First principles study of magnetism in nanographenes

A stable open-shell peri-hexacene with remarkable diradical character

[n]Peri-acenes ([n]PA) have attracted great interest as promising candidates for nanoelectronics and spintronics. However, the synthesis of large [n]PA (n > 4) is extremely challenging due to their intrinsic open-shell radical character and high reactivity. Herein, we report the successful synthesis and isolation of a derivative (1) of peri-hexacene in crystalline form. The structure of 1 is unequivocally confirmed by X-ray crystallographic analysis. Its ground state, aromaticity and photophysical properties are systematically studied by both experimental methods and theoretical calculations. Although the parent peri-hexacene is calculated to have a very large diradical character (y0 = 94.5%), 1 shows reasonable stability (t1/2 = 24 h under ambient conditions) due to the kinetic blocking. 1 exhibits an open-shell singlet ground state with a small singlet-triplet energy gap (-1.33 kcal/mol from SQUID measurements). 1 has also a narrow HOMO-LUMO energy gap (1.05 eV) and displays amphoteric redox behavior. This work opens new avenues for the design and synthesis of stable zigzag-edged graphene-like molecules with significant diradical character.

Peri-pentacene and Peri-hexacene Diradicaloids

Peri-acenes are valuable models for zigzag-edged graphene nanoribbons, but their synthesis poses significant challenges. In this study, stable derivatives of peri-pentacene (Peri-P) and peri-hexacene (Peri-H) were synthesized. Through kinetic blocking and a synergistic captodative effect, both compounds displayed remarkable stability under ambient air and light conditions. They show significant diradical character (y0), with y0 = 75.4% for Peri-P and y0 = 90.7% for Peri-H, alongside narrow singlet-triplet energy gaps of -1.68 ± 0.04 and -1.28 ± 0.02 kcal/mol, respectively. The structure of Peri-H was confirmed by X-ray crystallography, with bond-length analysis and theoretical calculations indicating a dominant structure featuring five aromatic sextet rings. Their optical and electrochemical properties were also studied and compared to those of smaller peri-acenes.

Pushing Up the Size Limit of Boron-doped peri-Acenes: Modular Synthesis and Characterizations

Heteroatom-doped peri-acenes (PAs) have recently attracted considerable attention considering their fascinating physical properties and chemical stability. However, the precise sole addition of boron atoms along the zigzag edges of PAs remains challenging, primarily due to the limited synthetic approach. Herein, we present a novel one-pot modular synthetic strategy toward unprecedented boron-doped PAs (B-PAs), including B-[4,2]PA (1 a-2), B-[4,3]PA (1 b-2) and B-[7,2]PA (1 c-3) derivatives, through efficient intramolecular electrophilic borylation. Their chemical structures are unequivocally confirmed with a combination of mass spectrometry, NMR, and single-crystal X-ray diffraction analysis. Notably, 1 b-2 exhibits an almost planar geometry, whereas 1 a-2 displays a distinctive bowl-like distortion. Furthermore, the optoelectronic properties of this series of B-PAs are thoroughly investigated by UV/Vis absorption and fluorescence spectroscopy combined with DFT calculation. Compared with their parent all-carbon analogs, the obtained B-PAs exhibit high stability, wide energy gaps, and high photoluminescence quantum yields of up to 84 %. This study reveals the exceptional ability of boron doping to finely tune the physicochemical properties of PAs, showcasing their potential applications in optoelectronics.

Persistent peri-Heptacene: Synthesis and In Situ Characterization

n‐peri‐Acenes (n‐PAs) have gained interest as model systems of zigzag‐edged graphene nanoribbons for potential applications in nanoelectronics and spintronics. However, the synthesis of n‐PAs larger than peri‐tetracene remains challenging because of their intrinsic open‐shell character and high reactivity. Presented here is the synthesis of a hitherto unknown n‐PA, that is, peri‐heptacene (7‐PA), in which the reactive zigzag edges are kinetically protected with eight 4‐tBu‐C₆H₄ groups. The formation of 7‐PA is validated by high‐resolution mass spectrometry and in situ FT‐Raman spectroscopy. 7‐PA displays a narrow optical energy gap of 1.01 eV and exhibits persistent stability (t_1/2≈25 min) under inert conditions. Moreover, electron‐spin resonance measurements and theoretical studies reveal that 7‐PA exhibits an open‐shell feature and a significant tetraradical character. This strategy could be considered a modular approach for the construction of next‐generation (3 N+1)‐PAs (where N≥3).

