

The enthalpy of formation for olympicene is calculated to be approximately 245 kJ/mol using computational methods. The compound demonstrates moderate thermal stability, decomposing above 400 °C through complex pathways characteristic of polycyclic aromatic hydrocarbons. Sublimation occurs at elevated temperatures under reduced pressure, facilitating purification through sublimation techniques.

Spectroscopic Characteristics

Nuclear magnetic resonance spectroscopy of olympicene reveals distinctive patterns consistent with its symmetric yet asymmetric structure. Proton NMR displays signals in the aromatic region between 7.0 and 8.5 ppm, with the methylene protons appearing as a singlet near 4.0 ppm. Carbon-13 NMR shows signals corresponding to 19 distinct carbon environments, with chemical shifts ranging from 20 ppm for the methylene carbon to 130-140 ppm for the aromatic carbons.

Infrared spectroscopy exhibits characteristic aromatic C-H stretching vibrations at 3050 cm⁻¹ and aliphatic C-H stretches at 2920 cm⁻¹ and 2850 cm⁻¹. The fingerprint region shows aromatic C=C stretching vibrations between 1600 cm⁻¹ and 1450 cm⁻¹. UV-Vis spectroscopy demonstrates absorption maxima at 280 nm, 320 nm, and 380 nm, corresponding to π-π* transitions within the conjugated system.

Chemical Properties and Reactivity

Reaction Mechanisms and Kinetics

Olympicene undergoes electrophilic aromatic substitution reactions preferentially at positions ortho and para to the methylene bridge, where electron density is highest. Reactions with electrophiles such as nitronium ions and acyl cations proceed with moderate rates, comparable to other polycyclic aromatic hydrocarbons with similar electron density distributions. The compound demonstrates relative resistance to oxidation but undergoes gradual photooxidation under ultraviolet light exposure.

Hydrogenation reactions occur selectively at the central non-aromatic ring, preserving the aromatic character of the peripheral benzene rings. Complete hydrogenation requires severe conditions due to the stability of the aromatic system. Olympicene participates in Diels-Alder reactions as a dienophile, particularly at the central ring, which exhibits enhanced reactivity compared to the aromatic rings.

Acid-Base and Redox Properties

The compound exhibits no significant acid-base character in aqueous systems due to the absence of ionizable functional groups. Olympicene demonstrates moderate resistance to reduction, with the first reduction potential measured at -1.8 V versus the standard hydrogen electrode. Oxidation occurs at potentials above +1.2 V, leading to the formation of radical cations that subsequently undergo dimerization or reaction with nucleophiles.

Synthesis and Preparation Methods

Laboratory Synthesis Routes

The synthesis of olympicene begins with pyrene-1-carboxaldehyde as the starting material. The first step involves preparation of a phosphonium salt through reaction of triphenylphosphine with ethyl bromoacetate in anhydrous toluene. Subsequent treatment with mild base generates the corresponding ylide, which undergoes Wittig reaction with pyrene-1-carboxaldehyde to produce an α,β-unsaturated ester.

Hydrogenation of the double bond using palladium on carbon catalyst in ethyl acetate affords the saturated ester. Saponification with potassium hydroxide followed by acidification yields the carboxylic acid, which is converted to the acid chloride using thionyl chloride. Intramolecular Friedel-Crafts acylation with aluminum chloride catalyst in dichloromethane produces a ketone intermediate. Reduction with lithium aluminum hydride in anhydrous ether gives the corresponding alcohol, which undergoes acid-catalyzed dehydration using ion exchange resin to furnish olympicene.

Analytical Methods and Characterization

Identification and Quantification

Olympicene is routinely characterized by combination of chromatographic and spectroscopic techniques. High-performance liquid chromatography with UV detection provides effective separation from related polycyclic aromatic hydrocarbons, with retention times typically between 15 and 20 minutes on reverse-phase C18 columns using acetonitrile/water mobile phases. Gas chromatography-mass spectrometry exhibits a molecular ion peak at m/z 240 with characteristic fragmentation patterns including loss of CH₂ (m/z 226) and sequential loss of acetylene units.

Quantitative analysis is achieved through calibrated HPLC methods with detection limits of approximately 0.1 μg/mL. Nuclear magnetic resonance spectroscopy serves as a definitive identification method, with the characteristic methylene proton signal providing a distinctive fingerprint. X-ray crystallography has been employed to confirm the molecular structure, though suitable crystals require careful preparation due to the compound's tendency to form microcrystalline aggregates.

Applications and Uses

Research Applications and Emerging Uses

Olympicene serves primarily as a model compound for fundamental studies in surface science and molecular imaging. The compound's distinctive five-ring structure makes it particularly suitable for scanning probe microscopy investigations, as demonstrated by IBM researchers in Zurich who obtained high-resolution images using non-contact atomic force microscopy. These studies provide insights into molecular adsorption on surfaces and intermolecular interactions at the nanoscale.

The molecule finds application in theoretical chemistry as a test system for computational methods addressing electronic structure and aromaticity in polycyclic systems. Its non-planar yet conjugated structure presents challenges for molecular orbital theories, making it valuable for benchmarking computational approaches. Olympicene derivatives with functional groups attached to the methylene bridge show potential as building blocks for molecular materials with tailored electronic properties.

Historical Development and Discovery

The conceptual design of olympicene emerged in March 2010 as a collaborative effort between computational chemists and synthetic organic chemists seeking to create a molecule that would symbolically celebrate the 2012 London Olympics. Graham Richards and Antony Williams proposed the basic structure based on computational modeling, while Anish Mistry and David Fox at the University of Warwick developed the synthetic route that first produced the compound in laboratory quantities.

The initial synthesis represented a significant achievement in organic synthesis due to the challenges associated with constructing the specific ring topology. Early characterization included scanning tunneling microscopy images that confirmed the molecular structure. Subsequent refinement of synthetic methods improved yields and purification protocols. The compound's popularity stems not only from its symbolic value but also from its utility as a model system for studying aromaticity and molecular imaging techniques.

Conclusion

Olympicene stands as a structurally unique polycyclic aromatic hydrocarbon whose distinctive five-ring topology has captured both scientific and popular interest. The compound exhibits characteristic aromatic properties while maintaining a non-aromatic central ring that provides synthetic challenges and interesting electronic characteristics. Its synthesis through a multi-step route demonstrates sophisticated organic synthesis techniques, while its applications in surface science and computational chemistry highlight its value as a model system.

Future research directions include development of more efficient synthetic routes, exploration of functionalized derivatives for materials applications, and continued use as a benchmark system for theoretical calculations. The compound's well-defined structure and relative stability make it particularly suitable for advanced microscopy studies, potentially contributing to improved understanding of molecular adsorption and self-assembly phenomena at surfaces.