Ring Currents and the PCP Rule

According to the recently discovered PCP rule, the intensity of cyclic conjugation in the fivemembered rings of polycyclic conjugated hydrocarbons related to acenaphthylene and fluoranthene increases with the number of phenyl-cyclopentadienyl (PCP) fragments present in the molecule. The validity of this regularity was first observed by studying the energy effects of cyclic conjugation in the fivemembered rings of these systems. In order to show that the PCP rule is not an artifact of the method employed, it is necessary to confirm its validity by other approaches. We now show that the PCP rule is in perfect agreement with calculated topological π-electron ring-currents, ostensibly an independent, quantitative, theoretical – albeit, it is here argued, likewise also graph-theoretical – measure of cyclic conjugation in the individual rings of polycyclic π-electron systems. In addition, we show that the molecularstructure dependency of the ring currents in acenaphthylene and fluoranthene congeners also agrees with other, earlier-established, regularities for cyclic conjugation

Correlations between Topological Ring-Currents, π-Electron Partitions, and Six-Centre Delocalisation Indices in Benzenoid Hydrocarbons

Comparison is made between three different indices that characterise the individual rings of a wide range of condensed benzenoid hydrocarbons. Two of these ("π-electron partitions" and the six-centre delocalisation-indices that have been called "Δ6-values") have been introduced only recently as potential indicators of what might be called "local aromaticity", whilst the third ("topological π-electron ring-currents") was suggested as a possible discriminator in this regard nearly fifty years ago. Whilst linear correlations between ring currents and π-electron partitions within certain restricted classes of ring types are good (with correlation coefficients of up to 0.998), agreement between the two indices over all classes of ring types is poor. Predictions arising from a consideration of π-electron partitions and Δ6-values seem, on the other hand, to be in somewhat better accord. It is therefore concluded that, despite its superficially intuitive appeal, the ring-current index is out of step both with π-electron partitions and Δ6-values as a general indicator of so-called "local aromaticity". (doi: 10.5562/cca1846

Utilization of K-region oxidation for the synthesis of functional materials

Pyrene with its long fluorescence lifetime, large Stokes shift and proclivity towards excimer formation has become the gold standard in the sensing of microenvironments using fluorescence spectroscopy. Derivatives of pyrene find applications in a broad range of fields. However, the design of more complex systems is hampered by the lack of selectivity one encounters when working with pyrene. Alkylation, halogenation and nitration produce practically inseparable isomers. Methods that desymmetrize pyrene are currently of high value. Known methods have recently been improved and applied to a number of systems within the Bodwell group. The optimization of a K-region oxidation reaction and utilization of desymmetrized pyrene in the synthesis of redox-active molecules and progress towards a 2,7-pyrenylene-ethynylene macrocycle is investigated in this thesis work. References [1] R. Rieger, K. Müllen, J. Phys. Org. Chem., 2010, 23, 315-325. [2] J. Andréasson, U. Pischel, Chem. Soc. Rev., 2015, 44, 1053-1069. [3] P. Conlon, C. J. Yang, Y. Wu, Y. Chen, K. Martinez, Y. Kim, N. Stevens, A. A. Marti, S. Jockusch, N. J. Turro, W. Tan, J. Am. Chem. Soc., 2008, 130, 336-342

Chemical and Photophysical Behaviour of π-Extended Tropyliums

Non-benzenoid annulenes have long fascinated organic chemists, due to their chemical properties, aromaticity, and photophysical behaviour. When incorporated into a polycyclic aromatic framework, such non-hexagonal rings can give rise to nonplanar structures with modified optical properties and improved solubility compared to planar, defect-free graphene. Among non-benzenoid annulenes, the tropylium cation has been of special interest, due to its unique blend of reactivity (owing to its positive charge) and stability (a result of its aromaticity). Indeed, this cation has found wide utility as a versatile ligand , stimulus-responsive dye , and an organocatalyst. Yet, reports of polycyclic aromatics featuring this heptagonal annulene are sparse. This Thesis employs tropylium and its neutral, nonaromatic homologue, cycloheptatriene (Figure 1) as key structural motifs within an extended π-framework to gain fundamental insights into the electronic and optical properties of non-benzenoid and charged polycyclic aromatics. We find that judicious engineering of strain into the framework of sterically overcrowded tropyliums can cause its aromaticity to rupture, forming an “aromatic-to-nonaromatic” equilibrium at room temperature. Moreover, modifying the conjugation length in a series of cycloheptatriene-rotors was found to vastly alter their photoluminescence properties, allowing for new modes of chemical reactivity. We also report our synthetic forays toward a highly warped redox-active warped nanographene, as well as preliminary findings on the excited-state proton transfer dynamics in a series of hydroxybenzotropyliums. 1. T. Murahashi, M. Fujimoto, M. A. Oka, Y. Hashimoto, T. Uemura, Y. Tatsumi, Y. Nakao, A. Ikeda, S. Sakaki and H. Kurosawa, Science, 2006, 313, 1104–1107 2. U. P. N. Tran, G. Oss, D. P. Pace, J. Ho and T. V. Nguyen, Chem. Sci., 2018, 9, 5145–5151. 3. D. J. M. Lyons, R. D. Crocker and T. V. Nguyen, Chem.—Eur. J., 2018, 24, 10959–10965. 4. P. K. Saha, A. Mallick, A. T. Turley, A. N. Bismillah, A. Danos, A. P. Monkman, A.- J. Avestro, D. S. Yufit and P. R. McGonigal, Nature Chem., 2023 15, 516–525. 5. A. T. Turley, P. K. Saha, A. Danos, Aisha N. Bismillah, A. P. Monkman, D. S. Yufit, B. F. E. Curchod, M. K. Etherington, and Paul R. McGonigal, Angew. Chem. Int. Ed., 2022, 61, e202202193