A Stable Triplet-Ground-State Conjugated Diradical Based on a Diindenopyrazine Skeleton

High-spin conjugated radicals have great potential in magnetic materials and organic spintronics. However, to obtain high-spin conjugated radicals is still quite challenging due to their poor stability. We report the successful synthesis and isolation of a stable triplet conjugated diradical, 10,12-diaryldiindeno[1,2-b:2',1'-e]pyrazine (m-DIP). With the m-xylylene analogue skeleton containing electron-deficient sp² -nitrogen atoms, m-DIP displays significant aromatic character within its pyrazine ring and its spin density mainly delocalizes on the meta-pyrazine unit, making it a triplet ground state conjugated diradical. Our work provides an effective "spin density tuning" strategy for stable high-spin conjugated radicals.

Unravelling the Open-Shell Character of Peripentacene on Au(111)

Polycyclic aromatic hydrocarbons (PAHs) are a family of organic compounds comprising two or more fused aromatic rings which feature manifold applications in modern technology. Among these species, those presenting an open-shell magnetic ground state are of particular interest for organic electronic, spintronic, and non-linear optics and energy storage devices. Within PAHs, special attention has been devoted in recent years to the synthesis and study of the acene and fused acene (periacene) families, steered by their decreasing HOMO-LUMO gap with length and predicted open-shell character above some size. However, an experimental fingerprint of such magnetic ground state has remained elusive. Here, we report on the in-depth electronic characterization of isolated peripentacene molecules on a Au(111) surface. Scanning tunnelling spectroscopy, complemented by computational investigations, reveals an antiferromagnetic singlet ground state, characterized by singlet-triplet inelastic excitations with an experimental effective exchange coupling (J_eff) of 40.5 meV. Our results deepen the fundamental understanding of organic compounds with magnetic ground states, featuring perspectives in carbon-based spintronic devices.

Hydrogenated polycyclic aromatic hydrocarbons (HnPAHs) as catalysts for hydrogenation reactions in the interstellar medium: a quantum chemical model

Density functional studies show that neutral H_nPAHs are able to catalyze the formation of water with no activation barrier.

Tailoring Bond Topologies in Open-Shell Graphene Nanostructures

Polycyclic aromatic hydrocarbons exhibit a rich spectrum of physicochemical properties depending on the size and, more critically, on the edge and bond topologies. Among them, open-shell systems-molecules hosting unpaired electron densities-represent an important class of materials for organic electronic, spintronic, and optoelectronic devices, but remain challenging to synthesize in solution. We report the on-surface synthesis and scanning tunneling microscopy- and spectroscopy-based study of two ultralow-gap open-shell molecules, namely peri-tetracene, a benzenoid graphene fragment with zigzag edge topology, and dibenzo[ a, m]dicyclohepta[ bcde, nopq]rubicene, a nonbenzenoid nonalternant structural isomer of peri-tetracene with two embedded azulene units. Our results provide an understanding of the ramifications of altered bond topologies at the single-molecule scale, with the prospect of designing functionalities in carbon-based nanostructures via engineering of bond topology.

Sequential BN-doping induced tuning of electronic properties in zigzag-edged graphene nanoribbons: a computational approach

We employed first-principles methods to elaborate doping induced electronic and magnetic perturbations in one-dimensional zigzag graphene nanoribbon (ZGNR) superlattices. Consequently, the incorporation of alternate boron and nitrogen (hole-electron) centers into the hexagonal network instituted substantial modulations to electronic and magnetic properties of ZGNR. Our theoretical analysis manifested some controlled changes to electronic and magnetic properties of the ZGNR by tuning the positions (array) of impurity centers in the carbon network. Subsequent DFT based calculations also suggested that the site-specific alternate electron-hole (B/N) doping could regulate the band-gaps of the superlattices within a broad range of energy. The consequence of variation in the width of ZGNR in the electronic environment of the system was also tested. The systematic analysis of various parameters such as the structural orientations, spin-arrangements, the density of states (DOS), band structures, and local density of states envisioned a basis for the band-gap engineering in ZGNR and attributed to its feasible applications in next generation electronic device fabrication.

Electron Pair Repulsion Responsible for the Peculiar Edge Effects and Surface Chemistry of Black Phosphorus

The electronic and optical properties of black phosphorus (black-P) are significantly modulated by fabricating the edges of this two-dimensional material. Electron lone pairs (ELPs) are ubiquitous in black-P, but their role in creating the edge effects of black-P is poorly understood. Using first-principle calculations, we report ELPs of black-P experience severe Coulomb repulsion and play a central role in creating the edge effects of black-P. We discover the outermost P atoms of the zigzag edges of black-PQDs are free of the Coulomb repulsion, but the P atoms of the armchair edges do experience the Coulomb repulsion. The Coulomb repulsion serves as a new chemical driving force to make electron donor-acceptor bonds with chemical groups bearing vacant orbitals. Our results provide insights into the mechanism responsible for the peculiar edge effects of black-P and highlight the opportunity to use the ELPs of black-P for their damage-free surface functionalization.

Building a 22-ring nanographene by combining in-solution and on-surface syntheses

A nanographene formed by the fusion of 22 benzene rings has been prepared by combining in-solution cycloaddition reactions and on-surface cyclodehydrogenations.

Semiconducting Self-Assembled Nanofibers Prepared from Photostable Octafluorinated Bisanthene Derivatives

Bisanthene is an important class of small two-dimensional polycyclic aromatic hydrocarbons with a zigzag-edged graphene nanoribbon character. Therefore, the functionalization and deep understanding of the structure-property relationship of bisanthene would provide an effective design for small organic molecular devices. In this study, octa- and tetrafluorinated bisanthene derivatives were synthesized for investigating the effect of electronegative fluorine substitution on the structure and physical property of bisanthene. Firstly, the octafluorinated bisanthene derivative has a twisted structure due to the steric repulsion of fluorine atoms at the bay region. Secondly, the absorption and fluorescence peak maxima are blueshifted with an increase in the degree of fluorine substitution. Notably, a triisopropylsilylethynyl-substituted octafluorinated derivative (F8) exhibited strong fluorescence at 657 nm with high fluorescence quantum yield (84 %). Additionally, cyclic voltammograms indicate the positive effect of fluorine substitution on the high highest occupied molecular orbital energy level of the molecules; thus, F8 molecule exhibited a remarkably increased photostability. Finally, the self-assembled behavior of fluorinated compounds was investigated by scanning electron microscopy and X-ray diffraction analysis. Specifically, F8 self-assembled to form bundles of long semicrystalline nanofibers exhibiting hole-transporting properties (3.4×10^-3  cm²  V^-1  s^-1 ).

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.

Polycyclic aromatic hydrocarbons with tunable optoelectronic properties promise applications in future high tech devices. Here, the authors demonstrate an efficient method for preparing a series of stable 2D twisted dibenzoterrylene-acenes, and investigate their electronic structures and geometries.

Zethrenes, extended p-quinodimethanes, and periacenes with a singlet biradical ground state

Researchers have studied polycyclic aromatic hydrocarbons (PAHs) for more than 100 years, and most PAHs in the neutral state reported so far have a closed-shell electronic configuration in the ground state. However, recent studies have revealed that specific types of polycyclic hydrocarbons (PHs) could have a singlet biradical ground state and exhibit unique electronic, optical, and magnetic activities. With the appropriate stabilization, these new compounds could prove useful as molecular materials for organic electronics, nonlinear optics, organic spintronics, organic photovoltaics, and energy storage devices. However, before researchers can use these materials to design new devices, they need better methods to synthesize these molecules and a better understanding of the fundamental relationship between the structure and biradical character of these compounds and their physical properties. Their biradical character makes these compounds difficult to synthesize. These compounds are also challenging to physically characterize and require the use of various experimental techniques and theoretic methods to comprehensively describe their unique properties. In this Account, we will discuss the chemistry and physics of three types of PHs with a significant singlet biradical character, primarily developed in our group. These structures are zethrenes, Z-shaped quinoidal hydrocarbons; hydrocarbons that include a proaromatic extended p-quinodimethane unit; and periacenes, acenes fused in a peri-arrangement. We used a variety of synthetic methods to prepare these compounds and stabilized them using both thermodynamic and kinetic approaches. We probed their ground-state structures by electronic absorption, NMR, ESR, SQUID, Raman spectroscopy, and X-ray crystallography and also performed density functional theory calculations. We investigated the physical properties of these PHs using various experimental methods such as one-photon absorption, two-photon absorption, transient absorption spectroscopy, electrochemistry, and spectroelectrochemistry. These systematic studies revealed that aromaticity played a very important role in determining their singlet biradical character, which is critically related to both their physical properties and their chemical reactivity. In particular, we found that Clar's aromatic sextet rule, which is useful for the closed-shell PAHs, can also predict the relative biradical character of benzenoid PH-based singlet biradicaloids. Other factors, such as structural flexibility of the biradical and quinoid resonance forms and the participation of the substitution in the π-conjugation, also influence the biradical character. These molecular materials demonstrate a number of unique properties such as near-infrared absorption, redox amphotericity, large two-photon absorption cross section, short excited state lifetime, stimuli-responsive magnetic activity, and singlet fission, which suggests promise for future applications.