Estimation method for the thermochemical properties of polycyclic aromatic molecules

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, 2005.Includes bibliographical references.Polycyclic aromatic molecules, including polycyclic aromatic hydrocarbons (PAHs) have attracted considerable attention in the past few decades. They are formed during the incomplete combustion of hydrocarbon fuels and are precursors of soot. Some PAHs are known carcinogens, and control of their emissions is an important issue. These molecules are found in many materials, including coal, fuel oils, lubricants, and carbon black. They are also implicated in the formation of fullerenes, one of the most. chemically versatile class of molecules known. Clearly, models that provide predictive capability for their formation and growth are highly desirable. Thlermochemical properties of the species in the model are often the most important parameter, particularly for high temperature processes such as the formation of PAH and other aromatic molecules. Thermodynamic consistency requires that reverse rate constants be calculated from the forward rate constants and from the equilibrium constants. The later are obtained from the thermochemical properties of reactants and products. The predictive ability of current kinetic models is significantly limited by the scarcity of available thermochemical data.(cont.) In this work we present the development of a Bond-Centered Group Additivity method for the estimation of the thermochemical properties of polycyclic aromatic molecules, including PAHs, molecules with the furan substructure, molecules with triple bonds, substituted PAHs, and radicals. This method is based on thermochemical values of about two hundred polycyclic aromatic molecules and radicals obtained from quantum chemical calculations at the B3LYP/6-31G(d) level. A consistent set of homodesmic reactions has been developed to accurately calculate the heat of formation from the absolute energy. The entropies calculated from the B3LYP/6-31G(d) vibrational frequencies are shown to be at least as reliable as the few available experimental values. This new Bond-Centered Group Additivity method predicts the thermochemistry of C₆₀ and C₇₀ fullerenes, as well as smaller aromatic molecules, with accuracy comparable to both experiments and the best quantum calculations. This Bond-Centered Group Additivity method is shown to extrapolate reasonably to infinite graphene sheets.(cont.) The Bond-Centered Group Additivity method has been implemented into a computer code within the automatic Reaction Mechanism Generation software (RMG) developed in our group. The database has been organized as a tree structure, making its maintenance and possible extension very straightforward. This computer code allows the fast and easy use of this estimation method by non-expert users. Moreover, since it is incorporated into RMG, it will allow users to generate reaction mechanisms that include aromatic molecules whose thermochemical properties are calculated using the Bond-Centered Group Additivity method. Exploratory equilibrium studies were performed (l. Equilibrium concentrations of individual species depend strongly on the thermochemistry of the individual species, emphasizing the importance of consistent thermochemistry for all the species involved in the calculations. Equilibrium calculations can provide many interesting insights into the relationship between PAH and fullerenes in combustion.by Joanna Yu.Ph.D

Tetrabenzo[8]circulene: Synthesis and Structural Properties of Polycyclic Aromatic Hydrocarbons with Negative Curvature