Closed-shell and open-shell 2D nanographenes

This chapter describes a series of two-dimensional (2D) expanded arene networks, also known as nanographenes, with either closed-shell or open-shell electronic structure in the ground state. These systems are further categorized into three classes on a basis of different edge structures: those with zigzag edges only, those with armchair edges only, and those possessing both. Distinctive physical properties of these 2D aromatic systems are closely related to their structural characteristics and provide great potential for them as materials for different applications.

The edges of graphene

The edge of two dimensional (2D) graphene, as the surface of a three dimensional (3D) crystal, plays a crucial role in the determination of its physical, electronic and chemical properties and thus has been extensively studied recently. In this review, we summarize the recent advances in the study of graphene edges, including edge formation energy, edge reconstruction, method of graphene edge synthesis and the recent progress on metal-passivated graphene edges and the role of edges in graphene CVD growth. We expect this review to provide a guideline for readers to gain a clear picture of graphene edges from several aspects, especially the catalyst-passivated graphene edges and their role in graphene CVD growth.

Large scale computational chemistry modeling of the oxidation of highly oriented pyrolytic graphite

Large scale molecular dynamics (MD) simulations are performed to study the oxidation of highly oriented pyrolytic graphite (HOPG) by hyperthermal atomic oxygen beam (5 eV). Simulations are performed using the ReaxFF classical reactive force field. We present here additional evidence that this method accurately reproduces ab initio derived energies relevant to HOPG oxidation. HOPG is modeled as multilayer graphene and etch-pit formation and evolution is directly simulated through a large number of sequential atomic oxygen collisions. The simulations predict that an oxygen coverage is first established that acts as a precursor to carbon-removal reactions, which ultimately etch wide but shallow pits, as observed in experiments. In quantitative agreement with experiment, the simulations predict the most abundant product species to be O2 (via recombination reactions), followed by CO2, with CO as the least abundant product species. Although recombination occurs all over the graphene sheet, the carbon-removal reactions occur only about the edges of the etch pit. Through isolated defect analysis on small graphene models as well as trajectory analysis performed directly on the predicted etch pit, the activation energies for the dominant reaction mechanisms leading to O2, CO2, and CO product species are determined to be 0.3, 0.52, and 0.67 eV, respectively. Overall, the qualitative and quantitative agreement between MD simulation and experiment is very promising. Thus, the MD simulation approach and C/H/O ReaxFF parametrization may be useful for simulating high-temperature gas interactions with graphitic materials where the microstructure is more complex than HOPG.

Half-metallicity of graphene nanoribbons and related systems: a new quantum mechanical El Dorado for nanotechnologies... or a hype for materials scientists?

In this work we discuss in some computational and analytical details the issue of half-metallicity in zig-zag graphene nanoribbons and nanoislands of finite width, i.e. the coexistence of metallic nature for electrons with one spin orientation and insulating nature for the electrons of opposite spin, which has been recently predicted from so-called first-principle calculations employing Density Functional Theory. It is mathematically demonstrated and computationally verified that, within the framework of non-relativistic and time-independent quantum mechanics, like the size-extensive spin-contamination to which it relates, half-metallicity is nothing else than a methodological artefact, due to a too approximate treatment of electron correlation in the electronic ground state.