Contorted polycyclic aromatic hydrocarbons have found increasing utility in the application of molecular electronics due to the surpamolecular properties that result from these non-planar structures. The [n]circulene series of molecules are particularly attractive members of the contorted aromatic family due to the unique structural implications that result from their changing value of n. For example, when n ≤ 5, the structures adopt a bowl-like shape; when n = 6, a planar structure is observed; and when 7 ≤ n ≤ 16, the compounds assume a saddle-like shape. Very few molecules exhibit the structural contortions that these contorted aromatics do – primarily because aromatic molecules desire to adopt highly planar conformations. Following the model of aromaticity developed by Erich Clar, we set our sights on the synthesis of tetrabenzo[8]circulene, the stabilized form of [8]circulene established through the addition of four fused benzenoid rings around the periphery of the molecule. The initial approach towards this structure employs a Diels-Alder [4 + 2] cycloaddition reaction and a palladium catalyzed arylation reaction as the key transformation steps. The results of these studies were promising, establishing the structural characterization of this new molecule and providing access to functionalized derivatives of the saddle-shaped structure. However, access towards these functionalized derivatives proved limiting, compelling us to investigate alternative synthetic methodologies. In the course of our studies, we established a new methodology towards 2,5-diarylthiophene-1-oxides, a key precursor to the Diels-Alder cycloaddition reaction. These reactive dienes are prepared from readily available arylacetylene precursors via zirconacyclopentadiene intermediates. The isolated yields of the desired thiophene-1-oxides are comparable to those obtained from previously established oxidation strategies while avoiding the formation of over-oxidation products. Of significant importance to scope of our work, this newly established methodology offers broader versatility providing products outfitted with electron-donating or electron-withdrawing groups. These new methodologies provided access to functionalized derivatives of the saddle-shaped molecule tetrabenzo[8]circulene in improved yield when coupled with a revised Diels-Alder/oxidative cyclodehydrogenation approach. This methodology affords products containing both electron-rich and electron-poor functional groups in a more efficient manner. The optoelectronic effects that result from the introduction of this functionality and investigations into the development of larger contorted aromatic systems are also discussed

Developing Methods for Growing Single-Chirality Carbon Nanotubes and Other Aromatic Systems

Thesis advisor: Lawrence T. ScottThe work described herein stems from an effort to develop a method for growing single-chirality carbon nanotubes from small hydrocarbon templates using a Diels-Alder cycloaddition/rearomatization strategy. Current technologies are incapable of producing significant amounts of homogeneous carbon nanotubes; therefore, much research has been put into the development of aromatic templates (belts and bowls), from which one type of nanotube might be grown (Chapter 1). Since no such functional template had yet been synthesized, the work in this dissertation developed reagents and methods for forming new benzene rings on aromatic test systems that would be analogous to the rim of a growing nanotube (Chapters 2 and 4). Theoretical investigations relating to nanotube dimensions (Chapter 3) were undertaken and paired with experimental work that would take into consideration the changing properties of growing tubes (Chapter 5). The test systems used for discovering new reagents for growth also became functional platforms for studies of new reactivity of polycyclic aromatic hydrocarbons (PAHs), such as bay-region oxidation (Chapter 6) and progress toward the synthesis of soluble graphene ribbons (Chapter 7). This PAH work also resulted in the observation of unique solid state properties in the crystal form (Chapter 8) and novel reactivity, generating five-membered rings by Scholl reactions of tethered PAHs (Chapter 9). Additional considerations for future nanotube templates and fullerene precursors also bore scrutiny (Chapter 10).Thesis (PhD) — Boston College, 2010.Submitted to: Boston College. Graduate School of Arts and Sciences.Discipline: Chemistry

The Hexadehydro-Diels–Alder (HDDA) Reaction-Enabled Bottom-up Synthesis of Elaborated Polycyclic Aromatics

University of Minnesota Ph.D. dissertation. May 2019. Major: Chemistry. Advisor: Thomas Hoye. 1 computer file (PDF); xvi, 482 pages.Polycyclic arenes are an important class of organic molecules with promising semiconducting properties. Their relatively low cost, band-gap tunability, and ease of fabrication render them suitable for a host of applications for the next-generation optoelectronics devices. The biggest challenge to realize the full potential of organic semiconductors is the chemical synthesis of atomically precise polycyclic aromatics. The current strategies to assemble these materials heavily rely on transition-metal catalyzed cross-coupling reactions of prefunctionalized arenes, which in a way limit the vision of material scientists when search for potential compounds. This thesis describes a complementary synthetic approach to polycyclic aromatic products from polyynes via the hexadehydro-Diels–Alder (HDDA) reaction. The HDDA reaction is a variant of the classic Diels–Alder reaction, which generates pristine benzyne intermediates purely thermally. This mechanistically intriguing transformation also has served as a great platform for many discoveries of fundamentally new reactivities. Here multiple aspects of the HDDA reaction are discussed: (1) reaction of perylenes with HDDA-benzynes and photochemical HDDA reactions, (2) accessing other reactive intermediates via HDDA-generated benzynes, (3) copper(I)- and BF3- catalyzed trapping reactions of benzynes, (4) rapid construction of polyacenes via the domino HDDA reaction, and (5) a cascade strategy for using classically generated benzynes as in situ diynophiles for accessing HDDA-naphthynes